Hypertension is defined as a mean resting arterial pressure of 140/90 mm Hg or higher and/or current use of antihypertensive drugs.
It is the most common public health problem in developed countries.
Often referred to as the “silent killer”, as affected individuals may be asymptomatic for many years and then suffer a fatal heart attack, it is a major risk factor for developing coronary artery disease, myocardial infarction, heart failure, stroke, and a leading cause of morbidity and mortality. However, among the risk factors for cardiovascular disease, it is the most modifiable.
It is often classified as primary or essential hypertension and secondary hypertension.
Primary hypertension, responsible for about 95 percent of cases, is probably the consequence of environmental factors, genetic factors, and their interaction. Among the environmental factors, diet plays a central role. Among the genetic factors, interest has focused on factors influencing the blood pressure response to salt intake, and several genotypes have been identified, many of which influence the renin-angiotensin-aldosterone system or renal salt handling.
Secondary hypertension is due to other diseases, usually endocrine, such as hyperthyroidism, hyperaldosteronism, and Cushing’s syndrome.

Contents

• Blood pressure levels and cardiovascular diseases
• Age-related hypertension
• Hypertension and childhood
• How to prevent hypertension
  • Role of potassium intake
  • Role of sodium intake
  • Role of body weight
  • Role of physical activity
• References

Blood pressure levels and cardiovascular disease

Above-optimal blood pressure levels, not yet in the hypertensive or prehypertensive range, confers an increased risk of cardiovascular disease, as shown by the fact that nearly one-third of blood pressure-related deaths from coronary heart disease are estimated to occur in non-hypertensive individuals with systolic blood pressure of 120-139 mm Hg, or diastolic blood pressure of 80-89 mm Hg. This means that the risk of cardiovascular disease increases throughout the blood pressure range, starting from 115/75 mm Hg.

Category	Blood pressure (mm Hg)
	Systolic	Diastolic
Optimal	< 120	< 80
Normal	< 130	< 85
Normal Hight	130 – 139	85 – 89
Grade 1 hypertension	140 – 159	90 – 99
Grade 2 hypertension	160 – 179	100 – 109
Grade 3 hypertension	≥180	≥110
Isolated systolic hypertension	≥140	≤ 90

Finally, pre-hypertensive individuals have a high risk, about 90%, of developing hypertension over time, although the transition is not inevitable.

Age-related hypertension

The prevalence of hypertension increases with increasing age, as shown by the fact that more than half of the adult population over 60 years old is hypertensive.
Age-related risk is a function of variables such as weight gain, low physical activity, excessive use of salt, fats and saturated fatty acids, alcohol, hypercholesterolemia, and low intake of fruits and vegetables, rather than of aging per se. For example, studies of vegetarians living in industrialized countries have shown that such dietary habits are associated with a lower increase in blood pressure with increasing age, and with a markedly lower blood pressure compared to non-vegetarians.

Hypertension and childhood

According to a study conducted by a team of researchers from Johns Hopkins University, prevention of hypertension starts in childhood.
Furthermore, a meta-analysis on studies from diverse populations, studies published between January 1970 and July 2006, have examined the tracking of blood pressure from childhood to adulthood showing that childhood blood pressure is associated with blood pressure in later life, and that a high values in childhood are likely to help predict hypertension in adulthood.
Finally, other studies have also shown that increased blood pressure among children is related to the growing obesity epidemic.

How to prevent hypertension

A downward trend in blood pressure has been documented in the USA over the last two decades, and the adoption of healthy lifestyle have contributed to this trend.
Lifestyle modifications that effectively lower blood pressure are:

• reduce the intake of salt and other forms of sodium;
• follow a diet rich in fresh fruit, vegetables, complex carbohydrates and low-fat dairy products;
• increase potassium intake by consuming fruit, vegetables and legumes;
• lose body weight if overweight, or prevent weight gain among those who are thin;
• increase physical activity of low or moderate intensity;
• stop smoking;

These changes are the first line of defense in preventing high blood pressure, and need not be made one at a time: the best results are achieved when they occur simultaneously, as demonstrated by two studies in which multicomponent interventions lowered blood pressure in hypertensive and nonhypertensive individuals.
Finally, it has been demonstrated that there is also a relationship between alcohol and hypertension.

Role of potassium intake

Potassium, an essential nutrient for humans, is the most abundant cation in intracellular fluids. It is therefore widely distributed in foods that come from living tissues, both animal and vegetable, but which have not undergone salting and/or drying. Cooking methods tend to lower the amount of potassium, as well.
Considering vegetables, the worst cooking method is boiling in plenty of water, for more than an hour, whereas the best is microwave cooking.

Potassium	Content
	>250 mg/100 g
Legumes	Dried legumes, such as chickpeas, beans, lentils, peas, and soybeans, and fresh beans.
Vegetables	Garlic, chard, cauliflower, cabbage, Brussels sprouts, broccoli, artichokes, cardoons, fennel, mushrooms, potatoes, tomatoes, spinach, zucchini.
Fruits	Avocados, apricots, bananas, fresh and dried chestnuts, watermelon, kiwi, melon, hazelnuts.
Dried fruits	Apricots, dates, figs, prunes, raisins, peanuts, almonds, walnuts, pine nuts, pistachios.
Dairy products	Milk powder (high in sodium).
	150-250 mg/100 g
Fresh legumes	Green beans, broad beans, peas.
Vegetables	Asparagus, beets, carrots, chicory, endive, lettuce, peppers, tomatoes, leeks, radishes, celery, tomato and carrot juice, pumpkin.
Fruits	Pineapple, oranges, raspberries, blueberries, loquats, pears, peaches, grapefruit, grapes.
Meat	Meat and fish products, both fresh and preserved; the latter should be avoided because of their high sodium content.

A high dietary potassium intake and blood pressure are inversely correlated, as demonstrated by animal studies, observational epidemiological studies, clinical trials, controlled feeding studies, such as the DASH Study and the OmniHeart trial, and meta-analysis. Furthermore, a high potassium intake also increases urinary sodium excretion.
The optimal strategy for increasing potassium intake is to consume foods naturally rich in the mineral, such as seasonal fruit and vegetables, and legumes, typical foods of the Mediterranean diet. It is therefore not difficult to reach the recommended daily intake, for the healthy population, equal to 4.7 g per day.

Role of sodium intake

Sodium is the most abundant cation in extracellular fluids, of which it strongly affects the osmotic pressure values.
There are three main source of sodium.
The most intuitive source is table salt, which represents up to 20 percent of the daily intake. It is important to note the terms salt and sodium are often used interchangeably, but this is incorrect. On a weight basis, salt is 40 percent sodium and 60 percent chlorine.
A second source is salt or sodium compounds added during food preparation or processing. Between 35 to 80 percent of the daily sodium intake comes from processed foods such as:

• processed, smoked or cured meat and fish;
• meat extracts, savory snacks, soy and hot sauce;
• pre-package frozen foods;
• canned soups and legumes;
• cheeses, especially long-ripened cheeses.

There are also many sodium-based food additives, often used as preservatives and flavour enhancers.
The third source is negligible, namely, the sodium naturally present in foods, generally low in fresh foods.
A high sodium intake contributes to the increase in blood pressure and the development of hypertension. This is supported by many epidemiological, animal, and migration studies, and meta-analysis, with the final evidence coming from carefully controlled dose-response studies. Furthermore, in primitive societies, where sodium intake is very low, people rarely develop hypertension, and blood pressure does not increase with increasing age.
Therefore, a reduction in sodium intake is recommended to prevent the development of hypertension. In view of the available food supply and the high daily sodium intake, a reasonable recommendation may be to limit its intake to 2.3 g per day, equal to 5.8 g per day of salt. How can this level be achieved?

• Using as little salt as possible when preparing food.
• Avoiding adding salt at the table.
• Avoiding highly salted, processed foods.

Clinical studies have documented that a reduced sodium intake is able to lower blood pressure even the setting of antihypertensive therapy, and can facilitate hypertension control.
Some components of the diet may modify the blood pressure response to sodium. A high dietary intake of foods rich in potassium and calcium may prevent or attenuate the increase in blood pressure for a given increase in sodium intake. Conversely, some data, mainly observed in animal models, suggest that a high sucrose intake could enhance salt sensitivity of blood pressure.

Note: high sodium intakes may contribute to the development of osteoporosis by increasing renal calcium excretion, particularly if daily calcium intake is low.

Role of body weight

Body weight, especially overweight and obesity, is a determinant of blood pressure at any age. Indeed:

• it has been estimated that the risk of developing high blood pressure is two to six times greater in overweight people than in normal weight people;
• there is a linear correlation between blood pressure and body weight or body mass index, which, if greater than 27, correlates with an increase in blood pressure;
• even when sodium intake is held constant, the correlation between change in weight and change in blood pressure is linear;
• 60 percent of hypertensive subjects are more than 20 percent overweight;
• the central distribution of body fat, as a determinant of blood pressure, with a waist circumference greater than 88 cm in women and 102 in men, is more important than the peripheral distribution of fat, both in men and women;
• weight loss, in both hypertensive and normotensive subjects, may reduce blood pressure, and the reduction occurs before, and without, achieving a desirable body weight.

Role of physical activity

Physical activity produces a drop in systolic and diastolic blood pressure. Therefore, for the primary prevention of hypertension, it is important to increase physical activity of low or moderate intensity for 30-45 minutes 3-4 times a week up to an hour most days, as recommended by the World Health Organization. Conversely, less active people are 30 to 50 percent more likely to develop hypertension than active people.

References

1. Appel L.J., Brands M.W., Daniels S.R., Karanja N., Elmer P.J. and Sacks F.M. Dietary approaches to prevent and treat HTN: a scientific statement from the American Heart Association. Hypertension 2006;47:296-08. doi:10.1161/01.HYP.0000202568.01167.B6
2. Bibbins-Domingo K., Chertow G.M., Coxson P.G., Moran A., Lightwood J.M., Pletcher M.J., and Goldman L. Projected effect of dietary salt reductions on future cardiovascular disease. N Engl J Med 2010;362:590-9. doi:10.1056/NEJMoa0907355
3. Cappuccio FP. Overview and evaluation of national policies, dietary recommendtions and programmes around the world aiming at reducing salt intake in the population. World Health Organization. Reducing salt intake in populations: report of a WHO forum and technical meeting. WHO Geneva 2007;1-60.
4. Chen J, Gu D., Jaquish C.E., Chen C., Rao D.C., Liu D., Hixson J.E., Lee Hamm L., Gu C.C., Whelton P.K. and He J. for the GenSalt Collaborative Research Group. Association between blood pressure responses to the cold pressor test and dietary sodium intervention in a chinese population. Arch Intern Med. 2008;168:1740-1746. doi:10.1001/archinte.168.16.1740
5. Chen X. and Wang Y. Tracking of blood pressure from childhood to adulthood. A systematic review and meta-regression analysis. Circulation 2008;117:3171-80. doi:10.1161/CIRCULATIONAHA.107.730366
6. Denton D., Weisinger R., Mundy N.I., Wickings E.J., Dixson A., Moisson P., Pingard A.M., Shade R., Carey D., Ardaillou R., Paillard F., Chapman J., Thillet J. & Michel J.B. The effect of increased salt intake on blood pressure of chimpanzees. Nature Med 1995;10:1009-1016. doi:10.1038/nm1095-1009
7. Ford E.S., Ajani U.A., Croft J.B., Critchley J.A., Labarthe D.R., Kottke T.E., Giles W.H, and Capewell S. Explaining the decrease in U.S. deaths from coronary disease, 1980-2000. N Engl J Med 2007;356:2388-98. doi:10.1056/NEJMsa053935
8. Geleijnse J.M., Witteman J.C., den Breeijen J.H., Hofman A., de Jong P., Pols H.A. and Grobbee D.E. Dietary electrolyte intake and blood pressure in older subjects: the Rotterdam Study. J Hyperten 1996;14:73741. doi:10.1097/00004872-199606000-00009
9. Gutiérrez O.M. Sodium- and phosphorus-based food additives: persistent but surmountable hurdles in the management of nutrition in chronic kidney disease. Adv Chronic Kidney Dis 2013;20(2):150-6. doi:10.1053/j.ackd.2012.10.008
10. Harlan W.R. and Harlan L.C. Blood pressure and calcium and magnesium intake. In: Laragh J.H., Brenner B.M., eds. Hypertension: pathophysiology, diagnosis and management. 2end ed. New York: Raven Press 1995;1143-1154
11. He F.J., Tan M., Ma Y., MacGregor G.A. Salt reduction to prevent hypertension and cardiovascular disease: JACC state-of-the-art review. J Am Coll Cardiol 2020;75(6):632-647. doi:10.1016/j.jacc.2019.11.055
12. Holmes E., Loo R.L., Stamler J., Bictash M., Yap I.K.S., Chan Q., Ebbels T., De Iorio M., Brown I.J., Veselkov K.A., Daviglus M.L., Kesteloot H., Ueshima H., Zhao L., Nicholson J.K. and Elliott P. Human metabolic phenotype diversity and its association with diet and blood pressure. Nature 2008;453:396-400. doi:10.1038/nature06882
13. Nugroho P., Andrew H., Kohar K., Noor C.A., Sutranto A.L. Comparison between the world health organization (WHO) and international society of hypertension (ISH) guidelines for hypertension. Ann Med 2022;54(1):837-845. doi:10.1080/07853890.2022.2044510
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15. Sesso H.D., Cook N.R., Buring J.E., Manson J.E. and Gaziano J.M. Alcohol consumption and the risk of hypertension in women and men. Hypertension 2008;51:1080-1087. doi:10.1161/HYPERTENSIONAHA.107.104968
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19. Tzoulaki I., Brown I.J., Chan Q., Van Horn L., Ueshima H., Zhao L., Stamler J., Elliott P., for the International Collaborative Research Group on Macro-/Micronutrients and Blood Pressure. Relation of iron and red meat intake to blood pressure: cross sectional epidemiological study. BMJ 2008;337:a258. doi:10.1136/bmj.a258
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Hypercholesterolemia can be caused by many factors, often present simultaneously.
What are they?

• A high intake of saturated fatty acids.
• Trans fatty acids.
• Overweight.
• Genetic causes.

And dietary cholesterol?
There is not a direct correlation between blood cholesterol and cholesterol intake. Dietary cholesterol may increase plasma cholesterol only when it is consumed with trans fats and saturated fatty acids.
However, if you want to reduce your cholesterol intake, we advise to reduce the use of animal products and/or use semi-skimmed or skimmed milk, light cheese, light yogurt, and lean meat.

Contents

• Saturated fatty acids
• Trans fatty acids
• Overweight
• Genetic causes
• References

Saturated fatty acids

A risky factor for hypercholesterolemia is a high intake of saturated fatty acids, a group of lipids that can be easily used for the endogenous synthesis of cholesterol.
These fatty acids are present in meat, diary products, and in abundance in vegetal fats and oils, such as margarine, palm oil, palm seed oil, and coconut oil, which are much used in the confectionery industry.
What to do:

• to eliminate the visible fat of meat, or buy lean cuts;
• to replace whole milk, butter, fat cheese, creams, and ice-creams with products which contain less fat, such as low-fat yogurt, semi-skimmed or skimmed milk, low-fat cheeses;
• to avoid confectionery products.

Trans fatty acids

Trans fatty acids or trans fats are an extremely risky factor, and not only for hypercholesterolemia.
Studies have observed a high atherogenic potential caused by changes in plasmatic lipoproteins, where a decrease of HDL levels, and an increase of LDL and triglyceride levels occur.
Where can they be found?

• In a lot of foods for kids.
• In baked industrial products, such as crackers, breadsticks, cakes, packed bread, and snacks.
• In a lot of industrial foods, such as soups, ready fresh or frozen meals, and mixtures to prepare pies and pizza.
• In bouillon cubes.
• In soft candies.
• In some corn flakes.
• In ice creams, in vegetal substitutes of cream, and in margarine.
• In a lot of preserves, jams included.

As regards to the content of saturated and trans fatty acids, there is often no difference between classic products and “natural” or “organic” ones.
What can we do?
To avoid to buy products that contain vegetal fats and/or hydrogenated fatty acids, and to avoid to buy fried products.

Overweight

A significant body fat gain contributes to hypercholesterolemia.
In a lot of people, the decrease in the intake of satured and trans fatty acids doesn’t reduce the cholesterolemia levels till weight starts to drop.
What to do:

• to reduce the intake of animal and vegetable fats;
• to reduce foods rich in simple sugars, such as sweets, soft drinks, desserts, candies, and cakes;
• do not win back calories you have eliminated in the preceding points by an excessive use of extravergin olive oil and starch, namely pasta, potatoes, rice, bread;
• to increase the physical activity;
• to increase the intake of fruit and vegetables.

Genetic causes

In this case, it needs a drug prescription by physician, which must be however combined with right nutritional advices.

References

1. Ascherio A., Katan M.B., Zock P.L., Stampfer M.J., Willett W.C. Trans fatty acids and coronary heart disease. N Engl J Med 1999;340:1994-1998. doi:10.1056/NEJM199906243402511
2. Benito-Vicente A., Uribe K.B., Jebari S., Galicia-Garcia U., Ostolaza H., Martin C. Familial hypercholesterolemia: the most frequent cholesterol metabolism disorder caused disease. Int J Mol Sci 2018;19(11):3426. doi:10.3390/ijms19113426
3. Fernandez M.L., Murillo A.G. Is there a correlation between dietary and blood cholesterol? Evidence from epidemiological data and clinical interventions. Nutrients 2022;14(10):2168. doi:10.3390/nu14102168
4. Hu F.B., Willett W.C. Optimal diet for prevention of coronary heart disease JAMA 2002;288:2569-2578. doi:10.1001/jama.288.20.2569
5. Lichtenstein A.H. Dietary fat, carbohydrate, and protein: effects on plasma lipoprotein patterns J. Lipid Res. 2006;47:1661-1667. doi:10.1194/jlr.R600019-JLR200
6. Lichtenstein A.H., Ausman L., Jalbert S.M., Schaefer E.J. Effect of different forms of dietary hydrogenated fats on serum lipoprotein cholesterol levels. N Engl J Med 1999;340:1933-1940. doi:10.1056/NEJM199906243402501
7. Mahan L.K., Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000
8. Mensink R.P., Katan M.B. Effect of dietary trans fatty acids on high-density and low-density lipoprotein cholesterol levels in healthy subjects. N Engl J Med 1990;323:439-445. doi:10.1056/NEJM199008163230703
9. Mozaffarian D., Katan M.B., Ascherio A., Stampfer M.J., Willett W.C. Trans fatty acids and cardiovascular disease. N Engl J Med 2006;354:1601-1613. doi:10.1056/NEJMra054035
10. Shils M.E., Olson J.A., Shike M., Ross A.C.: “Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999

The concept of mediterranean diet was developed in the 1950’s by the American physiologist Ancel Benjamin Keys.
Keys first described the relationship between diet and cardiovascular diseases or CVD, in particular thanks to the epidemiological study known as the Seven Countries Study, the first study that highlighted the mediterranean dietary pattern.
The Mediterranean diet, which is recognized as one of the healthiest dietary pattern, is rich in minimally processed plant foods, such as vegetables, legumes, cereals, preferably whole grain, with extra virgin olive oil as the main source of lipids. Therefore, it is a dietary pattern rich in antioxidant compounds, and with anti-inflammatory action.
The Seven Country Study was followed by many other studies that  highlighted, in populations of industrialized and non-industrialized countries, the protective role of this dietary pattern not only on CVD but also chronic-degenerative diseases, depressive disorders, as well as a correlation with improvements in learning ability. And it has been shown that greater adherence to the mediterranean diet is associated with improved health status and reduced mortality in general.
For all this, there is no scientific association that support that it is harmful for health.
In addition, thanks to the reduced consumption of meat, mediterranean diet improve public health by contributing to the reduction of greenhouse gas emissions.
Ultimately, the mediterranean diet is a dietary pattern that must be safeguarded and promoted, as opposed to the worldwide trends toward dietary uniformity.

Contents

• Ancel Keys and the Seven Countries Study
• Characteristics of the mediterranean diet
• Mediterranean diet and chronic diseases
• Role in the reduction of greenhouse gas emissions
• References

Ancel Keys and the Seven Countries Study

Keys identified the correlation between diet and cardiovascular disease risk in the early 1950s by comparing the rate of CVD occurrence of American business executives, and European populations just out of World War II. While in the former, well nourished subjects, the onset rates were high, in the latter, who were in a phase of food insecurity, the rates were low. These observations led Keys to hypothesize a correlation between dietary fat intake and deaths from cardiovascular diseases.
The subsequent observation of an extremely low frequency of coronary heart disease and certain cancers in the population of the island of Crete, Greece, in much of the rest of Greece population and in southern Italy compared to USA population, led Keys to hypothesize that the diet of those populations, characterized by a low content of animal fats, represented a protection factor, and to initiate the long-term observational study known as the Seven Countries Study, an epidemiological longitudinal study and the best known study on the Mediterranean diet.
This study showed:

• an inverse correlation between diet and the risk of death in general and from cardiovascular diseases;
• that saturated fats was the major dietary risk factor;
• that a Mediterranean-type diet led to a reduction in the risk of developing cardiovascular diseases.

Characteristics of the mediterranean diet

Mediterranean diet is a dietary pattern characterized by the consumption of large quantities of vegetables, legumes, fruits, cereals, preferably whole grain, and extra virgin olive oil, which ensures a good supply of fiber, antioxidants, phytosterols, polyphenols and unsaturated fatty acids.
Regarding products of animal origin, the consumption of meat, especially red meat and red meat products, as well as high-fat dairy products should be limited, whereas fish and seafood should be present.
Ethanol consumption should be moderate, primarily in the form of red wine and during meals.
And in the Greek population segment that participated to EPIC study, extra virgin olive oil, vegetables, legumes, a moderate intake of ethanol, together with a low consumption of meat and meat products are the dominant components predictor of lower mortality.
Cornerstone of the mediterranean diet is extra virgin olive oil. It is an excellent source of monounsaturated fatty acids and contains over 2000 different compounds, many with antioxidant activity.
Being the major source of lipids, when associated with a low consumption of high fat animal products, it ensures a high ratio of monounsaturated to saturated fatty acids, which improves lipid profile and glycemic control in diabetics. For a more extensive discussion on this topic the articles “Olive oil: chemical composition” and “Polyphenols in olive oil“.
However, it is misleading to focus on a single element of this eating pattern; it does not exist “the magic bullet” as shown by studies focused on a single element. People don’t eat a single nutrient but a complex of them and, more important, nutrients interact with each other in synergistic or antagonist ways. So, the health benefits of Mediterranean diet are due to all its components.

Mediterranean diet and chronic diseases

After the Seven Countries Study, many studies have shown the effectiveness of this dietary pattern in primary and secondary prevention of the main chronic diseases, from cardiovascular diseases to depressive disorders, as well as a reduction in mortality in general.
Here are some examples.

• A meta-analysis have evaluated the association between adherence to the mediterranean diet pattern, mortality, and incidence of diseases, showing that “greater adherence to a Mediterranean Diet is significantly associated with a reduced risk of overall mortality, cardiovascular mortality, cancer incidence and mortality, and incidence of Parkinson’s disease.” (Sofi F. at al. BMJ 2008, see References).
• A randomized multicenter study has demonstrated the its efficacy in primary prevention of cardiovascular events in subjects at high cardiovascular risk.
• It is related to a lower risk for Alzheimer’s disease and to its subsequent course and outcome: the higher adherence is associated with lower mortality and it is suggested a dose-response effect.
• Mounting evidence suggest a protective effect on weight gain.
• It has been reported an inverse association between adherence to this dietary pattern and the incidence of type 2 diabetes among initially healthy people and in patients who survived myocardial infarction.
• It is associated with a lower prevalence of the metabolic syndrome.
• Epidemiological and interventional studies have revealed a protective effect against mild chronic inflammation and its metabolic complications.
• There is evidence that adherence to the Mediterranean diet may have a protective role in the prevention of depressive disorders.

Role in the reduction of greenhouse gas emissions

The mediterranean diet is able to improve public health also by contributing to the reduction of greenhouse gas emissions, namely, carbon dioxide or CO₂, methane, nitrous oxide and similar, from the livestock sector, responsible for 4/5 of emissions related to agriculture. These emissions are greater than those due to transport, and second only to those of energy production. Adding to this that world population is growing, and that this growth is accompanied by an increase in per capita consumption of meat, with estimates that by 2030 there will be an increase in meat production of 85% compared to 2000, the role of the mediterranean diet in reducing greenhouse gas emissions is even more evident.
Analyzing in detail the greenhouse gas emission from cattle farming, the major contributor of the emissions in the livestock sector:

• about 40% comes from the loss of annual plants, grasses and trees that covered the land where the crop is grown;
• 32% from the methane emissions of animal waste, and by the animals themselves as a result of digestion;
• 14% from fertilizers to grow feed grain, 16 pounds of grain fodder for every kilogram of meat consumed;
• 14% from agricultural production generally.

Foods	Distance	Grams of CO2 equivalent
Patatoes	0,17 miles – 300 meters	59
Apples	0.2 miles – 320 meters	68
Asparagus	0.27 miles – 440 meters	91
Chicken	0.73 miles – 1.17 kilometers	249
Pork	2.52 miles – 4.1 kilometers	862
Beef	9.81 miles – 15.8 kilometers	3.360

The table above compares CO₂ emission from the production of different foods, considering portions of 225 g, with those from a gasoline car that travels about 12 km per litre of fuel.
So producing 225 grams of beef releases to the atmosphere an amount of greenhouse gases almost 13 times greater than that released producing an equal amount of chicken, and even 57 times greater if we consider potatoes.
To take another example, to produce 41 kilograms of beef, the amount annually consumed by the average American, it releases the same amount of CO₂ of a car traveling about 3,000 km.

References

1. Di Daniele N., Noce A., Vidiri M., Moriconi E., Marrone G., Annicchiarico-Petruzzelli M., D’Urso G., Tesauro M., Rovella V., De Lorenzo A. Impact of Mediterranean diet on metabolic syndrome, cancer and longevity. Oncotarget 2017;8:8947-8979. doi:10.18632/oncotarget
2. Estruch R., Ros E., Salas-Salvadó J., et al. Primary prevention of cardiovascular disease with a Mediterranean Diet. N Engl J Med 2013;368:1279-1290. doi:10.1056/NEJMoa1200303
3. Friel S., Dangour A.D., Garnett T., Lock K., Chalabi Z., Roberts I., Butler A., Butler C.D., Waage J., McMichael A.J. and Haines A. Public health benefits of strategies to reduce greenhouse-gas emissions: food and agriculture. Lancet 2009;374:2016-2025. doi:10.1016/S0140-6736(09)61753-0
4. Giugliano D. and Esposito K. Mediterranean Diet and Cardiovascular Health. Annals NY Acad Sci 2005;1056(1):253-260. doi:10.1196/annals.1352.012
5. Giugliano D. and Esposito K. Mediterranean diet and metabolic diseases. Curr Opin Lipidol 2008;19:63-68. doi:10.1097/MOL.0b013e3282f2fa4d
6. Keys A. Mediterranean diet and public health: personal reflections. Am J Clin Nutr 1995;61:1321S-1323S doi:10.1093/ajcn/61.6.1321S
7. Keys A., Aravanis C., Blackburn H., Buzina R., Djordjevic B.S., Dontas A.S., Fidanza F., Karvonen M.J., Kimura N., Menotti A., Mohacek I., Nedeljkovic S., Puddu V., Punsar S., Taylor H.L., Van Buchem F.S.P. Seven Countries: A Multivariate Analysis of Death and Coronary Heart Disease. Harvard University Press, Cambridge, Harvard University Press, ISBN: 0-674-80237-3, 1980. 381 pp.
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The human gastrointestinal tract is one of the most fierce and competitive ecological niches. It harbors viruses, eukaryotes, bacteria, and one member of Archaebacteria, Methanobrevibacter smithii.
Bacteria vary in proportion and amount all along the gastrointestinal tract; the greatest amount is found in the colon, which contains over 400 different species belonging to 9 phyla or divisions, of the 30 recognized phyla, hereafter referred as gut microbiota, which in turn is part of the larger human microbiota.
These are the phyla and some of their most represented genera.

• Actinobacteria, Gram-positive bacteria; Bifidobacterium, Collinsella, Eggerthella, and Propionibacterium.
• Bacteroidetes, Gram-negative bacteria; more than 20 genera including Bacteroides, Prevotella and Corynebacterium.
• Cyanobacteria, Gram-negative bacteria.
• Firmicutes, Gram-positive bacteria; at least 250 genera, including Mycoplasma, Bacillus, Clostridium, Dorea, Faecalibacterium, Ruminococcus, Eubacterium, Staphylococcus, Streptococcus, Lactobacillus, Lactococcus, Enterococcus, Sporobacter, and Roseburia.
• Fusobacteria, Gram-negative bacteria.
• Lentisphaerae, Gram-negative bacteria.
• Proteobacteria, Gram-negative bacteria; Escherichia, Klebsiella, Shigella, Salmonella, Citrobacter, Helicobacter, and Serratia.
• Spirochaeates, Gram-negative bacteria.
• Verrucomicrobia, Gram-negative bacteria.

The presence of a small subset of the bacterial world in the colon is the result of a strong selective pressure which acted, during evolution, on both the microbial colonizers, selecting organisms very well adapted to this environment, and the intestinal niche. And nevertheless, each individual harbors an unique bacterial community in his gut.
Despite the high variability existing both with regard to taxa and between individuals, it has been proposed, but not accepted by all researchers, that in most adults the bacterial gut microbiota can be classified into variants or “enterotypes”, on the basis of the ratio of the abundance of the genera Bacteroides and Prevotella. This seems to indicate that there is a limited number of well balanced symbiotic states, which could respond differently to factors such as diet, age, genetics, and drug intake.

Adult’s gut harbors a large and diverse community of DNA and RNA viruses made up of about 2,000 different genotypes, none of which is dominant. Indeed, the most abundant virus accounts for only about 6 percent of the community, whereas in infants the most abundant virus accounts over 40 percent of the community. The majority of DNA viruses are bacteriophages or phages, that is, viruses that infect bacteria. They are the most abundant biological entity on earth, with an estimated population of about 10³¹ units, whereas the majority of RNA viruses are plant viruses.

Contents

• Factors affecting gut microbiota composition and development
• Composition throughout life
• References

Factors affecting gut microbiota composition and development

The intestinal bacterial community is regulated by several factors, most of which are listed below.

• The diet of the host.
  It seems to be the most important factor.
  Traditionally considered sterile, mother’s milk harbors a rich microbiota consisting of more than 700 species, dominated by staphylococci, streptococci, bifidobacteria and lactic acid bacteria. Therefore, it is a major source for the colonization of the breastfed infant gut, and it was suggested that this mode of colonization is closely correlated with infant’s health status, because, among other functions, it could protect against infections and contribute to the maturation of the immune system. Breast milk affects intestinal microbiota also indirectly, through the presence of oligosaccharides with prebiotic activity that stimulate the growth of specific bacterial groups including staphylococci and bifidobacteria.
  A recent study has compared the intestinal microbiota of European and African children, respectively from Florence and a rural village in Burkina Faso, between the ages of 1 and 6 years old. It has highlighted the dominant role of diet over variables such as climate, geography, hygiene and health services; it was also observed the absence of significant differences in the expression of key genes regulating the immune function, which suggests a functional similarity between the two groups. Indeed infants, as long as they are breastfed, have a very similar gut microbiota, rich in Actinobacteria, mainly Bifidobacterium.
  The subsequent introduction of solid foods in the two groups, a Western diet rich in animal fats and proteins in European children, and low in animal proteins but rich in complex carbohydrates in African children, leads to a differentiation in the Firmicutes/Bacteroidetes ratio between the two groups. Gram-positive bacteria, mainly Firmicutes, were more abundant than Gram-negative bacteria in European children, whereas Gram-negative bacteria, mainly Bacteroidetes, prevailed over Gram-positive bacteria in African children.
  And the long-term diets are strongly associated to the enterotype partitioning. Indeed, it has been observed that:

a diet high in animal fats and proteins, i.e. a Western-type diet, leads to a gut microbiota dominated by the Bacteroides enterotype;
a diet high in complex carbohydrates, typical of agrarian societies, leads to the prevalence of the Prevotella enterotype.

Similar results emerged from the aforementioned study on children. In the Europeans, gut microbiota was dominated by taxa typical of Bacteroides enterotype, whereas in the Burkina Faso children, Prevotella enterotype dominates.
With short-term changes in the diet, 10 days, such as the switch from a low-fat and high-fiber diet to a high-fat and low-fiber diet and vice versa, changes were observed in the composition of the microbiome, within 24 hours, but no stable change in the enterotype partitioning. And this underlines as a long-term diet is needed for a change in the enterotypes of the gut microbiota.
Dietary interventions can also result in changes in the gut virome, which moves to a new state, that is, changes occur in the proportions of the pre-existing viral populations, towards which subjects on the same diet converge.

• pH, bile salts and digestive enzymes.
  The stomach, due to its low pH, is a hostile environment for bacteria, which are not present in high numbers, about 10²-10³ bacterial cells/gram of tissue. In addition to Helicobacter pylori, able to cause gastritis and gastric ulcers, microorganisms of the genus Lactobacillus are also present.
  Reached the duodenum, an increase in bacterial cell number occurs, 10⁴-10⁵ bacterial cells/gram of tissue; and similar bacterial concentrations are present in the jejunum and proximal ileum. The low number of microorganisms present in the small intestine is due to the inhospitable environment, consequent to the fact that there is the opening of the ampulla of Vater in the descending part of the duodenum, which pours pancreatic juice and bile into the duodenum, that is, pancreatic enzymes and bile salts, which damage microorganisms.
  In the terminal portion of the ileum, where the activities of pancreatic enzymes and bile salts are lower, there are about 10⁷ bacterial cells/gram of tissue, and up to 10¹²-10¹⁴ bacterial cells/gram of tissue in the colon, so that bacteria represent a large proportion, about 40 percent, of the fecal mass.
  The distribution of bacteria along the intestine is strategic. In the duodenum and jejunum, the amount of available nutrients is much higher than that found in the terminal portion of the ileum, where just water, fiber, and electrolytes remain. Therefore, the presence of large number of bacteria in the terminal portion of the ileum, and even more in the colon, is not a problem. The problem would be to find a high bacterial concentration in the duodenum, jejunum, and proximal parts of the ileum; and there is a disease condition, called small intestinal bacterial overgrowth or SIBO, in which the number of bacteria in the small intestine increases by about 10-15 times. This puts them in a position to compete with the host for nutrients and give rise to gastrointestinal disturbances such as diarrhea.
• The geographical position and the resulting differences in lifestyle, diet, religion etc.
  For example, a kind of geographical gradient occurs in the microbiota of European infants, with a higher number of Bifidobacterium species and some of Clostridium in Northern infants, whereas Southern infants have higher levels of Bacteroides, Lactobacillus and Eubacterium.
• The mode of delivery.
• The genetics of the host.
• The health status of the infant and mother.
  For example, in mothers with inflammatory bowel disease or IBD, Faecalibacterium prausnitzii, a bacterium that produces butyric acid, an important source of energy for intestinal cells, and with anti-inflammatory activity is depleted, whereas there is an increase in the number of adherent Escherichia coli.
• The treatment with antibiotics.
• Bacterial infections and predators.
  Bacteriocins, i.e. proteins with antibacterial activity, and bacteriophages.
  Phages play an important role in controlling the abundance and composition of the gut microbiota. In particular, they could play a major role in the colonization of the newborn, infecting the dominant bacteria thus allowing to another bacterial strain to become abundant.
  This model of predator-prey dynamics, called “kill the winner”, suggests that the blooms of a specific bacterial species would lead to blooms of their corresponding bacteriophages, followed by a decline in their abundance. Therefore, the most abundant bacteriophage genotype will not be the same at different times. And although some the gene sequences present in the infant gut virome are stable over the first three months of life, dramatic changes occur in the overall composition of the viral community between the first and second week of life. During this time period also the bacterial community is extremely dynamic.
• The competition for space and nutrients.

Composition throughout life

The development of the intestinal microbial ecosystem is a complex and crucial event in human life, highly variable from individual to individual, and influenced by the factors outlined above.

In utero, the gut is considered sterile, but is rapidly colonized by microbes at birth, as the infant is born with an immunological tolerance instructed by the mother.
However, recent studies show the presence of bacteria in the placental tissue, umbilical cord blood, fetal membranes and amniotic fluid from healthy newborns without signs of infection or inflammation. And for example, the meconium of premature infants, born to healthy mothers, contains a specific microbiota, with Firmicutes as the main phylum, and predominance of staphylococci, whereas Proteobacteria, in particular species such as Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, but also enterococci are more abundant in the faeces.
Note: The meconium is free of detectable viruses.
It seems that both vaginal and gut bacteria may gain access to the fetus, although via different route of entry: by ascending entry the vaginal ones, by dendritic cells of the immune system the gut ones. Therefore, there could exist a fetal microbiota.

Colonization occurs during delivery by a maternal inoculum, generally composed of aerobic and facultative bacteria (the newborn’s gut initially contains oxygen), then replaced by obligate anaerobes, bacteria typically present in adulthood, to which they have created a hospitable environment.
Furthermore, there is a small number of different taxa, with a relative dominance of the phyla Actinobacteria and Proteobacteria, that remains unchanged during the first month of life, but not in the subsequent ones as there is a large increase in variability and new genetic variants. Many studies underline that the initial exposure is important in defining the “trajectories” which will lead to the adult ecosystems. Additionally, these initial communities may act as a source of protective or pathogenic microorganisms.

Mother’s vaginal and fecal microbiotas are the main sources of inoculum in vaginally delivered infants. Indeed, infants harbor microbial communities dominated by species of the genera Lactobacillus, the most abundant genus in the vaginal microbiota and early gut microbiota, Bifidobacterium, Prevotella, or Sneathia. And it seems likely that anaerobes, such as members of the phyla Firmicutes and Bacteroidetes, not growing outside of their host, rely on the close contact between mother and offspring for transmission. Finally, due to the presence of oxygen in infant gut, the transmission of strict anaerobes could occur not directly at birth but at a later stage by means of spores.
The first bacteria encountered by infants born by caesarean section are those of the skin and hospital environment, and gut microbiota is dominated by species of the genera Corynebacterium, Staphylococcus and Propionibacterium, with a lower bacterial count and diversity in first weeks of life than infants born vaginally.
Further evidence supporting the hypothesis of vertical transmission is the similarity between the microbiota of meconium and samples obtained from possible sites of contamination.
These “maternal bacteria” do not persist indefinitely, and are replaced by other populations within the first year of life.
Objects, animals, mouths and skin of relatives, and breast milk are secondary sources of inoculum; and breast milk seems to have a primary role in determining the microbial succession in the gut.
The variation and diversity among children reflect instead the individuality of these microbial exposures.
Note: The delivery mode seems also to influence the immune system during the first year of life, perhaps via the influence on the development of gut microbiota. Infants born by cesarean section have:

• a lower bacterial count in stool samples at one month of age, mainly due to the higher number of bifidobacteria in infants born vaginally;
• a higher number of antibody secreting cells, which could reflect an excessive antigen exposure (the intestinal barrier would be more vulnerable to the passage of antigens).

Within a days after birth, a thriving community is established. This community is less stable over time and more variable in composition than that of adults. Very soon, it will be more numerous than that of the child’s cells, evolving according to a temporal pattern highly variable from individual to individual.
Viruses, absent at birth, reach about 10⁸ units/gram wet weight of faeces by the end of the first week of life, therefore representing a dynamic and abundant component of the developing gut microbiota. However, viral community has an extremely low diversity, like bacteria, and is dominated by phages, which probably influence the abundance and diversity of co-occurring bacteria, as seen above. The initial source of the viruses is unknown; of course, maternal and/or environmental inocula are among the possibilities. Notably, the earliest viruses could be the result of induction of prophages from the “newborn” gut bacterial flora, hypothesis supported by the observation that more than 25 percent of the phage sequences seem to be very similar to those of phages infecting bacteria such as Lactococcus, Lactobacillus, Enterococcus, and Streptococcus, which are abundant in breast milk.

By the end of the first month of life it is thought that the initial phase of rapid acquisition of microorganism is over.
In 1-month-old-infants, the most abundant bacteria belong to the genera Bacteroides and Escherichia, whereas Bifidobacterium, along with Ruminococcus, appear and grow to become dominant in the gastrointestinal tract of the breastfed infants between 1 and 11 months. Bifidobacteria such as Bifidobacterium longum subspecies infantis:

• are known to be closely related to breastfeeding;
• are among the best characterized commensal bacteria;
• are considered probiotics, that is, microorganisms which can confer health benefits to the host.

Their abundance confers also benefits through competitive exclusion, that is, they are an obstacle to colonization by pathogens. And indeed, Escherichia and Bacteroides can become preponderant if Bifidobacterium is not adequately present in the gut.
In contrast, bacteria of the genera Escherichia, such as E. coli), Clostridium, such as C. difficile, Bacteroides, such as B. fragilis, and Lactobacillus are present in higher levels in formula-fed infants than in breastfed infants.
Although breast-fed infants receive only breast milk until weaning, their microbiota can show a large variability in the abundances of bacterial taxa, with differences between individuals also with regard to the temporal patterns of variation. These variations may be due to diseases, treatments with antibiotics, changes in host lifestyle, random colonization events, as well as differences in immune responses to the gut colonizing microbes. However, it is not yet clear how these factors contribute to shape infant gut microbiota.
It seems that also the virome changes rapidly after birth, as the majority of the viral sequences present in the first week of life are not found after the second week. Moreover, the repertoire expands rapidly in number and diversity during the first three months. This is in contrast with the stability observed in the adult virome, where 95% of the sequences are conserved over time.

In normal condition, towards the end of the first year of life, babies have consumed an adult-like diet for a significant time period and should have developed a microbial community with characteristics similar to those found in the adult gut, such as:

• a more stable composition, phylogenetically more complex, and progressively more similar among different subjects;
• a preponderance of Firmicutes and Bacteroidetes, followed by Verrucomicrobia and a very low abundance of Proteobacteria;
• an increase in the levels of short-chain fatty acids, mainly acetic acid, propionic acid and butyric acid, and bacterial load in the feces;
• an increase of genes associated with xenobiotic degradation, vitamin biosynthesis, and carbohydrate utilization.

Interestingly, the significant turnover of taxa occurring from birth to the end of the first year is accompanied by a remarkable constancy in the overall functional capabilities.
Towards the end of the first year of life also the early viral colonizers were replaced by a community specific to the child.

The gut microbiota reaches maturity at about 2.5 years of age, fully resembling the adult gut microbiota.
The selection of the most adapted bacteria is the result of various factors.

• The transition to an adult diet.
• An increased fitness to the intestinal environment of the taxa that typically dominate the adult gut microbiota than the early colonizers.
• The significant changes in the intestinal environment, result of the developmental changes in the intestinal mucosa.
• The effects of the microbiota itself.

Therefore, the first 2-3 years of life are the most critical period in which you can intervene to shape the microbiota as best as possible, and so optimize child growth and development.

From a chaotic beginning, all this leads to the establishment of the gut ecosystem typical of the young adult, which is relatively stable over time until old age, viral, archaeal and eukaryotic components included, and dominated, at least in the western population, by members of the phyla Firmicutes, about 60% of the bacterial communities, Bacteroidetes and Actinobacteria, mainly belonging to the Bifidobacterium genus, each comprising about 10 percent of the bacterial community, followed by Proteobacteria and Verrucomicrobia. The genera Bacteroides, Clostridium, Faecalibacterium, Ruminococcus and Eubacterium make up, together with Methanobrevibacter smithii, the large majority of the adult gut microbial community.
It should be noted that different data were obtained from analysis of populations of African rural areas, as seen above.
And the gut microbiota is sufficiently similar among subjects to allow the identification of a shared core microbiome.
Stability and resilience, however, are subject to numerous variables among which, as previously said, diet seems to be one of the most important. Therefore, in order to maintain the stability of the gut microbiota, the variables have to be kept constant, or in the case of diseases prevented, also through vaccinations. However, the stability and resilience could be harmful if the dominant community is pathogenic.

The gut microbiota undergoes substantial changes in the elderly. In a study conducted in Ireland on 161 healthy people aged 65 years and over, the gut microbiota is distinct from that of younger adults in the majority of subjects, with a composition that seems to be dominated by the phyla Bacteroidetes, the main ones, and Firmicutes, with almost inverted percentages than those found in younger adults, although large variations across subjects were observed. And there are Faecalibacterium, about 6 percent of the main genera, followed by species of the genera Ruminococcus, Roseburia and Bifidobacterium ,the latter about 0.4 percent, among the most abundant genera.
Also the variability in the composition of the community is greater than in younger adults; this could be due to the increase in morbidities associated with aging and the subsequent increased intake of medications, as well as to changes in the diet.

References

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It has been known for almost a century that humans harbor a microbial ecosystem, known as human microbiota, remarkably dense and diverse, made up of a number of viruses and cells much higher than those of the human body, and that accounts for one to three percent of body weight. All the genes encoded by the human body’s microbial ecosystem, which are about 1,000 times more numerous than those of our genome, make up the human microbiome. Microorganisms colonize all the surfaces of the body that are exposed to the environment. Indeed, distinct microbial communities are found on the skin, in the vagina, in the respiratory tract, and along the whole intestinal tract, from the mouth up to rectum, the last part of the intestine.

Contents

• Composition
  • Bacteria
  • Viruses
  • Archaebacteria
  • Eukaryotes
• Function of the human microbiota
  • Commensals and pathogens
• Human Microbiome Project
• Effect of antibiotics
• References

Composition

The human microbiota consists of organisms from all taxa, namely, bacteria, viruses, archaea, and eukaryotes.

Bacteria

Bacteria are at least 100 trillion (10¹⁴) cells, a number ten times greater than that of the human body. They are found in very high concentration in the intestinal tract, up to 10¹²-10¹⁴/gram of tissue, where they form one of the most densely populated microbial habitats on Earth. In the gut, bacteria mainly belong to the Firmicutes, Bacteroidetes and Actinobacteria phyla. Fusobacteria (oropharynx), Tenericutes, Proteobacteria, and Verrucomicrobia are other phyla present in our body.
Note: Bacterial communities in a given body region resemble themselves much more across individuals than those from different body regions of the same individual; for example, bacterial communities of the upper respiratory tract are much more similar across individuals than those of the skin or intestine of the same individual.

Viruses

They are by far the most numerous organisms, being present with quadrillion units. The genomes of all the viruses harbored in the human body make up the human virome. In the past, viruses and eukaryotes have been studied focusing on pathogenic microorganisms, but in recent years the attention has also shifted on many non-pathogenic members of these groups. And many of the viral gene sequences found are new, which suggests that there is still much to learn about the human virome. Finally, just like for bacteria, there is considerable interpersonal variability.

Archaebacteria

Archaebacteria or Archaea are mainly those belonging to the order Methanobacteriales. Among the latter, Methanobrevibacter smithii is predominant in the human gut, representing up to 10 percent of all anaerobes.

Eukaryotes

Eukaryotes are also present, and the parasites of the genera Giardia and Entamoeba have probably been among the first to be identified. But there is also a great abundance and diversity of fungal species, belonging to genera such as Candida, Penicillium, Aspergillus, Hemispora, Fusarium, Geotrichum, Hormodendrum, Cryptococcus, Saccharomyces, and Blastocystis.

Function of the human microbiota

Sometimes referred to as “the forgotten organ”, human microbiota, mainly with its intestinal bacterial members, plays many important functions that can lead to nutritional, immunological, and developmental benefits, but can also cause diseases. Here are some examples.

• It is involved in the development of the gastrointestinal system of the newborn, as shown by experiments carried out on germ-free animals in which, for example, the thickness of the intestinal mucosa is thinner than that of colonized animals, therefore more easily subject to rupture.
• It contributes to energy harvest from nutrients, due to its ability to ferment indigestible carbohydrates, promote the absorption of monosaccharides and the storage of the derived energy. This has probably been a very strong evolutionary force that has played a major role in favor of the fact that these bacteria became our symbionts.
• It contributes to the maintenance of the acidic pH of the skin and in the colon.
• It is involved in the metabolism of xenobiotics and several polyphenols.
• It improves water and mineral absorption in the colon.
• It increases the speed of intestinal transit, slower in germ-free animals.
• It has an important role in resistance to colonization by pathogens, primarily in the vagina and gut.
• It is involved in the biosynthesis of isoprenoids and vitamins through the methylerythritol phosphate pathway.
• It stimulates angiogenesis.
• In the intestinal tract, it interacts with the immune system, providing signals for promoting the maturation of immune cells and the normal development of immune functions. And this is perhaps the most important effect of the symbiosis between the human host and microorganisms. Experiments carried out on germ-free animals have shown, for example, that:

macrophages, the cells that engulf pathogens and then present their antigens to the immune system, are found in much smaller amounts than those present in the colonized intestine, and if placed in the presence of bacteria they fail to find and therefore engulf them, unlike macrophages extracted from a colonized intestine;
there is not the chronic non-specific inflammation, present in the normal intestine as a result of the presence of bacteria (and of what we eat).

• Changes in its composition can contribute to the development of obesity and metabolic syndrome.
• It protects against the development of type I diabetes.
• Many diseases, both in children and adults, such as stomach cancer, lymphoma of mucosa-associated lymphoid tissue, necrotizing enterocolitis, an important cause of morbidity and mortality in premature babies, or chronic intestinal diseases, are, and others seem to be, related to the gut microbiota.

In conclusion, it seems very likely that the human body represents a superorganism, result of years of evolution and made up of human cells, and the resulting metabolic and physiological capacities, as well as an additional organ, the microbiota.

Commensals and pathogens

Based on the relationships with the human host, microorganisms may be classified as commensals or pathogens.
Commensals cause no harm to the host, with which they establish a symbiotic relationship that generally brings benefits to both.
On the contrary, pathogens are able to cause diseases, but fortunately represent a small percentage of the human microbiota. These microorganisms establish a symbiosis with the human host and benefit from it at the expense of the host. They can cause disease:

• if they move from their niche, such as the intestine, into another one where they do not usually reside, such as the vagina or bladder, as in the case of Candida albicans, normally present in the intestine, but in very small quantities;
• in patients with impaired immunological defenses, such as after an immunosuppressive therapy.

Human Microbiome Project

The bacterial component of the human microbiota is the subject of most studies including a large-scale project started in 2008 called “Human Microbiome Project”, whose aim is to characterize the microbiome associated with multiple body sites, such as the skin, mouth, nose, vagina and intestine, in 242 healthy adults.
These studies have shown a great variability in the composition of the human microbiota; for example, twins share less than 50 percent of their bacterial taxa at the species level, and an even smaller percentage of viruses. The factors that shape the composition of bacterial communities begin to be understood: for example, the genetic characteristics of the host play an important, although this is not true for the viral community. And metagenomic studies have shown that, despite the great interpersonal variability in microbial community composition, there is a core of shared genes encoding signaling and metabolic pathways. It appears namely that the assembly and the structure of the microbial community does not occur according to the species but the more functional set of genes. Therefore, disease states of these communities might be better identified by atypical distribution of functional classes of genes.

Effect of antibiotics

The microbiota in healthy adult humans is generally stable over time. However, its composition can be altered by factors such as dietary changes, urbanization, travel, and especially the use of broad-spectrum antibiotics. Here are some examples of the effect of antibiotic treatments.

• There is a long-term reduction in microbial diversity.
• The taxa affected vary from individual to individual, even up to a third of the taxa.
• Several taxa do not recover even after 6 months from treatment.
• Once the bacterial communities have reshaped, a reduced resistance to colonization occurs. This allows foreign and/or pathogen bacteria, able to grow more than the commensals, to cause permanent changes in human microbiota structure, as well as acute diseases, such as the dangerous pseudomembranous colitis, and chronic diseases, as it is suspected for asthma following the use and abuse of antibiotics in childhood. Moreover, their repeated use has been suggested to increase the pool of antibiotic-resistance genes in our microbiome. In support of this hypothesis, a decrease in the number of antibiotic-resistant pathogens has been observed in some European countries following the reduction in the number of antibiotics prescribed.

Finally, you must not underestimate the fact that the intestinal microflora is involved in many chemical transformations, and its alteration could be implicated in the development of cancer and obesity. However, regarding use of antibiotics, you should be underlined that if western population has a life expectancy higher than in the past is also because you do not die of infectious diseases!

References

1. Burke C., Steinberg P., Rusch D., Kjelleberg S., and Thomas T. Bacterial community assembly based on functional genes rather than species. Proc Natl Acad Sci USA 2011;108:14288-14293. doi:10.1073/pnas.1101591108
2. Clemente J.C., Ursell L.K., Wegener Parfrey L., and Knight R. The impact of the gut microbiota on human health: an integrative view. Cell 2012;148:1258-1270. doi:10.1016/j.cell.2012.01.035
3. Gill S.R., Pop M., Deboy R.T., Eckburg P.B., Turnbaugh P.J., Samuel B.S., Gordon J.I., Relman D.A., Fraser-Liggett C.M., and Nelson K.E. Metagenomic analysis of the human distal gut microbiome. Science 2006;312:1355-1359. doi:10.1126/science.1124234
4. Palmer C., Bik E.M., DiGiulio D.B., Relman D.A., and Brown P.O. Development of the human infant intestinal microbiota. PLoS Biol 2007;5(7):e177. doi:10.1371/journal.pbio.0050177
5. Turnbaugh P.J., Gordon J.I. The core gut microbiome, energy balance and obesity. J Physiol 2009;587:4153-4158. doi:10.1113/jphysiol.2009.174136
6. Zhang, T., Breitbart, M., Lee, W., Run, J.-Q., Wei, C., Soh, S., Hibberd, M., Liu, E., Rohwer, F., Ruan, Y. Prevalence of plant viruses in the RNA viral community of human feces. PLoS Biol 2006;4(1):e3. doi:10.1371/journal.pbio.0040003

Olive oil is a liquid fat obtained by pressing whole olives, the fruit of the olive tree (Olea europaea).
It is a staple ingredient in Mediterranean diet and is widely used for cooking, salad dressings, and other culinary purposes. Olive oil is also known for its health benefits, as it is rich in monounsaturated fats and antioxidants.
From a chemical point of view, we can identify in the olive oil two fractions, depending on the behavior in the presence of heating and strong alkaline solutions:

• the saponifiable fraction, which represents 98-99 percent of the total weight, is composed of lipids that form soaps in the above conditions;
• the unsaponifiable fraction, which represents the remaining 1-2 percent of the total weight, is composed of substances that fail to form soaps in the above conditions.

Contents

• Saponifiable fraction
  • Triglycerides
  • Diglycerides and monoglycerides
• Unsaponifiable fractions
  • Polyphenols
  • Hydrocarbons
  • Sterols
    • Common sterols or 4-alpha-desmethylsterols
    • 4-Methylsterols
    • Triterpene alcohols or 4,4-dimethylsterols
    • Triterpene dialcohols
  • Tocopherols
  • Pigments
  • Triterpene acids
    • Fatty alcohols
    • Diterpene alcohols
  • Volatile compounds
  • Minor components
• References

Saponifiable fraction

It is composed of saturated fatty acids and unsaturated fatty acids, esterified almost entirely to glycerol to form triglycerides or triacylglycerols. To a much lesser extent, diglycerides or diacylglycerols, monoglycerides or monoacylglycerols, and free fatty acids are also found.
Unsaturated fatty acids make up 75 to 85 percent of the total fatty acids. Oleic acid (O) and linoleic acid (L) are the most abundant ones; palmitoleic acid, eptadecenoic acid, gadoleic acid, and alpha-linolenic acid (Ln) are present in lower/trace amounts.

IOOC requirements for olive oil
Fatty acids	Number of carbons	Allowable range percent
Myristic acid	C14:0	<0.03
Palmitic acid	C16:0	7.5-20
Palmitoleic acid	C16:1	0.3-3.5
Heptadecanoic acid	C17:0	≤0.3
Heptadecenoic acid	C17:1	≤0.3
Stearic acid	C18:0	0.5-5.0
Oleic acid	C18:1	55.0-83.0
Linoleic acid	C18:2	2.5-21.0
Alpha-linolenic acid	C18:3	≤1.0
Arachidic acid	C20:0	≤0.6
Gadoleic acid	C20:1	≤0.4
Behenic acid	C22:0	≤0.2
Lignoceric acid	C24:0	≤0.2

Oleic acid is the major fatty acid in olive oils. According to the rules laid down by the International Olive Oil Council (IOOC), its concentration must range from 55 percent to 83 percent of total fatty acids.
Linoleic acid is the most abundant polyunsaturated fatty acid in olive oil; its concentration must vary between 2.5 percent and 21 percent (IOOC). Because of its high degree of unsaturation, it is subject to oxidation; this means that an oil high in linoleic acid becomes rancid easily, and thus it may be stored for a shorter time.
In mediterranean cuisine, olive oil is the main source of fat: therefore, oleic acid, among monounsaturated fatty acids, and linoleic acid, among polyunsaturated fatty acids, are the most abundant fatty acids.
alpha-Linolenic acid must be present in very low amount, according to the IOOC standards ≤1 percent. It is an omega-3 polyunsaturated fatty acid, which may have health benefits. However, because of to its high degree of unsaturation, higher than that of linoleic acid, it is very susceptible to oxidation, and therefore it promotes rancidity of the olive oil that contains it.
Saturated fatty acids make up 15 to 25 percent of the total fatty acids.
Palmitic acid (P), 5-20 percent, and stearic acid (S), 0.5-5 percent, are the most abundant saturated fatty acids; myristic acid, heptadecanoic acid, arachidic acid, behenic acid and lignoceric acid may be present in trace amounts.

The presence of fatty acids that should be absent or present in amounts different than those found is a marker of adulteration with other vegetable oils. On this regard, particular attention is paid to myristic, arachidic, behenic, lignoceric, gadoleic and alpha-linolenic acids, whose limits are set by IOOC.

Fatty acid composition is influenced by several factors.

• The climate.
• The latitude.
• The zone of production.
  Italian, Spanish and Greek olive oils are high in oleic acid and low in palmitic and linoleic acids, while Tunisian olive oils are high in palmitic and linoleic acids but lower in oleic acid. Therefore, oils can be divided into two groups:

one rich in oleic acid and low in palmitic and linoleic acids;
the other high in palmitic and linoleic acids and low in oleic acid.

• The cultivar.
• The degree of olive ripeness at the time of oil extraction.
  It should be noted that oleic acid is formed first in the fruit, and data seem to indicate a competitive relationship between oleic acid and palmitic, palmitoleic, and linoleic acids.

Triglycerides

As previously said, fatty acids in olive oil are almost entirely present as triglycerides.
In small percentage, they are also present as diglycerides, monoglycerides, and in free form.

During triglyceride biosynthesis, thanks to the presence of specific enzymes, only about 2 percent of glycerol binds palmitic acid in the sn-2 position. The percentage of stearic acid in the sn-2 position is very low, as well. For the most part, the sn-2 position is occupied by oleic acid.
On the contrary, if we consider oils that have undergone a nonenzymatic esterification, the percentage of palmitic acid in the sn-2 position increases significantly.
Note: sn = Stereospecific numbering

Among triglycerides present in significant proportions in olive oil, there are:

• OOO: 40-59 percent;
• POO: 12-20 percent;
• OOL: 12.5-20 percent;
• POL: 5.5-7 percent;
• SOO: 3- 7 percent.

POP, POS, OLnL, OLnO, PLL, PLnO are present in smaller amounts.
Trilinolein (LLL) is a triglyceride that contains three molecules of linoleic acid. Its low content is an indicator of an oil of good quality.
Triglycerides containing three saturated fatty acids or three molecules of alpha-linolenic acid have not been reported.

Diglycerides and monoglycerides

Their presence is due to an incomplete synthesis and/or a partial hydrolysis of triglycerides.
The content of diglycerides in virgin olive oil ranges from 1 percent to 2.8 percent. 1,2-Diglycerides prevail in fresh olive oil, representing over 80 percent of the diglycerides. During oil storage, isomerization occurs with a progressive increase of the more stable 1-3 isomers, which after about 10 months become the major isomers.
Therefore, the ratio 1,2/1,3-diglycerides may be used as an indicator of the age of the oil.
Monoglycerides are present in amounts lower than diglycerides, <0.25 percent, with 1-monoglycerides far more abundant than 2-monoglycerides.

Unsaponifiable fractions

It is composed of a large number of different molecules, very important from a nutritional point of view, as they contribute significantly to the health effects of olive oil.
Furthermore, they are responsible for the stability and the taste of olive oil, and are also used to detect adulteration with other vegetable oils.
This fraction includes tocopherols, sterols, polyphenols, pigments, hydrocarbons, aromatic and aliphatic alcohol, triterpene acids, waxes, and minor constituents.
Their content is influenced by factors similar to those seen for fatty acid composition, such as:

• the cultivar;
• the degree of ripeness of the olive;
• the zone of production;
• the crop year and olive harvesting practices;
• the storage time of olives;
• the oil extraction process;
• the storage conditions of the oil.

It should be noted that many of these compounds are not present in refined olive oils, as they are removed during the refining processes.

Polyphenols

They make up 18 to 37 percent of the unsaponifiable fraction.
They are a very heterogeneous group of molecules with nutritional and organoleptic properties; for example, oleuropein and hydroxytyrosol give oil its bitter and pungent taste.
For a more extensive discussion, see the article Polyphenols in olive oil.

Hydrocarbons

They make up 30 to 50 percent of the unsaponifiable fraction.
Squalene and beta-carotene are the main molecules.
Squalene, isolated for the first time from shark liver, is the major constituent of the unsaponifiable fraction, and constitutes more than 90 percent of the hydrocarbons. Its concentration ranges from 200 to 7500 mg/kg of olive oil.

It is an intermediate in the biosynthesis of the four-ring structure of steroids, and it seems to be responsible of several health effects of olive oil.
In the hydrocarbon fraction of virgin olive oil, n-paraffins, diterpene and triterpene hydrocarbons, isoprenoidal polyolefins are also found.
Beta-carotene acts both as antioxidant, protecting oil during storage, and as dye.

Sterols

They are important lipids of olive oil, and are:

• linked to many health benefits for consumers;
• important to the quality of the oil;
• widely used for checking its genuineness.
  On this regard, it is to underline that sterols are species-specific molecules; for example, the presence of high concentrations of brassicasterol, a sterol typically found in Brassicaceae (Cruciferae) family, such as rapeseed, indicates adulteration of olive oil with canola oil.

Four classes of sterols are present in olive oil: common sterols, 4-methylsterols, triterpene alcohols, and triterpene dialcohols. Their content ranges from 1000 mg/kg, the minimum value required by the IOOC standard, to 2000 mg/kg. The lowest values are found in refined oils because of the refining processes may cause losses up to 25 percent.

Common sterols or 4-alpha-desmethylsterols

Common sterols are present mainly in the free and esterified form; however they have been also found as lipoproteins and sterylglucosides.
The main molecules are beta-sitosterol, which makes up 75 to 90 percent of the total sterol, Δ5-avenasterol, 5 to 20 percent, and campesterol, 4 percent. Other components found in lower amounts or traces are, for example, stigmasterol, 2 percent, cholesterol, brassicasterol, and ergosterol.

4-Methylsterols

They are intermediates in the biosynthesis of sterols, and are present both in the free and esterified form. They are present in small amounts, much lower than those of common sterols and triterpene alcohols, varying between 50 and 360 mg/kg. The main molecules are obtusifoliol, cycloeucalenol, citrostadienol, and gramisterol.

Triterpene alcohols or 4,4-dimethylsterols

They are a complex class of sterols, present both in the free and esterified form. They are found in amounts ranging from 350 to 1500 mg/kg.
The main components are beta-amyrin, 24-methylenecycloartanol, cycloartenol, and butyrospermol; other molecules present in lower/trace amounts are, for example, cyclosadol, cyclobranol, germanicol, and dammaradienol.

Triterpene dialcohols

The main triterpene dialcohols found in olive oil are erythrodiol and uvaol.
Erythrodiol is present both in the free and esterified form; in virgin olive oil, its level varies between 19 and 69 mg/kg, and the free form is generally lower than 50 mg/kg.

Tocopherols

They make up 2 to 3 percent of the unsaponifiable fraction, and include vitamin E.
Of the eight E-vitamers, alpha-tocopherol represents about 90 percent of tocopherols in virgin olive oil. It is present in the free form and in very variable amount, but on average higher than 100 mg/kg of olive oil. Thanks to its in vivo antioxidant properties, its presence is a protective factor for health. Alpha-tocopherol concentration seems to be related to the high levels of chlorophylls and to the concomitant requirement for deactivation of singlet oxygen.
Beta-tocopherol, delta-tocopherol, and gamma-tocopherol are usually present in low amounts.

Pigments

In this group we find chlorophylls and carotenoids.
In olive oil, chlorophylls are present as phaeophytins, mainly phaeophytin a, namely, a chlorophyll from which magnesium has been removed and substituted with two hydrogen ions, and confer the characteristic green color to olive oil. They are photosensitizer molecules that contribute to the photooxidation of olive oil itself.
Beta-carotene and lutein are the main carotenoids in olive oil. Several xanthophylls are also present, such as antheraxanthin, beta-cryptoxanthin, luteoxanthin, mutatoxanthin, neoxanthin, and violaxanthin.
Olive oil’s color is the result of the presence of chlorophylls and carotenoids and of their green and yellow hues. Their presence is closely related.

Triterpene acids

They are important components of the olive, and are present in trace amounts in the oil.
Oleanolic and maslinic acids are the main triterpene acids in virgin olive oil: they are present in the olive husk, from which they are extracted in small amount during processing.

Aliphatic and aromatic alcohols

Fatty alcohols and diterpene alcohols are the most important ones.
Aliphatic alcohols have a number of carbon atoms between 20 and 30, and are located mostly inside the olive stones, from where they are partially extracted by milling.

Fatty alcohols

They are linear saturated alcohols with more than 16 carbon atoms.
They are found in the free and esterified form and are present, in virgin olive oil, in amount not generally higher than 250 mg/kg.
Docosanol (C22), tetracosanol (C24), hexacosanol (C26), and octacosanol (C28) are the main fatty alcohols in olive oil, with tetracosanol and hexacosanol present in larger amounts.
Waxes, which are minor constituents of olive oil, are esters of fatty alcohols with fatty acids, mainly of palmitic acid and oleic acid. They can be used as a criterion to discriminate between different types of oils; for example, they must be present in virgin and extra virgin olive oil at levels <150 mg/kg, according to the IOOC standards.

Diterpene alcohols

Geranylgeraniol and phytol are two acyclic diterpene alcohols, present in the free and esterified form. Among esters present in the wax fraction of extra virgin olive oil, oleate, eicosenoate , eicosanoate, docosanoate, and tetracosanoate have been found, mainly as phytyl derivatives.

Volatile compounds

More than 280 volatile compounds have been identified in olive oil, such as hydrocarbons, the most abundant fraction, alcohols, aldehydes, ketones, esters, acids, ethers and many others. However, only about 70 of them are present at levels higher than the perception threshold beyond which they may contribute to the aroma of virgin olive oil.

Minor components

Phospholipids are found among the minor components of olive oil; the main ones are phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol.
In the unfiltered oils, trace amounts of proteins may be found.

References

1. Gunstone F.D. Vegetable oils in food technology: composition, properties and uses. 2th Edition. Wiley J. & Sons, Inc., Publication, 2011
2. Caponio F., Bilancia M.T., Pasqualone A., Sikorska E., Gomes T. Influence of the exposure to light on extra virgin olive oil quality during storage. Eur Food Res Technol 2005;221:92-98. doi:10.1007/s00217-004-1126-8
3. Servili M., Sordini B., Esposto S., Urbani S., Veneziani G., Di Maio I., Selvaggini R. and Taticchi A. Biological activities of phenolic compounds of extra virgin olive oil. Antioxidants 2014;3:1-23. doi:10.3390/antiox3010001

ten is not a single protein but a mixture of cereal proteins, about 80 percent of its dry weight, for example gliadins and glutenins in wheat grains, lipids, 5-7 percent, starch, 5-10 percent, water, 5-8 percent, and mineral substances, <2 percent.
It forms when components naturally present in the grain of cereals, the caryopsis, and in their flours, are joined together by means of mechanical stress in aqueous environment, i.e. during the formation of the dough.
The term is also related to the family of proteins that cause problems for celiac patients.
Isolated for the first time in 1745 from wheat flour by the Italian chemist Jacopo Bartolomeo Beccari, it can be extracted from the dough by washing it gently under running water: starch, albumins and globulins, that are water-soluble, are washed out, and a sticky and elastic mass remains, precisely the gluten, which means glue in Latin.

Contents

• Cereals containing gluten
• Cereal grain proteins
• Wheat gluten proteins
  • Gliadins: extensibility and viscosity
  • Glutenins: elasticity and toughness
• Properties of wheat gluten
• Gluten-free cereals
• Oats and gluten
• References

Cereals containing gluten

It is present in:

• wheat, such as:

durum wheat (Triticum durum); groats and semolina for dry pasta making are obtained from it;
common wheat or bread wheat (Triticum aestivum), so called because it is used in bread and fresh pasta making, and in bakery products;

• rye (Secale cereale);
• barley (Hordeum vulgare);
• spelt, in the three species:

einkorn (Triticun monococcum);
emmer (Triticum dicoccum Schrank);
spelta (Triticum spelta);

• khorasan wheat (Triticum turanicum); a variety of it is Kamut®;
• triticale (× Triticosecale Wittmack), which is a hybrid of rye and common wheat;
• bulgur, which is whole durum wheat, sprouted and then processed;
• seitan, which is not a cereal, but a wheat derivative, also defined by some as “gluten steak”.

Given that most of the dietary intake of gluten comes from wheat flour, of which about 700 million tons per year are harvested, representing about 30 percent of the global cereal production, the following discussion will focus on wheat gluten, and mainly on its proteins.

Note: The term gluten is also used to indicate the protein fraction that remains after removal of starch and soluble proteins from the dough obtained with corn flour: however, this “corn gluten” is “functionally” different from that obtained from wheat flour.

Cereal grain proteins

The study of cereal grain proteins, as seen, began with the work of Beccari. 150 years later, in 1924, the English chemist Osborne T.B., which can rightly be considered the father of plant protein chemistry, developed a classification based on their solubility in various solvents.
The classification, still in use today, divides plant proteins into 4 families.

• Albumins, soluble in water.
• Globulins, soluble in saline solutions; for example avenalin of oat.
• Prolamins, soluble in 70 percent alcohol solution, but not in water or absolute alcohol.
  They include:

gliadins of wheat;
zein of corn;
avenin of oats;
hordein of barley;
secalin of rye.

They are the toxic fraction of gluten for celiac patients.

• Glutelins, insoluble in water and neutral salt solutions, but soluble in acidic and basic solutions.
  They include glutenins of wheat.

Cereals	Albumins	Globulins	Prolamins*	Glutelins**
Wheat	9	5	40	46
Corn	4	2	55	39
Barley	13	12	52	23
Oats	11	56	9	23
Rice	5	10	5	80
* Gliadins in wheat ** Glutenins in wheat

Albumins and globulins are cytoplasmic proteins, often enzymes, rich in essential amino acids, such as lysine, tryptophan and methionine. They are found in the aleurone layer and embryo of the caryopsis.
Prolamins and glutelins are the storage proteins of cereal grains. They are rich in glutamine and proline, but very low in lysine, tryptophan and methionine. They are found in the endosperm, and are the vast majority of the proteins in the grains of wheat, corn, barley, oat, and rye.
Although Osborne classification is still widely used, it would be more appropriate to divide cereal grain proteins into three groups: structural and metabolic proteins, storage proteins, and defense proteins.

Wheat gluten proteins

Proteins represent 10-14 percent of the weight of the wheat caryopsis, whereas about 80 percent of its weight consists of carbohydrates.
According to the Osborne classification, albumins and globulins represent 15-20 percent of the proteins, while prolamins and glutelins are the remaining 80-85 percent, composed respectively of gliadins, 30-40 percent, and glutenins, 40-50 percent. Therefore, and unlike prolamins and glutelins in the grains of other cereals, gliadins and glutenins are present in similar amounts, about 40 percent.

Technologically, gliadins and glutenins are very important. Why?
These proteins are insoluble in water, and in the dough, that contains water, they bind to each other through a combination of intermolecular bonds, such as:

• covalent bonds, i.e. disulfide bridges;
• noncovalent bonds, such as hydrophobic interactions, van der Waals forces, hydrogen bonds, and ionic bonds.

Thanks to the formation of these intermolecular bonds, a three-dimensional lattice is formed. This structure entraps starch granules and carbon dioxide bubbles produced during leavening, and gives strength and elasticity to the dough, two properties of gluten widely exploited industrially.
In the usual diet of the European adult population, and in particular in Italian diet that is very rich in derivatives of wheat flour, gliadin and glutenin are the most abundant proteins, about 15 g per day. What does this mean? It means that gluten-free diet engages celiac patients both from a psychological and social point of view.

Note: The lipids of the gluten are strongly associated with the hydrophobic regions of gliadins and glutenins and, unlike what you can do with the flour, they are extracted with more difficulty. The lipid content of the gluten depends on the lipid content of the flour from which it was obtained.

Gliadins: extensibility and viscosity

Gliadins are hydrophobic monomeric prolamins, of globular nature and with low molecular weight. On the basis of electrophoretic mobility in low pH conditions, they are separated into the following types:

• alpha/beta, and gamma, rich in sulfur, containing cysteines, that are involved in the formation of intramolecular disulfide bonds, and methionines;
• omega, low in sulfur, given the almost total absence of cysteine and methionine.

They have a low nutritional value and are toxic to celiac patients because of the presence of particular amino acid sequences in the primary structure, such as proline-serine-glutamine-glutamine and glutamine-glutamine-glutamine-proline.
Gliadins are associated with each other and with glutenins through noncovalent interactions; thanks to that, they act as “plasticizers” in dough making. Indeed, they are responsible for viscosity and extensibility of gluten, whose three-dimensional lattice can deform, allowing the increase in volume of the dough as a result of gas production during leavening. This property is important in bread-making.
Their excess leads to the formation of a very extensible dough.

Glutenins: elasticity and toughness

Glutenins are polymeric proteins, that is, formed of multiple subunits, of fibrous nature, linked together by intermolecular disulfide bonds. The reduction of these bonds allows to divide them, by SDS-PAGE, into two groups.

• High molecular weight (HMW) subunits, low in sulfur, that account for about 12 percent of total gluten proteins. The noncovalent bonds between them are responsible for the elasticity and tenacity of the gluten protein network, that is, of the viscoelastic properties of gluten, and so of the dough.
• Low molecular weight (LMW) subunits, rich in sulfur, namely, cysteine residues.
  These proteins form intermolecular disulfide bridges to each other and with HMW subunits, leading to the formation of a glutenin macropolymer.

Glutenins allow dough to hold its shape during mechanical (kneading) and not mechanical stresses (increase in volume due to both the leavening and the heat of cooking that increases the volume occupied by gases present) which is submitted. This property is important in pasta making.
If in excess, glutenins lead to the formation of a strong and rigid dough.

Properties of wheat gluten

From the nutritional point of view, gluten proteins do not have a high biological value, being low in lysine, an essential amino acid. Therefore, a gluten-free diet does not cause any important nutritional deficiencies.
On the other hand, it is of great importance in food industry: the combination, in aqueous solution, of gliadins and glutenins to form a three-dimensional lattice, provides viscoelastic properties, that is, extensibility-viscosity and elasticity-tenacity, to the dough, and then, a good structure to bread, pasta, and in general, to all foods made with wheat flour.
It has a high degree of palatability and a high fermenting power in the small intestine.
It is an exorphin: some peptides produced from intestinal digestion of gluten proteins may have an effect in central nervous system.

Gluten-free cereals

The following is a list of gluten-free cereals, minor cereals, and pseudocereals used as foods.

• Cereals

corn or maize (Zea mays)
rice (Oryza sativa)

• Minor cereals
  They are defined “minor” not because they have a low nutritional value, but because they are grown in small areas and in lower quantities than wheat, rice and maize.

Fonio (Digitaria exilis)
Millet (Panicum miliaceum)
Panic (Panicum italicum)
Sorghum (Sorghum vulgare)
Teff (Eragrostis tef)
Teosinte; it is a group of four species of the genus Zea. They are plants that grow in Mexico (Sierra Madre), Guatemala and Venezuela.

• Pseudocereals.
  They are so called because they combine in their botany and nutritional properties characteristics of cereals and legumes, therefore of another plant family.

Amaranth; the most common species are:

Amaranthus caudatus;
Amaranthus cruentus;
Amarantus hypochondriacus.

Buckwheat (Fagopyrum esculentum)
Quinoa (Chenopodium quinoa), a pseudocereal with excellent nutritional properties, containing fibers, iron, zinc and magnesium. It belongs to Chenopodiaceae family, such as beets.

• Cassava, also known as tapioca, manioc, or yuca (Manihot useful). It is grown mainly in the south of the Sahara and South America. It is an edible root tuber from which tapioca starch is extracted.

It should be noted that naturally gluten-free foods may not be truly gluten-free after processing. Indeed, the use of derivatives of gliadins in processed foods, or contamination in the production chain may occur, and this is obviously important because even traces of gluten are harmful for celiac patients.

Oats and gluten

Oats (Avena sativa) is among the cereals that celiac patients can eat. Recent studies have shown that it is tolerated by celiac patients, adult and child, even in subjects with dermatitis herpetiformis. Obviously, oats must be certified as gluten-free from contamination.

References

1. Beccari J.B. De Frumento. De bononiensi scientiarum et artium instituto atque Academia Commentarii, II. 1745:Part I.,122-127
2. Bender D.A. “Benders’ dictionary of nutrition and food technology”. 8th Edition. Woodhead Publishing. Oxford, 2006
3. Berdanier C.D., Dwyer J., Feldman E.B. Handbook of nutrition and food. 2th Edition. CRC Press. Taylor & Francis Group, 2007
4. Phillips G.O., Williams P.A. Handbook of food proteins. 1th Edition. Woodhead Publishing, 2011
5. Shewry P.R. and Halford N.G. Cereal seed storage proteins: structures, properties and role in grain utilization. J Exp Bot 2002:53(370);947-958. doi:10.1093/jexbot/53.370.947
6. Yildiz F. Advances in food biochemistry. CRC Press, 2009

During running, athletes burn calorie, and lose water and salts in amounts depending on various factors such as the technique, training level, environmental conditions, and physiological characteristics of each runner. The knowledge of these factors allows to plan an adequate diet both during workout and recovery, with the aim of optimizing performance.
Below we will analyze the energy expenditure of runners engaged in workouts on various distances, the amounts of carbohydrates, lipids, and proteins oxidized to meet the energy requirements, and which minerals are lost in sweat.

Contents

• Energy expenditure during running
  • Carbohydrate oxidation during workout
  • Lipid oxidation during workout
  • Protein oxidation during workout
• Water and minerals loss during running
• References

Energy expenditure during running

During running energy expenditure is equal to 0.85-1.05 kcal per kilogram per kilometer.
This range is due to the fact that athletes with a good technique spend less than those with a poor technique.
A 70 kilogram (154 pound) athlete has an energy expenditure per kilometer between:

70 x 0.85 x 1 = 59.5 kcal
and
70 x 1.05 x 1 = 73.5 kcal

The table shows the calculations to determine the energy expenditure of the athlete to run 10, 20, 30, and 40 kilometers.

Distance	Energy expenditure
10 km	0.85 x 70 x 10 = 595 kcal 1.05 x 70 x 10 = 735 kcal
20 km	0.85 x 70 x 20 = 1190 kcal 1.05 x 70 x 20 = 1470 kcal
30 km	0.85 x 70 x 30 = 1785 kcal 1.05 x 70 x 30 = 2205 kcal
40 km	0.85 x 70 x 40 = 2380 kcal 1.05 x 70 x 40 = 2940 kcal

Note: who has started running for a short time ago has an energy expenditure even higher than 1.05 kcal per kilogram per kilometer.

During running, the energy for muscle work derives from the oxidation of carbohydrates, lipids, and proteins. Carbohydrates and lipids are the main energy source, and their oxidation rate depends on the intensity of exercise: as it increases, the percentage of lipid oxidation decreases whereas that of carbohydrates increases, as summarized below.

Intensity	Fuel
30 percent VO2max	Mainly fats
40-60 percent VO2max	Fats and carbohydrates
75 percent VO2max	Mainly carbohydrates
80 percent VO2max	Almost exclusively carbohydrates

Note: The failure to use the suitable fuel can promote fatigue and lead to overtraining.

Then, when running above the anaerobic threshold, the oxidation of carbohydrates can provide the entire energy requirement. At marathon pace, carbohydrates provide 60-70 percent of the energy requirement, whereas at lower pace they provide less than 50 percent of energy requirement.
Below, the amounts of carbohydrates, lipids, and proteins oxidized during workout are analyzed. During workout ,the energy expenditure is covered for about 60 percent by carbohydrates, for about 40 percent by lipids, whereas the residual percentage, between 3 and 5 percent, by proteins.

Carbohydrate oxidation during workout

For a 70 kilogram runner the amount of carbohydrates oxidized per kilometer is between:

(0.6 x 59.5) /4 = 8.9 g/km
and
(0.6 x 73.5) /4 = 11 g/km

Note: carbohydrates provide, on average, 4 kcal per gram.
The table shows the calculations to determine the amount of carbohydrates oxidized when the athlete runs 10, 20, 30, and 40 kilometers.

Distance	Carbohydrate expenditure
10 km	[(0.85 x 70 x 10) x 0.6 ] / 4 = 89 g [(1.05 x 70 x 10) x 0.6 ] / 4 = 110 g
20 km	[(0.85 x 70 x 20) x 0.6] / 4 = 179 g [(1.05 x 70 x 20) x 0.6] / 4 = 221 g
30 km	[(0.85 x 70 x 30) x 0.6] / 4 = 268 g [(1.05 x 70 x 30) x 0.6] / 4 = 331 g
40 km	[(0.85 x 70 x 40) x 0.6] / 4 = 357 g [(1.05 x 70 x 40) x 0.6] / 4 = 441 g

Lipid oxidation during workout

By calculations similar to those for carbohydrates, we determine the amount of lipids oxidized per kilometer, which is between:

(0.4 x 59.5) / 9 = 2.6 g/km
and
(0.4 x 73.5) / 9 = 3.3 g/km

Note: lipids provide, on average, 9 kcal per gram.
The table shows the calculations to determine the amount of lipids oxidized when the athlete runs 10, 20, 30, and 40 kilometers.

Distance	Lipid expenditure
10 km	[(0.85 x 70 x 10) x 0.4] / 9 = 26 g [(1.05 x 70 x 10) x 0.4] / 9 = 33 g
20 km	[(0.85 x 70 x 20) x 0.4] / 9 = 53 g [(1.05 x 70 x 20) x 0.4] / 9 = 65 g
30 km	[(0.85 x 70 x 30) x 0.4] / 9 = 79 g [(1.05 x 70 x 30) x 0.4] / 9 = 98 g
40 km	[(0.85 x 70 x 40) x 0.4] / 9 = 106 g [(1.05 x 70 x 40) x 0.4] / 9 = 131 g

Protein oxidation during workout

Protein requirements of adults are equal to 0.9 grams per kilogram of body weight, and, for a 70 kilogram athlete is:

70 x 0.9 = 63 g

During workout the energy expenditure is covered for about 3-5 percent by protein oxidation.

The table shows the calculations to determine the amount of proteins oxidized when the athlete runs 10, 20, 30, and 40 kilometers, and proteins provide 3 percent of the energy requirement.

Distance	Protein expenditure (3 percent)
10 km	[(0.85 x 70 x 10) x 0.03)] / 4 = 4.5 g [(1.05 x 70 x 10) x 0.03)] / 4 = 5.5 g
20 km	[(0.85 x 70 x 20) x 0.03)] / 4 = 8.9 g [(1.05 x 70 x 20) x 0.03)] / 4 = 11 g
30 km	[(0.85 x 70 x 30) x 0.03)] / 4 = 13.4 g [(1.05 x 70 x 30) x 0.03)] / 4 = 16.5 g
40 km	[(0.85 x 70 x 40) x 0.03)] /4 = 17.9 g [(1.05 x 70 x 40) x 0.03)] /4 = 22.1 g

Note: proteins provide, on average, 4 kcal per gram.

For energy expenditure of 0.85 and 1.05 kcal per kilogram per kilometer, the average additional protein oxidation per kilogram to run 10, 20, 30, and 40 kilometers, rounded to the second decimal place, is:

• 10 km: [(4.5 + 5.5) / 2] / 70 = 0.07 g
• 20 km: [(4.5 + 5.5) / 2] / 70 = 0.14 g
• 30 km: [(4.5 + 5.5) / 2] / 70 = 0.21 g
• 40 km: [(4.5 + 5.5) / 2] / 70 = 0.29 g

Finally, adding the daily protein requirement of adults, the total protein requirement of a 70 kilogram runner, for the four distances, is:

• 10 km: 0.07 + 0.9 = 0.97 g
• 20 km: 0.14 + 0.9 = 1.04 g
• 30 km: 0.21 + 0.9 = 1.11 g
• 40 km: 0.29 + 0.9 = 1.19 g

By calculations similar to the previous ones, we determine the overall protein requirement when proteins provide 5 percent of the energy requirement.

• 10 km: 0.12 + 0.9 = 1.02 g
• 20 km: 0.24 + 0.9 = 1.14 g
• 30 km: 0.36 + 0.9 = 1.26 g
• 40 km: 0.48 + 0.9 = 1.38 g

Excluding athletes who run 30 kilometers or more every day, the values are slightly higher than 0.9 grams per kilogram of body weight.
In reality, the daily protein requirement is just slightly higher because a certain amount of nitrogen, hence proteins, is lost, as well as in the urine, also through sweating.

Water and minerals loss during running

Water losses depend on the amount of sweat produced, that depends on:

• air temperature and humidity;
• solar radiation.

The loss will be greater the higher these values are.
Finally, the amount of sweat produced is different from person to person.

Minerals lost in sweat are mostly:

• sodium and chlorine, about 1 gram per liter of sweat in heat acclimatized athletes;
• potassium, in an amount equal to about 15 percent of the sodium lost;
• magnesium, in an amount equal to about 1 percent of the sodium lost.

The amount of minerals lost depends on how much sweat is produced, and it increases in non-heat acclimatized athletes.

The table shows the values, in grams per liter, of the minerals lost in sweat for non-heat and heat-acclimated athletes.

	Non-heat acclimated athletes	Heat acclimated athetes
Sodium	1.38	0.92
Chlorine	1.5	1.00
Potassium	0.20	0.15
Magnesium	0.01	0.01
Total	3.09	2.08

Therefore, during physical activity, sodium is the mineral we need most of all.
After physical activity, runner, or who sweats heavily, tends to eat saltier food. This effect, known as selective hunger, was discovered, for sodium, in studies conducted on foundry workers. Probably, the selective hunger doesn’t not exist for potassium and magnesium.

References

1. Sawka M.N., Burke L.M., Eichner E.R., Maughan, R.J., Montain S.J., Stachenfeld N.S. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sport Exercise 2007;39(2):377-390. doi:10.1249/mss.0b013e31802ca597
2. Shirreffs S., Sawka M.N. Fluid and electrolyte needs for training, competition and recovery. J Sport Sci 2011;29:sup1, S39-S46. doi:10.1080/02640414.2011.614269

gamma-Linolenic acid (GLA), one of the omega-6 polyunsaturated fatty acids, like its precursor linoleic acid (the most abundant polyunsaturated fatty acid in human skin epidermis, where it’s involved in the maintenance of the epidermal water barrier), plays important roles in the physiology and pathophysiology of the skin.
Studies conducted on humans revealed that gamma-linolenic acid:

• improves skin moisture, firmness, roughness;
• decreases transepidermal water loss (one of the abnormalities of the skin in essential fatty acid deficiency animals).

Using guinea pig skin epidermis as a model of human epidermis (they are functionally similar), it was demonstrated that supplementation of animals with gamma-linolenic acid-rich foods results in a major production of PGE₁ and 15-HETrE in the skin (as previously demonstrated in in vitro experiments).
Because these molecules have both anti-inflammatory/anti-proliferative properties supplementation of diet with gamma-linolenic acid acid-rich foods may be an adjuncts to standard therapy for inflammatory/proliferative skin disorders.

Supplemental sources of GLA

The main supplemental sources of gamma-linolenic acid are oils of the seeds of:

• borage (20%-27% of the total fatty acids);
• black currant (from 15% to 19% of the total fatty acids);
• evening primrose (from 7% to 14% of the total fatty acids), and

Role of gamma-linolenic acid in lowering blood pressure

The relationship between dietary fatty acid intake and blood pressure mainly comes from studies conducted on genetically modified rats that spontaneously develops hypertension (a commonly used animal model for human hypertension).
In these studies many membrane abnormalities were seen so hypertension in rat model may be related to change in polyunsaturated fatty acid metabolism at cell membrane level.
About polyunsaturated fatty acids, several research teams have reported that gamma-linolenic acid reduces blood pressure in normal and genetically modified rats (greater effect) and it was purported by interfering with Renin-Angiotensin System (that promote vascular resistance and renal retention) altering the properties of the vascular smooth muscle cell membrane and so interfering with the action of angiotensin II.
Another possible mechanism of action of gamma-linolenic acid to lower blood pressure could be by its metabolite dihomo-gamma-linolenic acid: it may be incorporated in vascular smooth muscle cell membrane phospholipids, then released by the action of phospholipase A2 and transformed by COX-1 in PGE₁ that induces vascular smooth muscle relaxation.

Role gamma-linolenic acid in treatment of rheumatoid arthritis

In a study conducted by Leventhal et al. on 1993 it was demonstrated the dietary intake of higher concentration of borage oil (about 1400 mg of gamma-linolenic acid/day) for 24 weeks resulted in clinically significant reductions in signs and symptoms of rheumatoid arthritis activity.
In a subsequent study by Zurier et al. on 1996 the dietary intake of an higher dose (about 2.8 g/day gamma-linolenic acid) for 6 months reduced, in a clinically relevant manner, signs and symptoms of the disease activity; patients who remained for 1 year on the 2.8 g/day dietary gamma-linolenic acid exhibited continued improvement in symptoms (the use of gamma-linolenic acid also at the above higher dose is well tolerated, with minimal deleterious effects). These data underscore that the daily amount and the duration of gamma-linolenic acid dietary intake do correlate with the clinical efficacy.

References

1. Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008
2. Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008
3. Fan Y.Y. and Chapkin R.S. Importance of dietary γ-linolenic acid in human health and nutrition. J Nutr 1998;128:1411-1414. doi:10.1093/jn/128.9.1411
4. Leventhal L.J., Boyce E.G. and Zurier R.B. Treatment of rheumatoid arthritis with gammalinolenic acid. Ann Intern Med 1993;119:867-873. doi:10.7326/0003-4819-119-9-199311010-00001
5. Miller C.C. and Ziboh V.A. Gammalinolenic acid-enriched diet alters cutaneous eicosanoids. Biochem Biophys Res Commun 1988;154:967-974. doi:10.1016/0006-291X(88)90234-3
6. Zurier R.B., Rossetti R.G., Jacobson E.W., DeMarco D.M., Liu N.Y., Temming J.E., White B.M. and Laposata M. Gamma-linolenic acid treatment of rheumatoid arthritis. A randomized, placebocontrolled trial. Arthritis Rheum 1996;39:1808-1817. doi:10.1002/art.1780391106

Many studies have shown a direct, dose-dependent relationship between alcohol intake and blood pressure, particularly for intake above two drinks per day.
This relationship is independent of:

• age;
• salt intake;
• obesity;
• finally, it persists regardless of beverage type.

Furthermore, heavy consumption of alcoholic beverages for long periods of time is one of the factors predisposing to hypertension: from 5 to 7% of hypertension cases is due to an excessive alcohol consumption.
A meta-analysis of 15 randomized controlled trials has shown that decreasing alcoholic beverage intake intake has therapeutic benefit to hypertensive and normotensive with similar systolic and diastolic blood pressure reductions (in hypertensive reduction occurs within weeks).

Contents

• Alcohol intake and prevention of hypertension
  • Relationship between ethanol intake and blood pressure
• References

Alcohol intake and prevention of hypertension

Guidelines on the primary prevention of hypertension recommend that alcohol (ethanol) consumption in most men, in absence of other contra, should be less than 28 g/day, the limit in which it may reduce coronary heart disease risk.
The consumption limited to these quantities must be obtained by intake of drinks with low ethanol content, preferably at meals (drinking even lightly to moderately outside of meals increases the probability to have hypertension). This means no more than 680 ml or 24 oz of regular beer or 280 ml or 10 oz of wine (12% ethanol), especially in hypertension; for women and thinner subjects consumption should be halved¹.
To avoid intake of drinks with high ethanol content even though the total ethanol content not exceeding 28 g/day.

Relationship between ethanol intake and blood pressure

Anyway, uncertainty remains regarding benefits or risks attributable to light-to-moderate alcoholic beverage intake on the risk of hypertension.
In a study published on April 2008, the authors examined the association between ethanol intake and the risk of developing hypertension in 28848 women from “The Women’s Health Study” and 13455 men from the “Physicians’ Health Study”, (the follow-up lasted respectively for 10.9 and 21.8 years). The study confirms that heavy ethanol intake (exceeding 2 drinks/day) increases hypertension risk in both men and women but, surprisingly, found that the association between light-to-moderate alcohol intake (up to 2 drinks/day) and the risk of developing hypertension is different in women and men. Women have a potential reduced risk of hypertension from a light-to-moderate ethanol consumption with a J-shaped association²; men have no benefits of light-to-moderate ethanol consumption but an increased risk of hypertension.
However, guidelines for the primary prevention of hypertension limit alcohol consumption to less 2 drinks/day in men and less 1 drink/day in thinner subjects and women.

1. A standard drink contains approximately 14 g of ethanol i.e. a 340 ml or 12 oz of regular beer, 140 ml or 5 oz wine (12% alcohol), or 42 ml or 1,5 oz of distilled spirits (inadvisable).

2. Many studies have shown a J-shaped relationship between ethanol intake and blood pressure. Light drinker (no more than 28 g of ethanol/day) have lower blood pressure than teetotalers; instead, who consumes more than 28 g ethanol/day have higher blood pressure than non drinker. So alcohol is a vasodilator at low doses but a vasoconstrictor at higher doses.

References

1. Pickering T.G. New guidelines on diet and blood pressure. Hypertension 2006;47:135-136. doi:10.1161/01.HYP.0000202417.57909.26
2. Sesso H.D., Cook N.R., Buring J.E., Manson J.E. and Gaziano J.M. Alcohol consumption and the risk of hypertension in women and men. Hypertension 2008;51:1080-1087. doi:10.1161/HYPERTENSIONAHA.107.104968
3. Writing Group of the PREMIER Collaborative Research Group. Effects of comprehensive lifestyle modification on blood pressure control: main results of the PREMIER Clinical Trial. JAMA 2003;289:2083-2093. doi:10.1001/jama.289.16.2083
4. World Health Organization, International Society of Hypertension Writing Group. 2003 World Health Organization (WHO)/International Society of Hypertension (ISH) statement on management of hypertension. Guidelines and recommendations. J Hyperten 2003;21:1983-1992.

Trans fatty acids or TFA or trans-unsaturated fatty acids or trans fats are unsaturated fatty acids , a subclass of lipids, with at least one a double bond in the trans configuration.
Carbon-carbon double bonds show planar conformation, and so they can be considered as planes from whose opposite sides carbon chain attaches and continues. “The entry” and “the exit” of the carbon chain from the plain may occur on the same side of the plain, and in this case double bond is in cis configuration, or on opposite side, and in that case it is in trans configuration. This is an example of geometric isomerism, also called cis-trans isomerism.

Unsaturated fatty acids most commonly have their double bonds in cis configuration; the other, less common configuration is trans.
Cis bond causes a bend in the fatty acid chain, whereas the geometry of trans bond straightens the fatty acid chain, imparting a structure more similar to that of saturated fatty acids.

Contents

• Properties of fats rich in trans fatty acids
• Types of trans fatty acids
  • Industrial trans fatty acids
    • Partial hydrogenation of vegetable oils
  • Ruminant trans fatty acids
• Isomers
• Effects on human health
  • Effects at plasmatic level
  • Inflammation and endothelial-cell dysfunction
    • Other effects
  • Coronary heart disease
  • Diabetes
  • Ruminant trans fatty acids and the risk of CHD
• Trans fatty acids: legislation regulating their content
  • Food reformulation
• Food sources
  • Margarine
  • Vegetable shortenings
  • Vegetable oils
  • Prepared soups
  • Processed foods
  • Sauces
  • Human milk and infant foods
  • Fast foods and restaurant’s foods
• References

Properties of fats rich in trans fatty acids

Below, some distinctive characteristics of the fats rich in trans fats, that make them particularly suited for the production of margarines and vegetable shortening used in home and commercial cooking, and manufacturing processes.

• Bent molecules can’t pack together easily, but linear ones can do it.
  This means that trans fatty acids contribute, together with the geometrically similar saturated fatty acids, to the hardness of the fats in which they are, giving them a higher melting point.
  Heightening the melting point of fats means that it is possible to convert them from liquid form to semi-solids and solids at room temperature.
  Note: trans fats tend to be less solid than saturated fatty acids.
• They have a melting point, consistency and “mouth feel” similar to those of butter, a long shelf life at room temperature, and a flavor stability.
• They are stable during frying.

Types of trans fatty acids

Dietary TFA come from different sources briefly reviewed below.

• In industrialized countries, greater part of the consumed trans fatty acids, in USA about 80 percent of the total, are produced industrially, in varying amounts, during partial hydrogenation of edible oils containing unsaturated fatty acids.
• They are produced at home during frying with vegetable oils containing unsaturated fatty acids.
• They come from bacterial transformation of unsaturated fatty acids ingested by ruminants in their rumen.
• Another natural source is represented by some plant species, such as leeks, peas, lettuce and spinach, that contain trans-3-hexadecenoic acid, and rapeseed oil, that contains brassidic acid (22:1∆13t) and gondoic acid (20:1∆11t). In these sources trans fatty acids are present in small amounts.
• Very small amounts, less than 2 percent, are formed during deodorization of vegetable oils, a process necessary in the refining of edible oils. During this process trans fatty acids with more than one double bond are formed in small amounts. These isomers are also present in fried foods and in considerable amounts in some partially hydrogenated vegetable oils.

Industrial trans fatty acids

Hydrogenation is a chemical reaction in which hydrogen atoms react, in the presence of a catalyst, with a molecule.
The hydrogenation of unsaturated fatty acids involves the addition of hydrogen atoms to double bonds on the carbon chains of fatty acids. The reaction occurs in presence of metal catalyst and hydrogen, and is favored by heating vegetable oils containing unsaturated fatty acids.

Partial hydrogenation of vegetable oils

The process of hydrogenation was first discovered in 1897 by French Nobel prize in Chemistry, jointly with fellow Frenchman Victor Grignard, Paul Sabatier using a nickel catalyst.
Partially hydrogenated vegetable oils were developed in 1903 by a German chemist, Wilhelm Normann, who files British patent on “Process for converting unsaturated fatty acids or their glycerides into saturated compounds”. The term trans fatty acids or trans fats appeared for the first time in the Remark column of the 5th edition of the “Standard Tables of Food Composition” in Japan.
During partial hydrogenation, an incomplete saturation of the unsaturated sites on the carbon chains of unsaturated fatty acids occurs. For example, with regard to fish oil, trans fatty acid content in non-hydrogenated oils and in highly hydrogenated oils is 0.5 and 3.6 percent, respectively, whereas in partially hydrogenated oils is 30 percent.

But, most importantly, some of the remaining cis double bonds may be moved in their positions on the carbon chain, producing geometrical and positional isomers, that is, double bonds can be modified in both conformation and position.
Below, other changes that occur during partial hydrogenation are listed.

• Alpha-linolenic acid, the plant-based omega-3 polyunsaturated fatty acid, is largely destroyed.
• Cyclic monomers, as well as intramolecular linear dimmers, are also formed.

Partially hydrogenated vegetable oils were developed for the production of vegetable fats, a cheaper alternative to animal fats. In fact, through hydrogenation, oils such as soybean, safflower and cottonseed oils, which are rich in unsaturated fatty acids, are converted into semi-solid fats.
The first hydrogenated oil was cottonseed oil in USA in 1911 to produce vegetable shortening.
In the 1930’s, partial hydrogenation became popular with the development of margarine.
Currently, per year in USA, 6-8 billion tons of hydrogenated vegetable oil are produced.

Ruminant trans fatty acids

Ruminant trans fats are produced by bacteria in the rumen of the animals, for example cows, sheep and goats, using as a substrate a proportion of the relatively small amounts of unsaturated fatty acids present in their feedstuffs, that is, feed, plants and herbs. And, considering an animal that lives at least a year, and has the opportunity to graze and/or eat hay, there is a season variability in unsaturated fatty acids intake, and trans fats produced. In fact, in summer and spring, pasture plants and herbs may contain more unsaturated fatty acids than the winter feed supply.
Then, TFA are present at low levels in meat and full fat dairy products, typically <5 percent of total fatty acids, and are located in the sn-1 and sn-3 positions of the triacylglycerols, whereas in margarines and other industrially hydrogenated products they appear to be concentrated in the sn-2 position of the triacylglycerols.
Ruminant trans fatty acids are mainly monounsaturated fatty acids, with 16 to 18 carbon atoms, and constitute a small percentage of the trans fatty acids in the diet.

Isomers

The most important cluster of trans fatty acids is trans-C18:1 isomers, that is, fatty acids containing 18 carbon atoms plus one double bond, whose position varies between Δ6 and Δ16 carbon atoms. In both sources, the most common isomers are those with double bonds between positions Δ9 and Δ11.
However, even if these molecules are present both in industrial and ruminant TFA, there is a considerable quantitative difference. For example, vaccenic acid (C18:1 Δ11t) represents over 60 percent of the trans-C18:1 isomers in ruminant trans fatty acids, whereas in industrial ones elaidic acid (C18:1Δ9t) comprises 15-20 percent and C18:1 Δ10t and vaccenic acid over 20 percent each others.

Effects on human health

Ruminant trans fatty acids, in amounts actually consumed in diets, are not harmful for human health.
Conversely, consumption of industrial trans fats has neither apparent benefit nor intrinsic value, above their caloric contribution, and, from human health standpoint they are only harmful, having adverse effects on:

• serum lipid levels;
• endothelial cells;
• systemic inflammation;
• other risk factors for cardiovascular disease.

Moreover, they are positively associated with the risk of coronary heart disease (CHD), and sudden death from cardiac causes and diabetes.

Note: further in the text, we will refer to industrial trans fatty acids as trans fats or trans fatty acids.

Effects at plasmatic level

Low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C) plasma levels are well-documented risk markers for the development of coronary heart disease or CHD.

• High LDL-C levels are associated with an increased incidence of ischemic heart disease.
• High HDL-C levels are associated with a reduced incidence of the risk.

For this reason, the ratio between total cholesterol level and HDL-C is often used as a combined risk marker for these two components in relation to the development of heart disease: the higher the ratio, the higher the risk.

TFA, as previously said, have adverse effects on serum lipids.
These effects have been evaluated in numerous controlled dietary trials by isocaloric replacement of saturated fatty acids or cis-unsaturated fatty acids with trans fats. It was demonstrated that such replacement:

• raises LDL-C levels;
• lowers HDL-C levels, in contrast to saturated fatty acids that increase HDL-C levels when used as replacement in similar study;
• increases the ratio of total cholesterol to HDL-C, approximately twice that for saturated fatty acids, and, on the basis of this effect alone, trans fatty acids has been estimated to cause about 6% of coronary events in the USA.

Furthermore, trans fats:

• produce a deleterious increase in small, dense LDL-C subfractions, that is associated with a marked increased in the risk of CHD, even in the presence of relatively normal LDL-C;
• increase the blood levels of triglycerides, and this is an independent risk factor for CHD;
• increase levels of Lp(a)lipoprotein, another important coronary risk factor.

But on 2004 prospective studies have shown that the relation between the intake of trans fatty acids and the incidence of CHD is greater than that predicted by changes in serum lipid levels alone. This suggests that trans fats influence other risk factors for CHD, such as inflammation and endothelial-cell dysfunction.

Inflammation and endothelial-cell dysfunction

The role of inflammation in atherosclerosis, and consequently in CHD, is burgeoned in the last decade.
Interleukin-6, C-reactive protein (CRP), and an increased activity of tumor necrosis factor (TNF) system are markers of inflammation.
In women greater intake of trans fatty acids is associated with increased activity of TNF system, and in those with a higher body mass index with increased levels of interleukin-6 and CRP. For example, the difference in CRP seen with an average intake of trans fats of 2.1 percent of the total daily energy intake, as compared with 0.9 percent, correspond to an increased risk of cardiovascular disease of 30 percent. Similar results have been reported in patients with established heart disease, in randomized, controlled trials, in in vitro studies, and in studies in which it has been analyzed membrane levels of trans fatty acids, a biomarker of their dietary intake.
So, trans fats promote inflammation, and their inflammatory effects may account at least in part for their effects on CHD that, as seen above, are greater than would be predicted by effects on serum lipoproteins alone.
Attention: the presence of inflammation is an independent risk factor not only for CHD but also for insulin resistance, diabetes, dyslipidemia, and heart failure.

Another site of action of TFA may be endothelial function.
Several studies have suggested the association between greater intake of trans fats and increased levels of circulating biomarkers of endothelial dysfunction, such as E-selectin, sICAM-1, and sVCAM-1.

Other effects

In vitro studies have demonstrate that trans fats affect lipid metabolism through several pathways.

• They alter secretion, lipid composition, and size of apolipoprotein B-100 (apo B-100).
• They increase cellular accumulation and secretion of free cholesterol and cholesterol esters by hepatocytes.
• They alter expression in adipocytes of genes for peroxisome proliferator-activated receptor-gamma (PPAR-gamma), lipoprotein lipase, and resistin, proteins having a central roles in the metabolism of fatty acids and glucose.

Coronary heart disease

Industrial trans fats are independent cardiovascular risk factor.
Since the early 1990s attention has been focused on the effect of trans fatty acids on plasma lipid and lipoprotein concentrations.
Furthermore, four major prospective studies covering about 140,000 subjects, monitored for 6-14 years, have all found positive epidemiological evidence relating their levels in the diet, assessed with the aid of a detailed questionnaire on the composition of the diet, to the risk of CHD. These four studies are:

• The Health Professionals Follow-up study;
• The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study;
• The Nurses’ Health Study;
• The Zutphen Elderly Study.

These studies cover such different populations that the results very probably hold true for the populations as a whole.
A meta-analysis of these studies have shown that a 2 percent increase in energy intake from industrial TFA was associated with a 23 percent increase in the incidence of CHD. The relative risk of heart disease was 1.36 in “The Health Professionals Follow-up Study”, 1.14 in “The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study”; 1.93 (1.43-2.61) in “The Nurses’ Health Study”, and 1.28 (1.01-1.61) in “The Zutphen Elderly Study”.
So, there is a substantially increased risk even at low levels of intake: 2 percent of total energy intake, for a 2,000 Kcal diet is 40 Kcal or about 4-5 g of fat corresponding to a teaspoonful of fat!
Moreover, in three of the studies, the association between the intake of industrial trans fats and the risk of CHD was stronger than a corresponding association between the intake of saturated fatty acids and the risk of heart disease. In “The Zutphen Elderly Study”, this association was not investigated.
Because of the adverse effects of industrial trans fatty acids, for the same authors are unethical conducting randomized long-term trials to test their effects on the incidence of CHD.
So, avoidance of industrial trans fats, or a consumption of less 0.5% of total daily energy intake is necessary to avoid their adverse effects, far stronger on average than those of food contaminants or pesticide residues.

Further evidence
A study conducted in an Australia population with a first heart attack and no preceding history of CHD or hyperlipidemia has showed a positive association between levels of trans fatty acids in adipose tissue and the risk of nonfatal myocardial infarction.
It was shown that adipose tissue C18:1Δ7t, found in both animal and vegetable fats, was an independent predictor of a first myocardial infarction, that is, its adipose tissue level is still a predictor for heart disease after adjustment for total cholesterol. Again, it appears that only a minor part of the negative effects of trans fats occurs via plasma lipoproteins.
During the course of this study, mid-1996, TFA were eliminated from margarines sold in Australia. This was a unique opportunity to investigate the temporal relationship between trans fat intake and their adipose tissue levels. It was demonstrated that trans fats disappear from adipose tissue of both case-patients and controls with a rate about 15 percent of total trans fats/y.
Another study conduct in Costa Rica have found a positive association between myocardial infarction and trans fatty acids.
Interestingly, in a larger, community-based case-control study, levels of trans fats in red blood cell membranes were associated, after adjustment for other risk factors, with an increase in the risk of sudden cardiac death. Moreover, the increased risk appeared to be related to trans-C18:2 levels, that were associated with a tripling of the risk, but not with cell membrane levels of trans-C18:1, the major trans fatty acids in foods (see above).

Diabetes

In a prospective study covering 84,204 female nurses, from “The Nurses’ Health Study”, aged 34–59 y, analyzed from the 1980 to 1996, with no cancer, diabetes, or cardiovascular disease at base line, the intake of trans fatty acids was significantly related to the risk of developing type 2 diabetes. And, after adjustment for other risk factors trans fat intake was positively associated with the incidence of diabetes with a risk up to 39 percent greater.
Data from controlled intervention studies showed that TFA could impair insulin sensitivity in subjects with insulin resistance and type 2 diabetes (saturated fatty acids do the analogous response, with no significant difference between TFA and them) more than unsaturated fatty acids, in particular the isomer of conjugated linoleic acid (CLA) trans-10, cis-12-CLA. Be careful because some dietary supplements contain CLA isomers and may be diabetogenic and proatherogenic in insulin-resistant subjects.

No significant effect was seen in insulin sensitivity of lean, healthy subjects.

Ruminant trans fatty acids and the risk of CHD

Four prospective studies have evaluated the relation between the intake of ruminant trans fatty acids and the risk of CHD: no significant association was identified.
In another study published on 2008 was analyzed data from four Danish cohort studies that cover 3,686 adults enrolled between 1974 and 1993, and followed for a median of 18 years. In Denmark, consumption of dairy products is relatively high and the range of ruminant trans fat intake is relatively broad, up to 1.1 percent of energy. Conversely, in the other countries, ruminant trans fatty acid consumption for most people is substantially lower than 1 percent of energy, in USA about 0.5 percent of energy. After adjustment for other risk factors, no significant associations between ruminant TFA consumption and incidence of CHD were found, confirming, in a population with relatively high intake of ruminant trans fatty acids, conclusions of four previous prospective studies.
So ruminant trans fats, in amounts actually consumed in diets, do not raise CHD risk.
The absence of risk of CHD with trans fats from ruminants as compared with industrial trans fatty acids may be due to a lower intake. In the USA, greater part of trans fats have industrial origin (see above); moreover trans fat levels in milk and meats are relatively low, 1 to 8 percent of total fats.
The absence of a higher risk of CHD may be due also to the presence of different isomers. Ruminant and industrial sources share many common isomers, but there are some quantitative difference (see fig. 4):

• vaccenic acid level is higher in ruminant fats, 30-50 percent of trans isomers;
• trans-C18:2 isomers, present in deodorized and fried vegetable oils, as well as in some partially hydrogenated vegetable oils, are not present in appreciable amounts in ruminants fats.

Finally other, still unknown, potentially protective factors could outweigh harmful effects of ruminant trans fats.

Trans fatty acids: legislation regulating their content

USA
Until 1985 no adverse effects of trans fatty acids on human health was demonstrated, and in 1975 a Procter & Gamble study showed no effect of trans fats on cholesterol.
Their use in fast food preparation grew up from 1980’s, when the role of dietary saturated fats in increasing cardiac risk began clear. Then, it was led a successful campaign to get McDonald’s to switch from beef tallow to vegetable oil for frying its French fries. Meanwhile, studies began to raise concerns about their effects on health. On 1985 Food and Drug Administration (FDA) concluded that TFA and oleic acid affected serum cholesterol level similarly, but from the second half of 1985 their harmful began clear, and the final proof came from both controlled feeding trials and prospective epidemiologic studies.
On 2003 FDA ruled that food labels, for conventional foods and supplements, show their content beginning January 1, 2006. Notably, this ruling was the first substantive change to food labeling since the requirement for per-serving food labels information was added in 1990.
On 2005 The US Department of Agriculture made a minimized intake of trans fatty acids a key recommendation of the new food-pyramid guidelines.
The American Heart Association, on 2006, recommended to limit their intake to 1% of daily calorie consumption, and suggested food manufacturers and restaurants switch to other fats.
On 2006 New York City Board of Health announced trans fat ban in its 40,000 restaurants within July 1, 2008, followed by the state of California in 2010-2011.

Australia
After June 1996 they were eliminated from margarine sold in Australia, that before contributed about 50% of their dietary intake.

Europe
On March 11, 2003 the Danish government, after a debate started in 1994 and two new reports in 2001 and 2003, decided to phase out the use of industrial trans fats in food before the end of 2003. Two years later, however, the European Commission (EC) asked Denmark to withdraw this law, which was not accepted on the European Community level, unfortunately. However, in 2007, EC decided to closes its infringement procedure against Denmark because of increasing scientific evidence of the danger of this type of fatty acids.
The Danish example was followed by Austria and Switzerland in 2009, Iceland, Norway, and Hungary in 2011, and most recently, Estonia and Georgia in 2014. So, about 10 percent of the European Union population, about 500 million people, lives in countries where it is illegal to sell food high in industrial trans fats.
Governments of other European Union countries instead rely on the willingness of food producers to reduce trans fatty acid content in their products. This strategy has proved effective only for Western European countries.

Canada
Canada is considering legislation to eliminate them from food supplies, and, in 2005, ruled that pre-packaged food labels show their content.

Therefore, with the exception of the countries where the use of trans fats in the food industry was banned, the only way to reduce their intake in the other countries is consumer’s decision to choose foods free in such fatty acids, avoiding those known containing them, and always reading nutrition facts and ingredients because they may come from margarine, vegetable oil and frying. Indeed, for example in the USA, the producers of foods that contain less than 0.5 g of industrial trans fatty acids per serving can list their content as 0 on the packaging. This content is low but if a consumer eats multiple servings, he consumes substantial amount of them.

Be careful: food labels are not obligatory in restaurants, bakeries, and many other retail food outlets.

Food reformulation

Public health organizations, including the World Health Organization in September 2006, have recommended reducing the consumption of industrial trans fatty acids; only in USA the near elimination of these fatty acids might avoid between 72,000 and 280,000 of the 1.2 million of CHD events every year.
Food manufacturers and restaurants may reduce industrial TFA use choosing alternatives to partially hydrogenated oils.
In Denmark, their elimination from vegetable oils did not increase consumption of saturated fatty acids because they were mostly replaced with cis-unsaturated fatty acids. Moreover, there were no noticeable effects for the consumer: neither increase in the cost nor reduction in availability and quality of foods.
In 2009, Stender et al. have shown that industrial trans fatty acids in food such as French fries, cookies, cakes, and microwave-oven popcorn purchased in USA, South Africa, and many European Country can be replaced, at similar prices, with a mixture of saturated, monounsaturated, and polyunsaturated fatty acids. Such substitution has even greater nutritional benefit than one-to-one substitution of industrial trans fats with saturated fatty acids alone. However, be careful because only in French fries with low industrial trans fats the percentage of saturate fatty acids remains constant, whereas in cookies and cakes is in average +33 percentage points and microwave-oven popcorn +24 percentage points: saturated fatty acids are less dangerous than industrial trans fats but more than mono- and polyunsaturated fatty acids.
The same research group, analyzing some popular foods in Europe, purchased in supermarkets, even of the same supermarket chain, and fast food, namely, McDonald’s and Kentucky Fried Chicken (KFC), from 2005 to 2014, showed that their TFA content was reduced or even absent in several Western European countries while remaining high in Eastern and Southeastern Europe.
In 2010 Mozaffarian et al. evaluated the levels of industrial trans fats and saturated fatty acids in major brand-name U.S. supermarket and restaurant foods after reformulation to reduce industrial trans fatty acid content, in two time: from 1993 through 2006 and from 2008 through 2009. They found a generally reduction in industrial trans fat content without any substantial or equivalent increase in saturated fatty acid content.

Foods sources

Many foods high in trans fats are popularly consumed worldwide.
In USA greater part of these fatty acids comes from partially hydrogenated vegetable oils, with an average consumption from this source that has been constant since the 1960′s.
It should be noted that the following trans fatty acid values must be interpreted with caution because, as previously said, many fast food establishments, restaurants and industries may have changed, or had to change the type of fat used for frying and cooking since the analysis were done.
The reported values, unless otherwise specified, refer to percentage in trans fatty acids/ 100 g of fatty acids.

Margarine

Among foods with trans fats, stick or hard margarine had the highest percentage of them, but levels of these fatty acids have declined as improved technology allowed the production of softer margarines which have become popular. But there are difference in trans fatty acid content of margarine from different countries. Below some examples.

• The highest content, 13-16.5 percent, is found in soft margarine from Iceland, Norway, and the UK.
• Less content is found in Italy, Germany, Finland, and Greece, 5.1 percent, 4.8 percent, 3.2 percent, and 2.9 percent respectively).
• In Portugal, The Netherlands, Belgium, Denmark, France, Spain, and Sweden margarine trans fat content is less than 2 percent.

USA and Canada lag behind Europe, but in the USA, with the advent of trans fat labeling of foods and the greater knowledge of the risk associated with their consumption by the buyers, change is occurring. For this reason, at now, in the USA margarine is considered to be a minor contributor to the intake of TFA, whereas the major sources are commercially baked and fast food products like cake, cookies, wafer, snack crackers, chicken nuggets, French fries or microwave-oven popcorn (see below).

Vegetable shortenings

Trans fatty acid content of vegetable shortenings ranges from 6% to 50 percent, and varies in different country: in Germany, Austria and New Zealand it is less than France or USA.
However, like margarines, their trans fat content is decreasing. In Germany it decreased from 12% in 1994 to 6 percent in 1999, in Denmark is 7 percent (1996) while in New Zealand is about 6 percent (1997).

Vegetable oils

At now, non-hydrogenated vegetable oils for salad and cooking contain no or only small amounts of trans fats.
Processing of these oils can produce minimal level of them, ranged from 0.05g/100 food for extra virgin olive oil to 2.42 g/100 g food for canola oil. So, their contribution to trans fat content of the current food supply is very little.
One exception is represented by Pakistani hydrogenated vegetable oils whose TFA content ranges from 14% to 34%.

Prepared soups

Among foods with trans fats, prepared soups contain significant amount of them, ranging from 10 percent of beef bouillon to 35 percent of onion cream. So, they contribute great amount of such fatty acids to the diet if frequently consumed.

Processed foods

Thanks to their properties, trans fatty acids are used in many processed foods as cookies, cakes, croissants, pastries and other baked goods. And, baked goods are the greatest source of these fats in the North American diet. Of course, their trans fat content depends on the type of fat used in processing.

Sauces

Mayonnaise, salad dressings and other sauces contain only small or no-amounts of trans fats.

Human milk and infant foods

Trans fat content of human milk reflects the trans fat content of maternal diet in the previous day, is comprised between 1 and 7%, and is decreasing from 7.1 percent in 1998 to 4.6 percent in 2005/2006.
Infant formulas have trans fat values on average 0.1-4.5 percent, with a brand up to 15.7 percent.
Baby foods contain greater than 5 percent of trans fats.

Fast foods and restaurant’s foods

Vegetable shortenings high in trans fats are used as frying fats, so fast foods and many restaurant’s foods may contain relatively large amounts of them. Foods are fried pies, French fries, chicken nuggets, hamburgers, fried fish as well as fried chicken.
In articles published by Stender et al. from 2006 to 2009, it is showed that for French fries and chicken nuggets their content varies largely from nation to nation, but also within the same fast food chain in the same country, and even in the same city, because of the cooking oil used. For example, oil used in USA and Peru outlets of a famous fast food chain contained 23-24% of trans fats, whereas oil used in many European countries of the same fast food chain contained about 10 percent, with some countries, such as Denmark, as low as 5 percent and 1 percent.
And, considering a meal of French fries and chicken nuggets, in serving size of 171 and 160 g respectively, purchased at McDonald‘s in New York City, it contained over 10 g of TFA, while if purchased at KFC in Hungary they were almost 25 g.
Below, again from the work of Stender et al. it can see a cross-country comparison of trans fat contents of chicken nuggets and French fries purchased at McDonald ‘s or KFC.

Chicken nuggets and French fries from McDonald’s:

• less than 1 g only if the meals were purchased in Denmark;
• 1-5 g in Portugal, the Netherlands, Russia, Czech Republic, or Spain;
• 5-10 g in the United States, Peru, UK, South Africa, Poland, Finland, France, Italy, Norway, Spain, Sweden, Germany, or Hungary.

Chicken and French fries from KFC:

• less than 2 g if the meals were purchased UK (Aberdeen), Denmark, Russia, or Germany (Wiesbaden);
• 2-5 in Germany (Hamburg), France, UK (London or Glasgow), Spain, or Portugal;
• 5-10 in the Bahamas, South Africa, or USA;
• 10-25 g in Hungary, Poland, Peru, or Czech Republic.

References

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4. Asp N-G. Fatty acids in focus – the good and the bad ones. Scand J Food Nutr 2006;50:155-160. doi:10.1080/17482970601069094
5. Baylin A., Kabagambe E.K., Ascherio A., Spiegelman D., Campos H. High 18:2 trans-fatty acids in adipose tissue are associated with increased risk of nonfatal acute myocardial infarction in Costa Rican adults. J Nutr 2003;133:1186-1191. doi:10.1093/jn/133.4.1186
6. Chow C.K. Fatty acids in foods and their health implication. 3rd Edition. CRC Press Taylor & Francis Group, 2008.
7. Clifton P.M., Keogh J.B., Noakes M. Trans fatty acids in adipose tissue and the food supply are associated with myocardial infarction. J Nutr 2004;134:874-879. doi:10.1093/jn/134.4.874
8. Costa N., Cruz R., Graça P., Breda J., and Casal S. Trans fatty acids in the Portuguese food market. Food Control 2016;64:128-134. doi:10.1016/j.foodcont.2015.12.010
9. Eckel R.H., Borra S., Lichtenstein A.H., Yin-Piazza D.Y. Understanding the Complexity of Trans fatty acid reduction in the American diet. American Heart Association trans fat conference 2006 report of the trans fat conference planning group. Circulation 2007;115:2231-2246. doi:10.1161/CIRCULATIONAHA.106.181947
10. Hu F.B., Manson J.E., Stampfer M.J., Colditz G., Liu S., Solomon C.G., and Willett W.C. Diet, lifestyle, and the risk of type 2 diabetes mellitus in women. N Engl J Med 2001;345:790-797. doi:10.1056/NEJMoa010492
11. Hu F.B., Willett W.C. Optimal diet for prevention of coronary heart disease JAMA 2002;288:2569-2578. doi:10.1001/jama.288.20.2569
12. Lemaitre R.N., King I.B., Raghunathan T.E., Pearce R.M., Weinmann S., Knopp R.H., Copass M.K., Cobb L.A., Siscovick D.S. Cell membrane trans-fatty acids and the risk of primary cardiac arrest. Circulation 2002;105:697-701. doi:10.1161/hc0602.103583
13. Lemaitre R.N, King I.B, Mozaffarian D., Sootodehnia N., Siscovick D.S. Trans-fatty acids and sudden cardiac death. Atheroscler Suppl 2006; 7(2):13-5. doi:10.1016/j.atherosclerosissup.2006.04.003
14. Lichtenstein A.H. Dietary fat, carbohydrate, and protein: effects on plasma lipoprotein patterns J. Lipid Res. 2006;47:1661-1667. doi:10.1194/jlr.R600019-JLR200
15. Lichtenstein A.H., Ausman L., Jalbert S.M., Schaefer E.J. Effect of different forms of dietary hydrogenated fats on serum lipoprotein cholesterol levels. N Engl J Med 1999;340:1933-1940. doi:10.1056/NEJM199906243402501
16. Lopez-Garcia E., Schulze M.B., Meigs J.B., Manson J.E, Rifai N., Stampfer M.J., Willett W.C. and Hu F.B. Consumption of trans fatty acids is related to plasma biomarkers of inflammation and endothelial dysfunction. J Nutr 2005;135:562-566. doi:10.1093/jn/135.3.562
17. Masanori S. Trans Fatty Acids: Properties, Benefits and Risks J Health Sci 2002;48(1):7-13.
18. Mensink R.P., Katan M.B. Effect of dietary trans fatty acids on high-density and low-density lipoprotein cholesterol levels in healthy subjects. N Engl J Med 1990;323:439-445. doi:10.1056/NEJM199008163230703
19. Mozaffarian D. Commentary: Ruminant trans fatty acids and coronary heart disease-cause for concern? Int J Epidemiol 2008;37(1):182-184. doi:10.1093/ije/dym263
20. Mozaffarian D., Jacobson M.F., Greenstein J.S. Food Reformulations to reduce trans fatty acids. N Eng J Med 2010;362:2037-2039. doi:https://doi.org/10.1056/NEJMc1001841
21. Mozaffarian D., Katan M.B., Ascherio A., Stampfer M.J., Willett W.C. Trans fatty acids and cardiovascular disease. N Engl J Med 2006;354:1601-1613. doi:10.1056/NEJMra054035
22. Mozaffarian D., Pischon T., Hankinson S.E., Joshipura K., Willett W.C., and Rimm E.B. Dietary intake of trans fatty acids and systemic inflammation in women. Am J Clin Nutr 2004;79:606-612. doi:https://doi.org/10.1093/ajcn/79.4.606
23. Oh K., Hu F.B., Manson J.E., Stampfer M.J., Willett W.C. Dietary fat intake and risk of coronary heart disease in women: 20 years of follow-up of the Nurses’ Health Study. Am J Epidemiol 2005;161(7):672-679. doi:10.1093/aje/kwi085
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25. Oomen C.M., Ocke M.C., Feskens E.J., van Erp-Baart M.A., Kok F.J., Kromhout D. Association between trans fatty acid intake and 10-year risk of coronary heart disease in the Zutphen Elderly Study: a prospective population-based study. Lancet 2001; 357(9258):746-751. doi:10.1016/S0140-6736(00)04166-0
26. Pietinen P., Ascherio A., Korhonen P., Hartman A.M., Willett W.C., Albanes D., VirtamO J.. Intake of fatty acids and risk of coronary heart disease in a cohort of Finnish men: the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study. Am J Epidemiol 1997;145(10):876-887. doi:10.1093/oxfordjournals.aje.a009047
27. Risérus U. Trans fatty acids, insulin sensitivity and type 2 diabetes. Scand J Food Nutr 2006;50(4):161-165. doi:10.1080/17482970601133114
28. Salmerón J., Hu F.B., Manson J.E., Stampfer M.J., Colditz G.A., Rimm E.B., and Willett W.C. Dietary fat intake and risk of type 2 diabetes in women. Am J Clin Nutr 2001;73:1019-1026 doi:10.1093/ajcn/73.6.1019
29. Stender S., Astrup A., and Dyerberg J. A trans European Union difference in the decline in trans fatty acids in popular foods: a market basket investigation. BMJ Open 2012;2(5):e000859. doi:10.1136/bmjopen-2012-000859
30. Stender S., Astrup A., and Dyerberg J. Artificial trans fat in popular foods in 2012 and in 2014: a market basket investigation in six European countries. BMJ Open 2016;6(3):e010673. doi:10.1136/bmjopen-2015-010673
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Carbohydrate ingestion can improve endurance capacity and performance.
The ingestion of different types of carbohydrates, which use different intestinal transporters, can:

• increase total carbohydrate absorption;
• increase exogenous carbohydrate oxidation;
• and therefore improve performance.

Glucose and fructose

When a mixture of glucose and fructose is ingested, in the analyzed literature, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min, there is less competition for intestinal absorption compared with the ingestion of an iso-energetic amount of glucose or fructose, two different intestinal transporters being involved. Furthermore, fructose absorption is stimulated by the presence of glucose.

This can:

• contribute to a faster rate of the absorption of monosaccharides;
• increase the availability of exogenous carbohydrates in the bloodstream;
• cause the higher exogenous carbohydrate oxidation rates in fructose plus glucose combination compared to high glucose intake alone.

The combined ingestion of glucose and fructose allows to obtain exogenous carbohydrate oxidation rate around 1,26 g/min, therefore, higher than the rate reported with glucose alone (1g/min), also in high concentration.
The observed difference, namely, +0,26 g/min, can be fully attributed to the oxidation of ingested fructose.

Sucrose and glucose

The ingestion of sucrose and glucose, in the same conditions of the ingestion of glucose and fructose, therefore, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min, gives similar results.

Glucose, sucrose and fructose

Very high oxidation rates are found with a mixture of glucose, sucrose, and fructose, in the analyzed literature, respectively 1.2, 0.6 and 0.6 g/min, ratio 2:1:1, for total carbohydrate intake rate to 2.4 g/min; however, note the higher amounts of ingested carbohydrates.

Maltodextrin and fructose

High oxidation rates are also observed with combinations of maltodextrin and fructose, in the same conditions of the ingestion of glucose plus fructose, therefore, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min.
Such high oxidation rates can be achieved with carbohydrates ingested in a beverage, in a gel or in a low-fat, low protein, low-fiber energy bar.
The best combination of carbohydrates ingested during exercise seems to be the mixture of maltodextrin and fructose in a 2:1 ratio, in a 5 percent solution, and in a dose around 80-90 g/h.

Why?

• This mixture has the best ratio between amount of ingested carbohydrates and their oxidation rate and it means that smaller amounts of carbohydrates remain in the stomach or gut reducing the risk of gastrointestinal complication/discomfort during prolonged exercise.
• A solution containing a combination of multiple transportable carbohydrates and a carbohydrate content not exceeding 5% optimizes gastric emptying rate and improves fluid delivery.

Example of a 5 percent carbohydrate solution containing around 80-90 g of maltodextrin and fructose in a 2:1 rate; ingestion time around 1 h.

• 1.5 L solution: 80 g of carbohydrates, including around 55 g of maltodextrin and around 25 of fructose.
• 1.8 L solution: 90 g of carbohydrates, including 60 g of maltodextrin and 30 of fructose.

Conclusion

During prolonged exercise, when high exogenous carbohydrate oxidation rates are needed, the ingestion of multiple transportable carbohydrates is preferred above that of large amounts of a single carbohydrate.
The best mixture seems to be maltodextrin and , in a 2:1 ratio, in a 5 percent concentration solution, and at ingestion rate of around 80-90 g/h.

References

1. Burke L.M., Hawley J.A., Wong S.H.S., & Jeukendrup A. Carbohydrates for training and competition. J Sport Sci 2011;29:Sup1,S17-S27. doi:10.1080/02640414.2011.585473
2. Jentjens R.L.P.G., Moseley L., Waring R.H., Harding L.K., and Jeukendrup A.E. Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol 2004:96;1277-1284. doi:10.1152/japplphysiol.00974.2003
3. Jentjens R.L.P.G., Venables M.C., and Jeukendrup A.E. Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J Appl Physiol 2004:96;1285-1291. doi:10.1152/japplphysiol.01023.2003
4. Jeukendrup A.E. Carbohydrate feeding during exercise. Eur J Sport Sci 2008:2;77-86. doi:10.1080/17461390801918971
5. Jeukendrup A.E. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sport Sci 2011:29;sup1, S91-S99. doi:10.1080/02640414.2011.610348

Carotenoids are a class of fat-soluble pigments widely present in nature.
They are yellow, orange, and red organic compounds, made up of 8 isoprene units, and have many conjugated double bonds. Their hydrocarbon chain can undergo modifications which significantly influence the biological properties.
Discovered in the first half of the 1800s, carotenoids are synthesized by all photosynthetic organisms, including plants, macroalgae and microalgae, and by some non-photosynthetic organisms, such as some types of fungi, bacteria, and insects, such as pea aphids, some species of gall midges, and spider mites. In plants and microalgae they are synthesized and accumulated in plastids.
They form, with polyphenols and glucosinolates, the group of phytochemicals.
In the course of evolution, thanks to their chemical and physical properties, they have proved to be extremely versatile, as they can perform many functions in both plants and animals, among which the antioxidant activity is very important. Furthermore, in humans, some carotenoids serve as vitamin A precursor.
There are more than 750 different carotenoids, more than 100 have been found in fruits and vegetables, and about 40 are consumed in significant amounts by humans.
Thanks to their color, carotenoids are used as food additives.

Contents

• The discovery
• Chemical structure
• Isomerism
• Classification
• Solubility
• Color
• Role in plants
  • Quenching of singlet oxygen
  • Free radical scavenging activity
  • Light-harvesting role
• Food sources
• Food additives
• Carotenoids and human health
• Plasma carotenoids
• References

The discovery

Carotenoids were discovered in the first half of the nineteenth century.
The first carotenoid to be isolated was beta-carotene, in 1831, thanks to the work of the German pharmacist Heinrich Wilhelm Ferdinand Wackenroder, who called “carotene” the yellow pigment crystallized from the carrot root.
Jöns Jacob Berzelius, considered one of the fathers of modern chemistry, named the yellow pigments extracted from autumn leaves xanthophylls, from the Greek xanthos, which means yellow, and phyllon, which means leaf.
Mikhail Semyonovich Tsvet, an Italian-Russian botanist, who invented chromatography in 1906, managed to separate leaf pigments, namely, chlorophylles from carotenes and xanthophylls, thanks to column chromatography, and called the yellow-to-orange pigments carotenoids.

Chemical structure

Carotenoids are a class of lipids having as a basic structure a linear sequence of 8 isoprene units. Therefore, they are terpenes, specifically tetraterpenes, and then consist of 40 carbon atoms.
The isoprene units are joined in a 1,5 positional relationship, that is, head to tail, except in the middle of the hydrocarbon chain, where the positional relationship is 1,6, that is, tail to tail. This reverses the order making the molecule symmetrical.
Carotenoids are characterized by the presence of an extended double-bond system, 3 to 13 of which can be conjugated.

The hydrocarbon chain of carotenoids can undergo hydrogenation, dehydrogenation, introduction of oxygen atoms, cyclization of one or both ends to form an ionone rings, and may be esterification with fatty acids. Such modifications allow the formation of many different structures, and most of them are described by the chemical formula C₄₀H₅₆O_n, with 0≤n≤6.
Traditionally, carotenoids have common names related to the biological source from which they are extracted, although they can be named using IUPAC nomenclature.

Isomerism

Carotenoids have many double bonds which can exhibit cis–trans isomerism, also known as geometric isomerism. Although in theory their double bonds can be in a cis– or trans-configuration, in nature the trans-configuration prevails, as it is thermodynamically more stable than cis-configuration, thanks to the lower steric hindrance between the substituents.
The rotation around the simple bonds, or sigma bonds, allows the formation of conformational isomers, as well. When the double bonds are on the same side with respect to the sigma bond, namely, cis in reference to the single bond, the conformation is called s-cis conformation; conversely, when the double bonds are on opposite sides with respect to the sigma bond, namely, trans in reference to the single bond the conformation is called s-trans conformation. In nature, acyclic carotenoids are mainly in the s-trans conformation, as it has the least steric hindrance. When the polyene chain ends with cyclic structures, the rotation around the C6-C7 sigma bond gives rise to the formation of a 6-s-cis isomer with less steric hindrance and therefore energetically favored.
It should be noted that the steric hindrance and the dimensions of the carotenoids influence the ability to interact with enzymes and to form supramolecular structures.
Most carotenoids also have a chirality center or a chirality axis, which causes optical isomers to exist. In these cases, to give a unique and unambiguous name to the to the molecule, the RS system is used instead of the Fischer-Rosanoff convention, which is used only for carbohydrates and amino acids.

Classification

Carotenoids can be classified on the basis of the presence or absence of oxygen or cyclic structures.
Based on the presence or absence of oxygen, they are divided into xanthophylls and carotenes.
Carotenes, which are oxygen-free molecules, are made up of only carbon and hydrogen atoms, and have chemical formula C₄₀H₅₆. Examples of carotenes are alpha-carotene, beta-carotene, delta-carotene, zeta-carotene, phytoene, and lycopene.
Xanthophylls also contain oxygen atoms, have chemical formula C₄₀H₅₆O_n, with 0˂n≤6, and can, in turn, be divided into:

• hydroxycarotenoids, which contain at least one hydroxyl group, such as alpha-cryptoxanthin, beta-cryptoxanthin, zeaxanthin and lutein;
• epoxycarotenoids, which have at least one epoxy group, such as antheraxanthin, auroxanthin and luteoxanthin;
• ketocarotenoids, which have one or more carbonyl groups, such as astaxanthin and canthaxanthin.

Depending on the presence or absence of rings in the molecule, carotenoids can be divided into cyclic and acyclic.

• Cyclic carotenoids contain one or two cyclic structures, and, compared to acyclic carotenoids, are shorter, but with a greater steric hindrance and a greater space occupied. Examples are alpha-carotene, beta-carotene, gamma-carotene and delta-carotene.
• Acyclic carotenoids are formed by a linear carbon chain. Examples are lycopene, zeta-carotene, phytoene, and phytofluene.

Finally, there are also uncommon or species-specific carotenoids, such as bixin, capsanthin and capsorubin.

Solubility

Carotenoids are hydrophobic molecules, therefore soluble in organic solvents, solubility which varies according to the substituents present. Given their hydrophobicity, they are found in cell membranes. In most cases, the xanthophylls have the polar groups at the ends of the polyene chain, and, when present in cell membranes, in order to minimize the energy of the system, they arrange themselves in such a way that the polar groups are in contact with the polar groups of the lipid bilayer.
Carotenoids can also have access to the aqueous environment if in association with proteins, to which they bind non-covalently. In plasma, they are transported by lipoproteins.

Color

The conjugated double bond system, acting as a chromophore, is responsible for the color of carotenoids. However, only carotenoids that have at least seven conjugated double bonds are colored; hence, phytofluene, which has five conjugated bonds, or phytoene, which has three conjugated double bonds, are colorless.
As the number of conjugated double bonds increases, the color changes from yellow to orange to red; for example, lutein, alpha-cryptoxanthin, and violaxanthin are yellow, alpha-carotene, beta-carotene, and gamma-carotene are orange, and lycopene is red.

Role in plants

Carotenoids play many roles in plants.
They contribute to cell protection against oxidative damage, increase the absorption of sunlight, are the precursors of phytohormones, such as abscisic acid, and are involved in signaling pathways. Furthermore, with chlorophylls and anthocyanins, a class of flavonoids, the major type of polyphenols, they contribute to the color of leaves, fruits, vegetables, and grains, color which, attracting animals, promotes pollination and seed dispersion. Finally, they can act as repellent agents for phytophages and pathogens, as well.

Quenching of singlet oxygen

In organisms that carry out oxygenic photosynthesis, carotenoids contribute to the protection from damage caused by excessive solar radiation. When the sunlight arriving at the photosystem II antenna complex exceeds the conversion capacity of the reaction centers, the excess energy can cause chlorophyll to remain in its excited state, which can lead to the formation of its triplet state. The excess energy can be transferred from chlorophyll to molecular oxygen causing the formation of singlet oxygen, a reactive oxygen species, capable of damaging the photosynthetic apparatus and, more generally, nucleic acids, proteins, membrane unsaturated fatty acids, as well as enzyme cofactors.
Carotenoids can prevent oxidative damage caused by singlet oxygen in two ways.
They can directly quench the singlet oxygen. In this case the energy is transferred to the carotenoid by the singlet oxygen, which decays to the triplet ground state, while the carotenoid reach the triplet state, then releasing the excess energy in the form of heat. The capacity of quench singlet oxygen increases with the number of conjugated double bonds, being maximal at nine or more conjugated double bonds. In vitro studies seem to indicate that monocyclic carotenoids are able to quench singlet oxygen more efficiently than acyclic structures, and that the presence of carbonyl groups, such as in astaxanthin, improves the antioxidant potential compared to those with hydroxyl groups, probably thanks to the extended system of conjugated double bonds. It seems that each carotenoid, before undergoing degradation reactions, is able to quench about 1000 singlet oxygen molecules.
Carotenoids are also able to prevent the formation of singlet oxygen by absorbing the energy of the excited chlorophyll, so turning into a triplet state. As the triplet state of carotenoids has an energy level lower than that of singlet oxygen, it spontaneously decays to the fundamental state.

Free radical scavenging activity

Carotenoids are able to scavenge free radicals, such as hydroxyl, peroxyl, and superoxide radicals, and nitric oxide radical. Their action can occur in four different ways.

• Carotenoid can donate an electron to the radical cation, producing the radical cation of the carotenoid, while the radical becomes a stable molecule. The carotenoid radical cation can be regenerated to parent carotenoid by other cellular antioxidants, such as vitamin E, vitamin C and glutatione.
• A proton can be transferred from the carotenoid to the radical species, which becomes a neutral molecule, while the carotenoid becomes a radical anion.
• The neutralization of the free radicals can also occur as a result of addition reactions, as in the case of the neutralization of hydroxyl and peroxyl radicals.
• An hydrogen atom can be transfer from carotenoid to the reactive oxygen species, which leads to a neutral-resonance-stabilized carotenoid radical.

Light-harvesting role

Carotenoids are present in the antenna complexes as accessory light-harvesting pigments, mainly associated with antenna proteins.
Thanks to the system of conjugated double bonds, they are able to absorb sunlight in the range of 400-500 nm. Then, the absorbed energy is conveyed to the reaction centers.
The absorption spectra of individual carotenoids depend on the length of the conjugation network and the type of functional groups present.
The main carotene involved in light-harvesting is beta-carotene, present in the core of photosystems I and II of all organisms that carry out an oxygenic photosynthesis, whereas lutein, violaxanthin, neoxanthin and zeaxanthin, the main xanthophylls in plants, are bound to the light-harvesting complexes.

Food sources

Humans are not able to synthesize carotenoids. They take them from food, and store them mainly in liver and adipose tissue, but also in lung, kidney, brain, and bone, where they play many functions.
Of all the carotenoids identified, about a hundred have been found in foods consumed by humans, while, if we consider the single food item, generally there are one to five main carotenoids, accompanied by others present in traces or in any case small quantities.
Fruits and vegetables are the major source of carotenoids in the human diet. Among vegetables, leafy greens are rich in beta-carotene, the most abundant carotenoid in human diet and tissues, and lutein, followed by violaxanthin and neoxanthin, carrot root is rich in beta-carotene, pumpkin in alpha-carotene, tomato in lycopene, and peppers in capsanthin and capsorubin. Carotenoids in fruits are very varied, and those in ripe fruit can be different from those in unripe fruit. Furthermore, most of the carotenoids present in ripe fruits are esterified with fatty acids. However, those of some fruits, in particular fruits that remain green when ripe, such as kiwis, undergo limited or absent esterification.
Chicken eggs, thanks to the fact that laying hens mostly eat corn, are a good source of some carotenoids, such as lutein and zeaxanthin, which are responsible for the yellow-orange color of egg yolk.
Finally, mollusks and crustaceans, which together with fish are the main sources of long-chain omega-3 polyunsaturated fatty acids, such as eicosapentaenoic acid and docosahexaenoic acid, are the main source of carotenoids produced by microalgae and rarely found in plants, such as astaxanthin in shrimp, but also in salmon.

Food additives

Carotenoids used in food industry as additives, unlike naturally occurring carotenoids, are relatively unstable, being susceptible to the action of oxygen and sunlight, auto-oxidation, and dispersion into food ingredients which facilitates their degradation. Furthermore, at body and food storage temperatures the added carotenoids, due to their high melting point, are in crystalline form; to overcome this problem, they are used in the form of an oil-in-water emulsion.
Thanks to their coloring power, they are used as colours in foods which, due to processing or storage, lose part of their natural colour, or to standardize the colour of the products, as in the case of beverages, fruit juices or sausages, or to intensify their colour, thus improving their appearance.
Carotenoids are also precursors of compounds responsible for the flavor and aroma of some foods, hence, they can be used as flavour enhancers.
Furthermore, some carotenoids are vitamin A precursors and are used to fortify foods.
Finally, it should be emphasized that, when added to food, carotenoids are not powerful antioxidants.

Carotenoids and human health

In humans, carotenoids play important roles.
The most important role is to be precursors of vitamin A, which controls the expression of nearly 700 genes. Not all carotenoids are vitamin A precursors; only those with an unsubstituted beta-ionone ring are vitamin A precursors. The most important is beta-carotene which, having two unsubstituted beta-ionone rings, is converted into two molecules of the vitamin. Alpha-carotene and beta-cryptoxanthin, having only an unsubstituted beta-ionone ring, provide one molecule of vitamin A, hence, they have 50 percent provitamin A activity. The importance of provitamin A carotenoids is underlined by the fact that vitamin A deficiency can lead to night blindness and xerophthalmia, delay the growth and regeneration of the mucous membranes, increases the mortality rate caused by infectious diseases due to immune-related disorders, and is the single most important cause of childhood blindness in developing countries.
Carotenoids contribute to human health also by preventing the development of numerous chronic diseases through different mechanisms than provitamin A activity, such as antioxidant and anti-inflammatory actions, and photoprotection. Here are some examples. It has been observed that supplementation with lutein and zeaxanthin, two xanthophylls which accumulate in the macula lutea, is associated with an improvement in visual function and a reduction in the risk of progression of age-related macular degeneration. Lutein and zeaxanthin also seem to have a preventive effect on the development of cataracts and retinopathies, and lutein also a positive effect on cognitive functions. Among carotenes, the consumption of lycopene–rich foods has been associated with a reduction in the incidence of certain types of cancer, such as prostate and stomach cancer, and a reduction in cardiovascular risk.

Plasma carotenoids

Alpha-carotene, beta-carotene, lycopene, lutein, beta-cryptoxanthin and zeaxanthin make up more than 90 percent of the carotenoids present in plasma.
Carotenoid plasma concentrations depend on several factors including methods of cooking foods, the amount of lipids in the diet, individual variables regarding the processes of lipid digestion and lipid absorption, in which bile salts play an essential role, and vitamin A levels in the body. Considering the latter factor, a subject with low vitamin A levels may have a high conversion rates of provitamin A carotenoids, and this could be reflected in a lower plasma concentration.

References

1. Amengual J. Bioactive properties of carotenoids in human health. Nutrients 2019;11(10):2388. doi:10.3390/nu11102388
2. Archetti, M., Döring T.F., Hagen S.B., Hughes N.M., Leather S.R., Lee D.W., Lev-Yadun S., Manetas Y., Ougham H.J. Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends Ecol Evol 2009;24(3):166-73. doi:10.1016/j.tree.2008.10.006
3. Böhm V., Lietz G., Olmedilla-Alonso B., Phelan D., Reboul E., Bánati D., Borel P., Corte-Real J., de Lera A.R., Desmarchelier C., Dulinska-Litewka J., Landrier J.F., Milisav I., Nolan J., Porrini M., Riso P., Roob J.M., Valanou E., Wawrzyniak A., Winklhofer-Roob B.M., Rühl R., Bohn T. From carotenoid intake to carotenoid blood and tissue concentrations – implications for dietary intake recommendations. Nutr Rev 2021;79(5):544-573. doi:10.1093/nutrit/nuaa008
4. Burrows T.L., Williams R., Rollo M., Wood L., Garg M.L., Jensen M., Clare Collins C.E. Plasma carotenoid levels as biomarkers of dietary carotenoid consumption: a systematic review of the validation studies. J Nutr Intermed Metab 2015;2(1-2):15-64. doi:10.1016/j.jnim.2015.05.001
5. de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, 2010
6. Desmarchelier C., Borel P. Overview of carotenoid bioavailability determinants: from dietary factors to host genetic variations. Trends Food Sci Technol 2017;69:270-280 doi:10.1016/j.tifs.2017.03.002
7. Eggersdorfer M., Wyss A. Carotenoids in human nutrition and health. Arch Biochem Biophys 2018;652:18-26. doi:10.1016/j.abb.2018.06.001
8. Gruszecki W.I., Strzałka K. Carotenoids as modulators of lipid membrane physical properties. Biochim Biophys Acta 2005;1740(2):108-15. doi:10.1016/j.bbadis.2004.11.015
9. Leyva-Porras C., Román-Aguirre M., Cruz-Alcantar, P., Pérez-Urizar, J.T., Saavedra-Leos M.Z. Application of antioxidants as an alternative improving of shelf life in foods. Polysaccharides 2021;2:594-607. doi:10.3390/polysaccharides2030036
10. Namitha K.K., Negi P.S. Chemistry and biotechnology of carotenoids. Crit Rev Food Sci 2010;50:728-760. doi:10.1080/10408398.2010.499811
11. Ramel F., Birtic S., Cuiné S., Triantaphylidès C., Ravanat J.L., Havaux M. Chemical quenching of singlet oxygen by carotenoids in plants. Plant Physiol 2012;158(3):1267-78. doi:10.1104/pp.111.182394
12. Rodriguez-Amaya D.B. Food carotenoids: chemistry, biology and technology. Wiley J. & Sons, 2015.
13. Saini R.K., Prasad P., Lokesh V., Shang X., Shin J., Keum Y.S., Lee J.H. Carotenoids: dietary sources, extraction, encapsulation, bioavailability, and health benefits – A review of recent advancements. Antioxidants 2022;11(4):795. doi:10.3390/antiox11040795
14. Sourkes T. L. The discovery and early history of carotene. Bull Hist Chem 2009;34(1)
15. Swapnil P., Meena M., Kumar Singh S., Praveen Dhuldhaj U., Harish, Marwal A. Vital roles of carotenoids in plants and humans to deteriorate stress with its structure, biosynthesis, metabolic engineering and functional aspects. Curr Plant Biol 2021;26: 100203. doi:10.1016/j.cpb.2021.100203
16. Xu P., Chukhutsina V.U, Nawrocki W.J., Schansker G, Bielczynski L.W., Lu Y., Karcher D., Bock R., Croce R. Photosynthesis without β-carotene. eLife 2020;9:e58984. doi:10.7554/eLife.58984

Carbohydrate loading is a good strategy to increase fuel stores in muscles before the start of the competition.

What does the muscle “eat” during endurance exercise?

Muscle “eats” carbohydrates, in the form of liver and muscle glycogen, carbohydrates ingested during the exercise or just before that, and fat.
During endurance exercise, the most likely contributors to fatigue are dehydration and carbohydrate depletion, especially of muscle and liver glycogen.
To prevent the “crisis” due to the depletion of muscle and liver carbohydrates, it is essential having high glycogen stores before the start of the activity.

What does affect glycogen stores?

• The diet in the days before the competition.
• The level of training (well-trained athletes synthesize more glycogen and have potentially higher stores, because they have more efficient enzymes).
• The activity in the day of the competition and the days before (if muscle doesn’t work it doesn’t lose glycogen). Therefore, it is better to do light trainings in the days before the competition, not to deplete glycogen stores, and to take care of nutrition.

The “Swedish origin” of carbohydrate loading

Very high muscle glycogen levels (the so-called glycogen supercompensation) can improve performance, i.e. time to complete a predetermined distance, by 2-3% in the events lasting more than 90 minutes, compared with low to normal glycogen, while benefits seem to be little or absent when the duration of the event is less than 90 min.
Well-trained athletes can achieve glycogen supercompensation without the depletion phase prior to carbohydrate loading, the old technique discovered by two Swedish researchers, Saltin and Hermansen, in 1960s.
The researchers discovered that muscle glycogen concentration could be doubled in the six days before the competition following this diet:

• three days of low carb menu (a nutritional plan very poor in carbohydrates, i.e. without pasta, rice, bread, potatoes, legumes, fruits etc.);
• three days of high carbohydrate diet, the so-called carbohydrate loading (a nutritional plan very rich in carbohydrates).

This diet causes a lot of problems: the first three days are very hard and there may be symptoms similar to depression due to low glucose delivery to brain, and the benefits are few.
Moreover, with the current training techniques, the type and amount of work done, we can indeed obtain high levels of glycogen: above 2.5 g/kg of body weight.

The “corrent” carbohydrate loading

If we compete on Sunday, a possible training/nutritional plan to obtain supercompensation of glycogen stores can be the following:

• Wednesday, namely four days before the competition, moderate training and then dinner without carbohydrates;
• from Thursday on, namely the three days before the competition, hyperglucidic diet and light trainings.

The amount of dietary carbohydrates needed to recover glycogen stores or to promote glycogen loading depends on the duration and intensity of the training programme, and they span from 5 to 12 g/kg of body weight/d, depending on the athlete and his activity. With higher carbohydrate intake you can achieve higher glycogen stores but this does not always results in better performance; moreover, it should be noted that glycogen storage is associated with weight gain due to water retention (approximately 3 g per gram of glycogen), and this may not be desirable in some sports.

References

1. Burke L.M., Hawley J.A., Wong S.H.S., & Jeukendrup A. Carbohydrates for training and competition. J Sport Sci 2011;29:Sup1,S17-S27. doi:10.1080/02640414.2011.585473
2. Hargreaves M., Hawley J.A., & Jeukendrup A.E. Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance. J Sport Sci 2004;22:31-38. doi10.1080/0264041031000140536
3. Jeukendrup A.E., C. Killer S.C. The myths surrounding pre-exercise carbohydrate feeding. Ann Nutr Metab 2010;57(suppl 2):18-25. doi:10.1159/000322698
4. Moseley L., Lancaster G.I, Jeukendrup A.E. Effects of timing of pre-exercise ingestion of carbohydrate on subsequent metabolism and cycling performance. Eur J Appl Physiol 2003;88:453-458. doi:10.1007/s00421-002-0728-8

“The acid-ash hypothesis posits that protein and grain foods, with a low potassium intake, produce a diet acid load, net acid excretion, increased urine calcium, and release of calcium from the skeleton, leading to osteoporosis.” (Fenton et al., 2009, see References).
Is it true?
Calcium, present in bones in form of carbonates and phosphates, represents a large reservoir of base in the body. In response to an acid load such as the high protein diets these salts are released into the circulation to bring about pH homeostasis. This calcium is lost in the urine and it has been estimated that the quantity lost with the such diet over time could be as high as almost 480 g over 20 years or almost half the skeletal mass of calcium!
Even these losses of calcium may be buffered by ingestion of foods that are alkali rich as fruit and vegetables, and on-line information promotes an alkaline diet for bone health as well as a number of books, a recent meta-analysis has shown that the causal association between osteoporotic bone disease and dietary acid load is not supported by evidence and there is no evidence that the alkaline diet is protective of bone health, but it is protective against the risk for kidney stones.

Note: it is possible that fruit and vegetables are beneficial to bone health through mechanisms other than via the acid-ash hypothesis.

What is the role of proteins?

Excess dietary proteins with high acid renal load may decrease bone density, if not buffered by ingestion of foods that are alkali rich, that is fruit and vegetables. However, an adequate protein intake is needed for the maintenance of bone integrity. Therefore, increasing the amount of fruit and vegetables may be necessary rather than reducing protein too much.
Therefore it is advisable to consume a normo-proteic diet rich in fruits and vegetables and poor in sodium, such as medieterranean diet, eating foods with a negative acid load together with foods with a positive acid load. Example: pasta plus vegetables or meats plus vegetables and fruits .

Alkaline diet and muscle mass

As we age, there is a loss of muscle mass, which predispose to falls and fractures. A diet rich in potassium, obtained from fruits and vegetables, as well as a reduced acid load, results in preservation of muscle mass in older men and women.

Alkaline diet and growth hormone

In children, severe forms of metabolic acidosis are associated with low levels of growth hormone with resultant short stature; its correction with potassium or bicarbonate citrate increases growth hormone significantly and improves growth. In postmenopausal women, the use of enough potassium bicarbonate in the diet to neutralize the daily net acid load resulted in a significant increase in growth hormone and resultant osteocalcin.
Improving growth hormone levels may reduce cardiovascular risk factors, improve quality of life, body composition, and even memory and cognition.

Conclusion

Alkaline diet may result in a number of health benefits.

• Increased fruits and vegetables would improve the potassium/sodium ratio and may benefit bone health, reduce muscle wasting, as well as mitigate other chronic diseases such as hypertension and strokes.
• The increase in growth hormone may improve many outcomes from cardiovascular health to memory and cognition.
• The increase in intracellular magnesium is another added benefit of the alkaline diet; for example, magnesium, required to activate vitamin D, would result in numerous added benefits in the vitamin D systems.

It should be noted that one of the first considerations in an alkaline diet, which includes more fruits and vegetables, is to know what type of soil they were grown in since this may significantly influence the mineral content and therefore their buffering capacity.

References

1. Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res 2009;24(11):1835-1840. doi:10.1359/jbmr.090515
2. Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash diet hypothesis. Nutr J 2009;8:41. doi:10.1186/1475-2891-8-41
3. Fenton T.R., Tough S.C., Lyon A.W., Eliasziw M., Hanley D.A. “Causal assessment of dietary acid load and bone disease: a systematic review and meta-analysis applying Hill’s epidemiologic criteria for causality.” Nutr J 2011;10:41. doi:10.1186/1475-2891-10-41
4. Schwalfenberg G.K. The alkaline diet: is there evidence that an alkaline pH diet benefits health? J Environ Public Health 2012; Article ID 727630. doi:10.1155/2012/727630

Life depends on appropriate pH levels around and in living organisms and cells.
We requires a tightly controlled pH level in our serum of about 7.4 to avoid metabolic acidosis and survive. As a comparison, in the past 100 years the pH of the ocean has dropped from 8.2 to 8.1 because of increasing carbon dioxide deposition with a negative impact on life in the ocean.

Even the mineral content of the food we eat is considerable influenced by the pH of the soil in which plants are grown. The ideal pH of soil for the best overall availability of essential nutrients is between 6 and 7: an acidic soil below pH of 6 may have reduced magnesium and calcium, and soil above pH 7 may result in chemically unavailable zinc, iron, copper and manganese.

Metabolic acidosis and agricultural and industrial revolutions

In the human diet, there has been considerable change from the hunter gather civilization to the present in the pH and net acid load. With the agricultural revolution, in the last 10,000 years, and even more recently with industrialization, in the last 200 years, it has been seen:

• an increase in sodium compared to potassium, as the ratio potassium/sodium has reversed from 10 to 1 to a ratio of 1 to 3 in the modern diet, and in chloride compared to bicarbonate;
• a poor intake of magnesium and fiber;
• a large intake of simple carbohydrates and saturated fatty acids.

This results in a diet that may induce metabolic acidosis which is mismatched to the genetically determined nutritional requirements.
Moreover, with aging, there is a gradual loss of renal acid-base regulatory function and a resultant increase in diet-induced metabolic acidosis.
Finally, a high protein low-carbohydrate diet with its increased acid load results in very little change in blood chemistry, and pH, but results in many changes in urinary chemistry: urinary calcium, undissociated uric acid, and phosphate are increased, while urinary magnesium, urinary citrate and pH are decreased.
All this increases the risk for kidney stones.

pH as a protective barrier

The human body has an amazing ability to maintain a steady pH in the blood with the main compensatory mechanisms being renal and respiratory.
The pH in the body vary considerably from one area to another. The highest acidity is found in the stomach (pH of 1.35 to 3.5) and it aids in digestion and protects against opportunistic microbial organisms. The skin is quite acidic (pH 4-6.5) and this provides an acid mantle as a protective barrier to the environment against microbial overgrowth (this is also seen in the vagina where a pH of less than 4.7 protects against microbial overgrowth).
The urine have a variable pH from acid to alkaline depending on the need for balancing the internal environment.

Organ, fluid or membrane	pH	Function of pH
Skin natural	Natural pH is between 4 and 6.5	Barrier protection from microbes
Urine	4.6 to 8.0	Limit overgrowth of microbes
Gastric	1.35 to 3.5	Breakdown proteins
Bile	7.6 to 8.8	Neutralize stomach acid, aid in digestion
Pancreatic fluid	8.8	Neutralize stomach acid, aid in digestion
Vaginal fluid	<4.7	Limit overgrowth of opportunistic microbes
Cerebrospinal fluid	7.3	Bathes the exterior of the brain
Intracellular fluid	6.0 -7.2	Due to acid production in cells
Serum venous	7.35	Tightly regulated
Serum arterial	7.4	Tightly regulated
Modified from: Schwalfenberg G.K.; see in References

References

1. Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res 2009;24(11):1835-1840. doi:10.1359/jbmr.090515
2. Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash diet hypothesis. Nutr J 2009;8:41. doi:10.1186/1475-2891-8-41
3. Fenton T.R., Tough S.C., Lyon A.W., Eliasziw M., Hanley D.A. Causal assessment of dietary acid load and bone disease: a systematic review and meta-analysis applying Hill’s epidemiologic criteria for causality. Nutr J 2011;10:41. doi:10.1186/1475-2891-10-41
4. Schwalfenberg G.K. The alkaline diet: is there evidence that an alkaline pH diet benefits health? J Environ Public Health 2012; Article ID 727630. doi:10.1155/2012/727630

The international scientific literature is unanimous in setting the lower limit for the daily caloric intake to 1200 kcal for women and 1500 kcal for men (adults).
To make negative the daily caloric intake, and therefore lose body weight, but expecially lose body fat, evaluation of actual caloric needs of the subject will be alongside:

• the correct distribution of meals during the day;
• an increased physical activity, by which the negative balance can be achieved without major sacrifices during meals.

This will make weight loss easier and protect from subsequent weight gains and yo-yo effect.
Ultimately, there must be a change in lifestyle.

Contents

• Body fat and miracle diets
• Body fat and reduction of energy intake
  • Reduction in energy intake and role of carbohydrates
  • What goes in when carbohydrates goes out?
  • Body fat and the entry in a phase of famine/disease
• Yo-yo effect
• References

Body fat and miracle diets

So, the best strategy for losing body fat is not a drastic reduction in caloric intake, nor follow constrictive or “strange” diets, such as hcg diet plan, sacred heart diet, paleo diet, etc., that require to eliminate or greatly reduce the intake of certain macronutrients, mostly carbohydrates.
Such conducts can be:

• very stressful from psychological point of view;
• not passable for long periods;
• hazardous to health because of inevitable nutrient deficiencies.

Finally, they do not ensure that all the weight lost is only or almost only body fat and are often followed by substantial increases in body weight and/or by yo-yo effect.
Why?

Body fat and reduction of energy intake

An excessive reduction of energy intake means eating very little and this determines the risk, high, not to take adequate amounts of the various essential nutrients, that is, what we can’t synthesize, such as vitamins, certain amino acids, essential fatty acids, a class of lipids, and minerals, including e.g. calcium, essential for bone metabolism at every stage of life, or iron, used in many body functions as the transport of oxygen to the tissues. This results in a depression of metabolism and hence a reduction in energy expenditure.
Whether the reduction in energy intake is excessive, or even there are periods of fasting, it adds insult to injury because a proportion of free fatty mass will be lost. How?

Reduction in energy intake and role of carbohydrates

Glucose is the only energy source for red blood cells and some brain areas, while other brain areas can also derive energy from ketone bodies, which are a product of the metabolism of fatty acids.
At rest, brain extracts 10 percent of the glucose from the bloodstream, a significant amount, about 75 mg/min., considering that its weight is about 1.5 kg. To maintain a constant glycemia, and thus ensure a constant supply of glucose to tissues, we needs to take carbohydrates or alternatively amino acids, both easily obtained from foods.
In the case of a low or absent dietary intake of carbohydrates, whereas after about 18 hours liver glycogen, which releases glucose into circulation, depletes, body synthesizes de novo glucose from certain amino acids through a process called gluconeogenesis (actually this metabolic pathway is active even after a normal meal but increases its importance in fasting).
But what’s the main source of amino acids in the body when their dietary intake is low or absent? Endogenous proteins, and there is a hierarchy in their use that is before we consume the less important and only after the most important ones. For the first digestive enzymes, pepsin, chymotrypsin, elastase, carboxypeptidase and aminopeptidase, around 35-40 g, will be used; successively liver and pancreas slow down their synthesis activities for export proteins and unused amino acids are directed to gluconeogenesis. It’s clear that these are quite modest reserves of amino acids and it is the muscle that will undertake to provide the required amounts of amino acids that is proteolysis of muscle proteins begins.
Note: Anyway, there is no absolute sequentiality in the degradation of several proteins, there is instead a plot in which, proceeding, some ways lose their importance and others will buy. So, to maintain constant glycemia the protein component of muscle is reduced, including skeletal muscle that is a tissue that represents a fairly good portion of the value of the basal metabolism and that, with exercise, can significantly increase its energy consumption: thus essential for weight loss and subsequent maintenance. It is as if the engine capacity was reduced.
One thing which we don’t think about is that heart is a muscle that may be subject to the same processes seen for skeletal muscle.
Ultimately make glucose from proteins, also food-borne, is like heat up the fire-place burning the furniture of the eighteenth century, amino acids, having available firewood, dietary carbohydrates.
Therefore, an adequate intake of carbohydrates with diet prevents excessive loss of proteins, namely, there is a saving effect of proteins played by carbohydrates.
Mammals, and therefore humans, can’t synthesize glucose from fats.

What goes in when carbohydrates goes out?

The elimination or substantial reduction in carbohydrate intake in the diet results in an increased intake of proteins, lipids, including cholesterol, because it will increase the intake of animal products, one of the main defects in hyperproteic diet.
In the body there are no amino acids reserves, thus they are metabolized and, as a byproduct of their use, ammonia is formed and it’ll be eliminated as toxic. For this reason high-protein diets imply an extra work for liver and kidneys and also for this they are not without potential health risks.
An increased fat intake often results into an increased intake of saturated fatty acids, trans fatty acids, and cholesterol, with all the consequences this may have on cardiovascular health.
What has been said so far should not induce to take large amounts of carbohydrates; this class of macronutrients should represent 55-60% of daily calories, fats 25-30% (primarily olive oil) and the remainder proteins: thus a composition in macronutrient that refers to prudent diet or mediterranean diet.

Body fat and the entry in a phase of famine/disease

A excessive reduction in caloric intake is registered by our defense mechanisms as an “entry” in a phase of famine/disease.
The abundance of food is a feature of our time, at least in industrialized countries, while our body evolved over hundreds of thousands of years during which there was no current abundance: so it’s been programmed to try to overcome with minimal damage periods of famine. If caloric intake is drastically reduced it mimics a famine: what body does is to lower consumption, lower the basal metabolism, that is, consumes less and therefore also not eating much we will not get great results. It is as if a machine is lowered the displacement, it’ll consume less (our body burns less body fat).

Yo-yo effect

Yo-yo effect or weight cycling, namely, repeated phases of loss and weight gain, appears related to excess weight and accumulation of fat in the abdomen.
Several studies suggest a link in women with:

• increased blood pressure;
• hypercholesterolemia;
• gallbladder disease;
• significant increase in binge eating disorder;
• a sense of depression with regard to weight.

Lastly, yo-yo effect is related to a greater easy to gain weight than those who are not subject to it. In this regard there should be emphasized that the weight cycling occurs over years, during which, aging, the rate of metabolism inevitably tends to decrease: this could make more difficult the subsequent losses.

References

1. Cereda E., Malavazos A.E., Caccialanza R., Rondanelli M., Fatati G. and Barichella M. Weight cycling is associated with body weight excess and abdominal fat accumulation: a cross-sectional study. Clin Nutr 2011;30(6):718-723. doi:10.1016/j.clnu.2011.06.009
2. Montani J-P., Viecelli A.K., Prévot A. & Dulloo A.G. Weight cycling during growth and beyond as a risk factor for later cardiovascular diseases: the ‘repeated overshoot’ theory. Int J Obes (Lond) 2006;30:S58-S66. doi:10.1038/sj.ijo.0803520
3. Ravussin E., Lillioja S., Knowler W.C., Christin L., Freymond D., Abbott W.G.H., Boyce V., Howard B.V., and Bogardus C. Reduced rate of energy expenditure as a risk factor for body-weight gain. N Engl J Med 1988;318:467-472. doi:10.1056/NEJM198802253180802
4. Sachiko T. St. Jeor S.T. St., Howard B.V., Prewitt T.E., Bovee V., Bazzarre T., Eckel T.H., for the AHA Nutrition Committee. Dietary Protein and Weight Reduction. A Statement for Healthcare Professionals From the Nutrition Committee of the Council on Nutrition, Physical Activity, and Metabolism of the American Heart Association. Circulation 2001;104:1869-1874. doi:10.1161/hc4001.096152

Muscle glycogen is an important energy source for prolonged moderate to high intensity exercise, an importance that increases during high-intensity interval exercise, common in training session of swimmers, runners, rowers or in team-sport players, or during resistance exercise. For example, considering marathon, about 80% of energy needed comes from carbohydrate oxidation, for the most part skeletal muscle glycogen.
Fatigue and low muscle glycogen levels are closely correlated, but the underlying molecular mechanisms remain elusive. One hypothesis is that there is a minimum glycogen concentration that is “protected” and is not used during exercise, perhaps to ensure an energy reserve in case of need. Due to the closely relationship between skeletal muscle glycogen depletion and fatigue, its re-synthesis rate during post-exercise is one of the most important factors in determining necessary recovery time.
Finally, the highly trained athlete has muscle glycogen stores potentially higher and is also able to synthesize it faster due to more efficient enzymes.

To synthesize glycogen it is necessary to ingest carbohydrates; but how many, which, when, and how often?

Contents

• The two phases of muscle glycogen synthesis after exercise
  • The first phase
  • The second phase
  • Muscle glycogen synthesis rate and ingestion of carbohydrates and proteins
• The two phases of muscle glycogen synthesis: molecular mechanisms
  • Glycogen synthesis rate and insulin
  • Insulin and preferential carbohydrate storage
• References

The two phases of muscle glycogen synthesis after exercise

In order to restore as quickly as possible muscle glycogen depots, it is useful to know that, as a result of training sessions that deplete muscle glycogen to values below 75 percent those at rest and not fasting, glycogen synthesis occurs in two phases.
To know and therefore take advantage of the biphasicity is important for those athletes who are engaged in more daily training sessions, or who otherwise have little time for recovery between a high intensity exercise and the subsequent one, less than 8 hours, in order to maximize glycogen synthesis and achieve the optimal performance during a second close exercise session.
The two phases are characterized by:

• a different sensitivity to circulating insulin levels;
• a different velocity.

The first phase

Theis phase, immediately following the end of an activity and lasting 30-60 minutes, is insulin-independent, i.e. glucose uptake by muscle cell as glycogen synthesis are independent from hormone action.
This phase is characterized by an elevated rate of synthesis that however decreases rapidly if you do not take in carbohydrates: the maximum rate is in the first 30 minutes, then declines to about one fifth in 60 minutes, and to about one ninth in 120 minutes from the end of exercise.
How is it possible to take advantage of this first phase to replenish muscle glycogen stores as much as possible? By making sure that the greatest possible amount of glucose arrives to muscle in the phase immediately following to the end of exercise, best if done within the first 30 minutes.

• What to ingest?
  High glycemic index, but easy to digest and absorb, carbohydrates.
  Therefore, it is advisable to replace foods, even though of high glycemic index, that need some time for digestion and the subsequent absorption, with solutions/gel containing for example glucose and/or sucrose. These solutions ensure the maximal possible absorption rate and resupply of glucose to muscle because of they contain only glucose and are without fiber or anything else that could slow carbohydrate digestion and the following absorption of monosaccharides, that is, they are capable of producing high blood glucose levels in a relatively short time.
  It is also possible to play on temperature and concentration of the solution to accelerate the gastric transit.
  It should be further underlined that the use of these carbohydrate solutions is recommended only when the recovery time from a training/competition session causing significant depletion of muscle glycogen and the following one is short, less than 8 hours.
• How many carbohydrates do you need?
  Many studies has been conducted to find the ideal amount of carbohydrates to ingest.
  If in post-exercise the athlete does not eat, glycogen synthesis rate is very low, while if he ingests adequate amounts of carbohydrates immediately after cessation of exercise, synthesis rate can reach a value over 20 times higher.
  From the analysis of scientific literature it seems reasonable to state that, as a result of training sessions that deplete muscle glycogen stores as seen above (<75 percent of those at rest and not fasting), the maximum synthesis rate is obtained by carbohydrate intake, with high glycemic index and high digestion and absorption rates, equal to about 1.2 g/kg of body weight/h for the next 4-5 hours from the end of exercise.
  In this way, the amount of glycogen produced is higher than 150 percent compared to the ingestion of 0.8 g/kg/h.
  Because further increases, up to 1.6 g/kg/h, do not lead to further rise in glycogen synthesis rate, the carbohydrate amount equal to 1.2 g/kg/h can be considered optimum to maximize the resynthesis rate of muscle glycogen stores during post-exercise.
• And the frequency of carbohydrate ingestion?
  It was observed that if carbohydrates are ingested frequently, every 15-30 minutes, it seems there is a further stimulation of muscle glucose uptake as of muscle glycogen replenishment compared with ingestion at 2-hours intervals. Particularly, ingestions in the first post-exercise hours seem to optimize glycogen levels.

The second phase

The second phase begins from the end of the first, lasts until the start of the last meal before the next exercise, hence, from several hours to days, and is insulin-dependent i.e. muscle glucose uptake and glycogen synthesis are sensitive to circulating hormone levels.
Moreover, you observe a significant reduction in muscle glycogen synthesis rate: with adequate carbohydrate intake the synthesis rate is at a value of about 10-30 percent lower than that observed during the first phase.
This phase can last for several hours, but tends to be shorter if:

• carbohydrate intake is high;
• glycogen synthesis is more active;
• muscle glycogen levels are increased.

In order to optimize the resynthesis rate of glycogen, experimental data indicate that meals with high glycemic index carbohydrates are more effective than those with low glycemic index carbohydrates; but if between a training/competition session and the subsequent one days and not hours spend, the evidences do not favor high glycemic index carbohydrates as compared to low glycemic index ones as long as an adequate amount is taken in.

Muscle glycogen synthesis rate and ingestion of carbohydrates and proteins

The combined ingestion of carbohydrates and proteins, or free insulinotropic amino acids, allows to obtain post-exercise glycogen synthesis rate that does not significantly differ from that obtained with larger amounts of carbohydrates alone. This could be an advantage for the athlete who may ingest smaller amount of carbohydrates, therefore reducing possible gastrointestinal complications commons during training/competition afterward to their great consumption.
From the analysis of scientific literature it seems reasonable to affirm that, after an exercise that depletes at least 75 percent of muscle glycogen stores, you can obtain a glycogen synthesis rate similar to that reached with 1.2 g/kg/h of carbohydrates alone (the maximum obtainable) with the coingestion of 0.8 g/kg/h of carbohydrates and 0.4 g/kg /h of proteins, maintaining the same frequency of ingestion, therefore every 15-30 minutes during the first 4-5 hours of post-exercise.

The two phases of muscle glycogen synthesis: molecular mechanisms

The biphasicity is consequence, in both phases, of an increase in:

• glucose transport rate into cell;
• the activity of glycogen synthase, the enzyme that catalyzes glycogen synthesis.

However, the molecular mechanisms underlying these changes are different.
In the first phase, the increase in glucose transport rate, independent from insulin presence, is mediated by the translocation, induced by the contraction, of glucose transporters, called GLUT4, on the cytoplasmatic membrane of the muscle cell.
In addition, the low glycogen levels also stimulate glucose transport as it is believed that a large portion of transporter-containing vesicles are bound to glycogen, and therefore they may become available when its levels are depleted.
Finally, the low muscle glycogen levels stimulate glycogen synthase activity too: it has been demonstrated that these levels are a regulator of enzyme activity far more potent than insulin.
In the second phase, the increase in muscle glycogen synthesis is due to insulin action on glucose transporters and on glycogen synthase activity of muscle cell. This sensibility to the action of circulating insulin, that can persist up to 48 hours, depending on carbohydrate intake and the amount of resynthesized muscle glycogen, has attracted much attention: it is in fact possible, through appropriate nutritional intervention, to increase the secretion in order to improve glycogen synthesis itself, but also protein anabolism, reducing at the same time the protein-breakdown rate.

Glycogen synthesis rate and insulin

The coingestion of carbohydrates and proteins (or free amino acids) increases postprandial insulin secretion compared to carbohydrates alone (in some studies there was an increase in hormone secretion 2-3 times higher compared to carbohydrates alone).
It was speculated that, thanks to the higher circulating insulin concentrations, further increases in post-exercise glycogen synthesis rate could be obtained compared to those observed with carbohydrates alone, but in reality it does not seem so. In fact, if carbohydrate intake is increased to 1.2 g/kg/h plus 0.4 g/kg/h of proteins no further increases in glycogen synthesis rate are observed if compared to those obtained with the ingestion of carbohydrates alone in the same amount, 1,2 g/kg/h, that, as mentioned above, like the coingestion of 0,8 g/kg/h of carbohydrates and 0,4 g/kg/h of proteins, allows to attain the maximum achievable rate in post-exercise, or in isoenergetic quantities, that is, 1.6 g/kg.

Insulin and preferential carbohydrate storage

The greater circulating insulin levels reached with the coingestion of carbohydrates and proteins, or free amino acids, might stimulate the accumulation of ingested carbohydrates in tissues most sensitive to its action, such as liver and previously worked muscle, thus resulting in a more efficient storage, for the purposes of sport activity, of the same carbohydrates.

References

1. Beelen M., Burke L.M., Gibala M.J., van Loon J.C. Nutritional strategies to promote postexercise recovery. Int J Sport Nutr Exerc Metab 2010:20(6);515-532. doi:10.1123/ijsnem.20.6.515
2. Berardi J.M., Noreen E.E., Lemon P.W.R. Recovery from a cycling time trial is enhanced with carbohydrate-protein supplementation vs. isoenergetic carbohydrate supplementation. J Intern Soc Sports Nutrition 2008;5:24. doi:10.1186/1550-2783-5-24
3. Betts J., Williams C., Duffy K., Gunner F. The influence of carbohydrate and protein ingestion during recovery from prolonged exercise on subsequent endurance performance. J Sports Sciences 2007;25(13):1449-1460. doi:10.1080/02640410701213459
4. Howarth K.R., Moreau N.A., Phillips S.M., and Gibala M.J. Coingestion of protein with carbohydrate during recovery from endurance exercise stimulates skeletal muscle protein synthesis in humans. J Appl Physiol 2009:106;1394–1402. doi:10.1152/japplphysiol.90333.2008
5. Jentjens R., Jeukendrup A. E. Determinants of post-exercise glycogen synthesis during short-term recovery. Sports Medicine 2003:33(2);117-144. doi:10.2165/00007256-200333020-00004
6. Millard-Stafford M., Childers W.L., Conger S.A., Kampfer A.J., Rahnert J.A. Recovery nutrition: timing and composition after endurance exercise. Curr Sports Med Rep 2008;7(4):193-201. doi:10.1249/JSR.0b013e31817fc0fd
7. Price T.B., Rothman D.L., Taylor R., Avison M.J., Shulman G.I., Shulman R.G. Human muscle glycogen resynthesis after exercise: insulin-dependent and –independent phases. J App Physiol 1994:76(1);104–111. doi:10.1152/jappl.1994.76.1.104
8. Schweitzer G.G., Kearney M.L., Mittendorfer B. Muscle glycogen: where did you come from, where did you go? J Physiol 2017;595(9):2771-2772. doi:10.1113/JP273536
9. van Loon L.J.C., Saris W.H.M., Kruijshoop M., Wagenmakers A.J.M. Maximizing postexercise muscle glycogen synthesis: carbohydrate supplementation and the application of amino acid or protein hydrolysate mixtures. Am J Clin Nutr 2000;72: 106-111. doi:10.1093/ajcn/72.1.106

Omega-6 polyunsaturated fatty acids are the major polyunsaturated fatty acids or PUFA in the Western diet (about 90% of all of them in the diet), being components of most animal and vegetable fats.

Contents

• Synthesis
  • Omega-6 PUFA: from linoleic acid to arachidonic acid
• References

Synthesis

Within the omega-6 family, linoleic acid is one of the most important and widespread fatty acids and the precursor of all omega-6 polyunsaturated fatty acids. It is produced de novo from oleic acid, an omega-9 fatty acid, only by plant in a reaction catalyzed by Δ12-desaturase, i.e. the enzyme that forms the omega-6 polyunsaturated fatty acid family from omega-9 one.
Δ12-desaturase catalyzes the insertion of the double bond between carbon atoms 6 and 7, numbered from the methyl end of the molecule.
Linoleic acid, together with alpha-linolenic acid, is a primary product of plant polyunsaturated fatty acids synthesis.

Animals, lacking Δ12-desaturase, can’t synthesize it, and all the omega-6 polyunsaturated fatty acid family de novo, and they are obliged to obtain it from plant foodstuff and/or from animals that eat them; for this reason omega-6 polyunsaturated fatty acid are considered essential fatty acids, so called EFA. The essentiality of omega-6 polyunsaturated fatty acids, in particular just the essentiality of linoleic acid, was first reported in 1929 by Burr and Burr.

Omega-6 PUFA: from linoleic acid to arachidonic acid

Animals are able to elongate and desaturase dietary linoleic acid in a cascade of reactions to form very omega-6 polyunsaturated fatty acids.
Linoleic acid is first desaturated to gamma-linolenic acid, another important omega-6 fatty acid with significant physiologic effects, in the reaction catalyzed by Δ6-desaturase. It is thought that the rate of this reaction is limiting in certain conditions like in the elderly, under certain disease states and in premature infants; for this reason, and because it is found in relatively small amounts in the diet, few oils containing it, such as black currant, evening primrose, and borage oils, have attracted attention.
In turn gamma-linolenic acid may be elongated to dihomo-gamma-linolenic acid by an elongase. The enzyme catalyzes the addition of two carbon atoms from glucose metabolism to lengthen the fatty acid chain that may be further desaturated in a very limited amount to arachidonic acid, in a reaction catalyzed by another rate limiting enzyme, Δ5-desaturase.
Arachidonic acid can be elongated and desaturated to adrenic acid.
It should be noted that polyunsaturated fatty acids in the omega-6 family, and in any other n-families, can be interconverted by enzymatic processes only within the same family, not among families.
C-20 polyunsaturated fatty acids belonging to omega-6 and omega-3 polyunsaturated fatty acids are the precursors of eicosanoids (prostaglandins, prostacyclin, thromboxanes, and leukotrienes), powerful, short-acting, local hormones.
While the deprivation of omega-3 polyunsaturated fatty acids causes dysfunction in a wide range of behavioral and physiological modalities, the omission in the diet of omega-6 polyunsaturated fatty acids results in manifest systemic dysfunction.
In plant seed oils omega-6 fatty acids with chain length longer than 18 carbons are present only in trace while arachidonic acid is found in all animal tissues and animal-based food products.

References

1. Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008
2. Aron H. Uber den Nahvert (On the nutritional value). Biochem Z. 1918;92:211–233 (German)
3. Bender D.A. “Benders’ dictionary of nutrition and food technology”. 2006, 8th Edition. Woodhead Publishing. Oxford
4. Bergstroem S., Danielsson H., Klenberg D. and Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-PC4008.
5. Burr G.O. and Burr M.M. A new deficiency disease produced by the rigid exclusion of fat from the diet. Nutr Rev 1973;31(8):148-149. doi:10.1111/j.1753-4887.1973.tb06008.x
6. Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008
7. Rosenthal M.D., Glew R.H. Mediacal biochemistry. Human metabolism in health and disease. John Wiley & Sons, Inc. 2009
8. Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012
9. Van D., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-841

Omega-3 polyunsaturated fatty acids or omega-3 PUFAs or omega-3 fatty acids are unsaturated fatty acids, hence lipids, that have a double bond three carbons from the methyl end of the carbon chain.^[3] For humans, the most important omega-3 PUFAs are:

• Alpha-linolenic acid (ALA) or 18:3n-3, with 18 carbon atoms and 3 double bonds;
• eicosapentaenoic acid (EPA) or 20:5n-3, with 20 carbon atoms and 5 double bonds;
• docosahexaenoic acid (DHA) or 22:6n-3, that, with 22 carbon atoms and 6 double bonds, is the most complex.

EPA and DHA are termed long-chain polyunsaturated fatty acids (LC-PUFAs).
Mammals cannot synthesize linoleic acid or LA and alpha-linolenic acid, the precursors to omega-6 polyunsaturated fatty acids and omega-3 PUFAs, respectively, due to the lack of two desaturases: delta-12 desaturase (EC 1.14.19.6) and delta-15 desaturase or omega-3 desaturase or fatty acid desaturase 3 (EC 1-14.19.13).^[9] Such desaturases insert double bonds at positions 6 and 3 from the methyl end of the molecule, respectively. Linoleic acid and alpha-linolenic acid are therefore essential fatty acids.^[34] Humans and many other animals can produce, from dietary ALA, all the other omega-3 polyunsaturated fatty acids. Then, such omega-3 PUFAs become essential in the absence of dietary ALA, and for this reason they are termed conditionally essential fatty acids.^[9]
EPA and DHA are important structural components of cell membranes, where they are mainly found, especially in muscle and nerve tissues.^[35] Conversely, many other fatty acids are stored mainly in adipose tissue triglycerides.
DHA is the main component of cell membrane phospholipids of neural tissues of vertebrates, including photoreceptor of the retina, where it performs important functions.^{[22][26][29][32]} In addition to their structural functions, omega-3 PUFAs are substrates for the production of bioactive lipid mediators with anti-inflammatory action, such as eicosanoids, maresins, resolvins, and protectins.^[10][21]
Omega-3 polyunsaturated fatty acids are essential in neurological development of the fetus, and their intake during pregnancy is especially important in the third trimester of pregnancy, when significant brain growth occurs.^[8] In the course of life their intake has been associated with a reduction in the risk of developing many chronic diseases, particularly cardiovascular diseases.^[1][27][33]
The major dietary sources for humans are fishery products, especially those obtained from cold waters.^[3][9][36]

Contents

• Synthesis
  • Omega-3 synthesis in humans
• Do other animals need EPA and DHA?
• Benefits for humans
• Omega-3/Omega-6 ratio
• Effects at the molecular level of EPA and DHA
• Major sources of EPA and DHA for humans
• Omega-3 and culinary treatments
• References

Synthesis

Alpha-linolenic acid, the precursor for the synthesis of omega-3 polyunsaturated fatty acids, is produced from linoleic acid, an omega-6 PUFAs, in the plastids of phytoplankton and vascular terrestrial plants, where delta-15 desaturase inserts a double bond between carbon 3 and 4 from the methyl end of LA.^[3][9] In turn, ALA undergoes desaturation reactions, catalyzed by delta-5 desaturase (EC 1.14.19.44) and delta-6 desaturase (EC 1.14.19.3), elongation reactions, catalyzed by elongase 5 (EC 2.3.1.199) and elongase 5 and/or by elongase 2 (EC 2.3.1.199), and a limited beta-oxidation in peroxisomes, to produce DHA.^[6][8] Note that vascular terrestrial plants do not have the ability to synthesize long chain omega-3 PUFA, such as EPA and DHA.^[23]

The enzymes that catalyze the conversion of ALA to DHA are shared with the synthetic pathways leading to the synthesis of omega-6, omega-7 and omega-9 PUFAs.^[9] Omega-3 PUFAs appear to be the preferred substrates for delta-5 desaturase and delta-6 desaturase. However, because in many Western diets there is a high intake of linoleic acid relative to alpha-linolenic acid intake, the omega-6 pathway would be preferred over the other pathways.^[38] This could be one of the explanations for the low conversion rate of alpha-linolenic acid into the other omega-3 PUFAs, although the synthesis of arachidonic acid (ARA) from linoleic acid seems to be very low, too. Note that both the omega-3 and omega-6 families inhibit the synthesis of omega-9 polyunsaturated fatty acids.

Omega-3 synthesis in humans

Humans, like many other animals, can convert alpha-linolenic acid to docosahexaenoic acid, a metabolic pathway found mainly in the liver and cerebral microcirculation of the hematoencephalic barrier, but also in the cerebral endothelium and astrocytes.^[18] It is common opinion that humans, like other terrestrial animals, have a limited capacity to synthesize LC-PUFAs, and therefore need an adequate intake of EPA and DHA from food.^[7]
It has been shown that the yield of the synthesis decreases along the pathway: the rate of conversion of alpha-linolenic acid to eicosapentaenoic acid is low, and the limiting factor seems to be the activity of delta-6 desaturase, and the rate of conversion to docosahexaenoic acid is extremely low.^[7] However recent studies have demonstrated the existence of a marked polymorphism in the fatty acid desaturase (FADS) gene cluster, especially for the contiguous genes FADS1 and FADS2 coding for delta-5 desaturase and delta-6 desaturase, respectively, which are present on chromosome 11q12.2.^[6]
By analyzing genome-wide sequencing data from Bronze Age individuals and present-day Europeans, a comprehensive overview was obtained of the changes in allele frequency of FADS genes.^[25] In European populations, the transition from a hunter-gatherer society to an agricultural society would have resulted in an increase in the intake of linoleic acid and alpha-linolenic acid, and a reduction in the intake of EPA and ARA. Natural selection would then have favored the haplotype associated with the increase in the expression of FADS1 and the decrease in the expression of FADS2.^[6] This pattern is opposite to that found in the Greenlander Inuit, where it is hypothesized that natural selection would have favored alleles associated with a decrease in the rate of conversion of linoleic acid and alpha-linolenic acid into LC-PUFAs, in order to compensate for their relatively high dietary intake in such population.^[13]

Do other animals need EPA and DHA?

Organisms lacking delta-15 desaturase cannot synthesize alpha-linolenic acid and hence the other omega-3 PUFAs, and, if needed, must obtain it from dietary sources. However, many animals do not need to get EPA and DHA from diet.
Terrestrial herbivorous vertebrates satisfy their need for long chain omega-3 polyunsaturated fatty acids by synthesizing them from alpha-linolenic acid obtained from the green parts of plants.^[19]
And there are animals that do not need EPA and DHA, and practically do not have them. These include terrestrial insects, that have very low levels of EPA. In such animals, EPA is synthesized from dietary alpha-linolenic acid and used for eicosanoid production.^[16]
Conversely, aquatic insects have high levels of EPA, whereas DHA is practically absent.^[31]
Some classes of phytoplankton, such as Cryptophyceae and Dinophyceae, are very rich in EPA and DHA, whereas Bacillariophyceae or diatoms are very rich in EPA. In general, microalgae are the primary producers of EPA and DHA, and then, aquatic ecosystems are the main source of omega-3 LC-PUFAs in the biosphere.^[2][28] EPA and DHA are then transferred from these microalgae along the food chain, from invertebrates to fish, and from fish to terrestrial animals, including humans. So, from microalgae to humans.^[16]

Benefits for humans

Omega-3 polyunsaturated fatty acids are essential components of a healthy and balanced diet.^[36] They are needed throughout development, starting from fetal life, and are associated with health improvements and reduced risk of disease.^[8][26]] Indeed, many epidemiological studies have associated high intake of EPA and DHA with a lower cardiovascular mortality, especially for cardiac diseases, than predicted, probably due to the improvements in many risk factors such as plasma levels of triglycerides, HDL-cholesterol, C-reactive protein, blood pressure, both systolic and diastolic, and heart rate.^[1][27]
EPA and DHA have also been shown to be useful in the treatment of diseases such as rheumatoid arthritis, and could be useful in the treatment of other inflammatory conditions such as asthma, psoriasis or inflammatory bowel disease, due to their ability to modulate many aspects of the inflammatory processes.^[33]
Conversely, LC-PUFAs seem to have little or no effects on measures of glucose metabolism, such as insulin, insulin resistance, fasting glucose, and glycated haemoglobin, or on type 2 diabetes.^[5]

Omega-3/Omega-6 ratio

Epidemiological studies suggest that the consumption of a diet with a low omega-3/omega-6 ratio has had a negative impact on human health, contributing to the development, together with other risk factors such as sedentary life and smoking, of the main classes of diseases.^[8] Indeed, a lower incidence of cancer, autoimmunity and coronary heart disease has been observed in populations whose diet has a high omega-3/omega-6 ratio, such as Eskimos and Japanese, populations with a high fish consumption.^[20][37]
Despite these evidences, Western diet has become rich in saturated fatty acids and omega-6 polyunsaturated fatty acids, and poor in omega-3 polyunsaturated fatty acids, with an omega-3/omega-6 ratio between 1:10 and 1:20, then, far from the recommended ratio of 1:5.^[33]
The low value of the omega-3/omega-6 ratio is due to several factors, some of which are listed below.

• Although wild plant foods are generally high in omega-3 PUFAs, crops high in omega-6 PUFAs have been much more successful in industrial agriculture than those high in omega-3 PUFAs.^[3]
• Low consumption of fishery products and fish oils.^[33]
• The high consumption of animals raised on corn-based feed, such as chickens, cattle, and pigs. Added to this is the fact that the omega-3 PUFA content of some farmed fish species is lower than that of their wild counterparts.^[9]
• The high consumption of oils rich in omega-6 PUFAs and poor in omega-3 PUFAs, such as safflower, sunflower, soybeans and corn oils.^[4]

Note: there is no evidence that the omega-3/omega-6 ratio is important for prevention and treatment of type 2
diabetes mellitus.^[5]

Effects at the molecular level of EPA and DHA

In recent years, the molecular mechanisms underlying the functional effects attributed to omega-3 polyunsaturated fatty acids, especially to EPA and DHA, are being clarified, and most of these require their incorporation into membrane phospholipids.^[23]
Omega-3 PUFAs are structural components of cell membranes where they play an essential role in regulating fluidity.^[35] Due to this effect, omega-3, especially EPA and DHA, can modulate cellular responses that depend upon membrane protein functions. This is particularly important in the eye, where DHA allows for optimal activity of rhodopsin, a photoreceptor protein.^[29] The effect on membrane fluidity is essential for animals living in cold water, as EPA and DHA also have an antifreeze function.
EPA and DHA can modify the formation of lipid raft, microdomains with a specific lipid composition that act as platforms for receptor activities and the initiation of intracellular signaling pathways. By modifying lipid raft formation, they affect intracellular signaling pathways in different cell types, such as neurons, immune system cells, and cancer cells. In this way, EPA and DHA can modulate the activation of transcription factors, such as NF-κB, PPARs and SREBPs, and so the corresponding gene expression patterns. This is central to their role in controlling adipocyte differentiation, the metabolism of fatty acids and triacylglycerols, and inflammation.^[8]
EPA, DHA, and ARA are substrates for the synthesis of bioactive lipid mediators, such as eicosanoids, that are involved in the regulation of inflammation, immunity, platelet aggregation, renal function, and smooth muscle contraction.^[21]
Eicosanoids produced from arachidonic acid, that is the major substrate for their synthesis, have important physiological roles, but an excessive production has been associated with numerous disease processes.^[8] The increase in EPA and DHA content in membrane phospholipids is paralleled by a reduction in ARA content and associated with a decreased production of lipid mediators form ARA and an increased production of lipid mediators from the two omega-3. Moreover, among the molecules derived from EPA and DHA, there are eicosanoids analogous to those produced from ARA, but with lower activity, resolvins, and, from DHA, protectins and maresins. These molecules appear to be responsible for many of the immune-modulating and anti-inflammatory actions attributed to the omega-3 polyunsaturated fatty acids EPA and DHA.^[10]
EPA and DHA can also play a role in the non-esterified form, acting directly through receptors coupled to G proteins, modulating their activity.^[30]
Finally, they can reduce the intestinal absorption of omega-6 PUFAs, and, at the enzymatic level, competitively inhibit cyclooxygenase-1 or COX-1 (EC 1.14.99.1) and lipoxygenases, and compete with omega-6 PUFAs for acyltransferases.^[3]

Major sources of EPA and DHA for humans

In general, fish and aquatic invertebrates, such as molluscs and crustaceans, are the major sources of EPA and DHA for humans.^[3][9][36] These animals can get EPA and DHA from food, namely, from phytoplankton, or synthesize them from alpha-linolenic acid.^[16] Moreover, DHA is present in high concentrations in many fish oils, too, especially those from coldwater fish. However, it should be underscored that such oils are also high in saturated fatty acids.^[24] For those who do not eat fishery products, good sources of omega-3 LC-PUFAs are the liver of terrestrial animals and several birds of the order Passeriformes.^[14][15]
Regarding the recommended intake of omega-3 polyunsaturated fatty acids, it is not yet clear what it is. The following table shows the values suggested by Food and Agriculture Organization (FAO) of the United Nations and the European Food Safety Authority (EFSA).^[11][12]

	Population subgroup	Recommendation
FAO	Adult males and non-pregnant or non-lactating adult females	Minimum of 250 mg EPA + DHA daily
FAO	Pregnant or lactating females	Minimum of 300 mg EPA + DHA daily of which at least 200 mg should be DHA
FAO	Children aged 2-4 years	100-150 mg EPA + DHA daily
FAO	Children aged 4-6 years	150-200 mg EPA + DHA daily
FAO	Children aged 6-10 years	200-250 mg EPA + DHA daily
EFSA	Adult males and non-pregnant adult females	Adequate intake is 250 mg EPA + DHA daily
EFSA	Pregnant females	An additional 100-200 mg DHA daily
EFSA	Infants and children aged 6 months to 2 years	100 mg DHA daily
EFSA	Children aged 2-18 years	‘‘Consistent with adults’’

Omega-3 and culinary treatments

As omega-3 polyunsaturated fatty acids are particularly susceptible to oxidation due to heating, cooking and other culinary treatments could reduce their content.^[3] However, this is only partially true. In food, EPA and DHA are not in free form but mainly esterified into membrane phospholipids and, in such form, are much less susceptible to oxidation.^[9]
Considering the content of EPA and DHA, to express it as a percentage of the total fatty acids instead of as absolute content, namely, mg/g wet weight, leads to erroneous conclusions. For example, a fatty fish like salmon has a high EPA + DHA content, ~8 mg/g wet weight, and expressed as a percentage of the total fatty acids ~20 percent; conversely Atlantic code has a low EPA + DHA content, ~3 mg/g of wet weight, but, if expressed as a percentage of the total fatty acids ~40 percent. Atlantic code has a high percentage of EPA + DHA because is a lean fish, whereas in fatty fish EPA + DHA content is diluted by the high fatty acid content of the adipose tissue of the animal.^[17]
And when EPA + DHA content is expressed in mg/g of product, no decrease in LC-PUFAs content is observed following most common culinary treatments.^[16]

References

1. ^ ^{a b} AbuMweis S., Jew S., Tayyem R. Agraib L. Eicosapentaenoic acid and docosahexaenoic acid containing supplements modulate risk factors for cardiovascular disease: a meta-analysis of randomised placebo-control human clinical. J Hum Nutr Diet 2018;31(1):67-84. doi:10.1111/jhn.12493
2. ^{^} Ahlgren G., Lundstedt L., Brett M., Forsberg C. Lipid composition and food quality of some freshwater phytoplankton for cladoceran zooplankters. J Plankton Res 1990;12(4):809-18. doi:10.1093/plankt/12.4.809
3. ^ ^{a b c d e f g} Akoh C.C. and Min D.B. Food lipids: chemistry, nutrition, and biotechnology. 3th ed. 2008
4. ^{^} Blasbalg T.L., Hibbeln J.R., Ramsden C.E., Majchrzak S..F, Rawlings R.R. Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century. Am J Clin Nutr 2011;93(5):950-62. doi:10.3945/ajcn.110.006643
5. ^ ^{a b} Brown T.J., Brainard J., Song F., Wang X., Abdelhamid A., Hooper L. Omega-3, omega-6, and total dietary polyunsaturated fat for prevention and treatment of type 2 diabetes mellitus: systematic review and meta-analysis of randomised controlled trials. BMJ 2019;366:l4697. doi:10.1136/bmj.l4697
6. ^ ^{a b c} Buckley M.T., Racimo F., Allentoft M.E., et al. Selection in Europeans on fatty acid desaturases associated with dietary changes. Mol Biol Evol 2017;34(6):1307-1318. doi:10.1093/molbev/msx103
7. ^ ^{a b} Burdge G.C., Wootton S.A. Conversion of α-linolenic acid to eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in young women. Br J Nutr 2002;88(4):411-420. doi:10.1079/BJN2002689
8. ^ ^{a b c d e f} Calder P.C. Very long-chain n-3 fatty acids and human health: fact, fiction and the future. Proc Nutr Soc 2018 77(1):52-72. doi:10.1017/S0029665117003950
9. ^ ^{a b c d e f g h} Chow Ching K. Fatty acids in foods and their health implication. 3th ed. 2008
10. ^ ^{a b} Duvall M.G., Levy B.D. DHA- and EPA-derived resolvins, protectins, and maresins in airway inflammation. Eur J Pharmacol 2016;785:144-155. doi:10.1016/j.ejphar.2015.11.001
11. ^{^} EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA). Scientific opinion on dietary reference values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids, and cholesterol. 2010. doi:10.2903/j.efsa.2010.1461
12. ^{^} FAO. Global recommendations for EPA and DHA intake (As of 30 June 2014)
13. ^{^} Fumagalli M., Moltke I., Grarup N., Racimo F., Bjerregaard P., Jørgensen M.E., Korneliussen T.S., Gerbault P., Skotte L., Linneberg A., Christensen C., Brandslund I., Jørgensen T., Huerta-Sánchez E., Schmidt E.B., Pedersen O., Hansen T., Albrechtsen A., Nielsen R.. Greenlandic Inuit show genetic signatures of diet and climate adaptation. Science 2015;349(6254):1343-7. doi:10.1126/science.aab2319
14. ^{^} Gladyshev M.I., Makhutova O.N., Gubanenko G.A., Rechkina E.A., Kalachova G.S., Sushchik N.N. Livers of terrestrial production animals as a source of long-chain polyunsaturated fatty acids for humans: an alternative to fish? Eur J Lipid Sci Technol 2015;117:1417-1421. doi:10.1002/ejlt.201400449
15. ^{^} Gladyshev M.I., Popova O.N., Makhutova O.N., Zinchenko T.D., Golovatyuk L.V., Yurchenko Y.A., Kalachova G.S., Krylov A.V., Sushchik N.N. Comparison of fatty acid compositions in birds feeding in aquatic and terrestrial ecosystems. Contemp Probl Ecol 2016;9:503-513. doi:10.1134/S1995425516040065
16. ^ ^{a b c d} Gladyshev M.I. and Sushchik N.N. Long-chain omega-3 polyunsaturated fatty acids in natural ecosystems and the human diet: assumptions and challenges. Biomolecules 2019;9(9):485. doi:10.3390/biom9090485
17. ^{^} Gladyshev M.I., Sushchik N.N., Tolomeev A.P., Dgebuadze Y.Y. Meta-analysis of factors associated with omega-3 fatty acid contents of wild fish. Rev Fish Biol Fish 2018;28,:277-299. doi:10.1007/s11160-017-9511-0
18. ^{^} Kim H-Y. Novel metabolism of docosahexaenoic acid in neural cells. J Biol Chem 2007;282(26):18661-18665. doi:10.1074/jbc.R700015200
19. ^{^} Kouba M., Mourot J. A review of nutritional effects on fat composition of animal products with special emphasis on n-3 polyunsaturated fatty acids. Biochimie 2011;93(1):13-7. doi:10.1016/j.biochi.2010.02.027
20. ^{^} Kromann N., Green A. Epidemiological studies in the Upernavik district, Greenland. Incidence of some chronic diseases 1950-1974. Acta Med Scand 1980;208(5):401-6. doi:10.1111/j.0954-6820.1980.tb01221.x
21. ^ ^{a b} Kuda O. Bioactive metabolites of docosahexaenoic acid. Biochimie 2017;136:12-20. doi:10.1016/j.biochi.2017.01.002
22. ^{^} Lauritzen L., Hansen H.S., Jørgensen M.H., Michaelsen K.F. The essentiality of long chain n-3 fatty acids in relation to development and function of the brain and retina. Prog Lipid Res 2001;40(1-2):1-94. doi:10.1016/s0163-7827(00)00017-5
23. ^ ^{a b} Lee J.M., Lee H., Kang S. and Park W.J. Fatty acid desaturases, polyunsaturated fatty acid regulation, and biotechnological advances. Nutrients 2016;8(1):23. doi:10.3390/nu8010023
24. ^{^} Mason R.P., Sherratt S.C.R. Omega-3 fatty acid fish oil dietary supplements contain saturated fats and oxidized lipids that may interfere with their intended biological benefits. Biochem Biophys Res Commun 2017;483(1):425-429. doi:10.1016/j.bbrc.2016.12.127
25. ^{^} Mathieson I., Lazaridis I., Rohland N., Mallick S., Patterson N., Roodenberg S.A., Harney E., Stewardson K., Fernandes D., Novak M., et al. Genome-wide patterns of selection in 230 ancient Eurasians. Nature 2015;528:499-503. doi:10.1038/nature16152
26. ^ ^{a b} McCann J.C., Ames B.N. Is docosahexaenoic acid, an n-3 long-chain polyunsaturated fatty acid, required for development of normal brain function? An overview of evidence from cognitive and behavioral tests in humans and animals. Am J Clin Nutr 2005;82(2):281-95. doi:10.1093/ajcn.82.2.281
27. ^ ^{a b} Mensink R.P., Zock P.L., Kester A.D., Katan M.B. Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials. Am J Clin Nutr 2003;77(5):1146-55. doi:10.1093/ajcn/77.5.1146
28. ^{^} Muller-Navarra D.C. Biochemical versus mineral limitation in Daphnia. Limnol Oceanogr 1995;40:1209-1214. doi:10.4319/lo.1995.40.7.1209
29. ^ ^{a b} Niu S.L., Mitchell D.C., Lim S.Y., Wen Z.M., Kim H.Y., Salem N. Jr, Litman B.J. Reduced G protein-coupled signaling efficiency in retinal rod outer segments in response to n-3 fatty acid deficiency. J Biol Chem 2004;279(30):31098-104. doi:10.1074/jbc.M404376200
30. ^{^} Oh D.Y., Talukdar S., Bae E.J., Imamura T., Morinaga H., Fan WQ, Li P., Lu W.J., Watkins S.M., Olefsky J.M. GPR120 is an omega-3 fatty acid receptor mediating potent anti-inflammatory and insulin-sensitizing effects. Cell 2010 142(5):687-698. doi:10.1016/j.cell.2010.07.041
31. ^{^} Popova O.N., Haritonov A.Y., Sushchik N.N., Makhutova O.N., Kalachova G.S., Kolmakova A.A., Gladyshev M.I. Export of aquatic productivity, including highly unsaturated fatty acids, to terrestrial ecosystems via Odonata. Sci Total Environ 2017;581-582:40-48. doi:10.1016/j.scitotenv.2017.01.017
32. ^{^} Salem N. Jr, Litman B., Kim H.Y., Gawrisch K. Mechanisms of action of docosahexaenoic acid in the nervous system. Lipids 2001;36(9):945-59. doi:10.1007/s11745-001-0805-6
33. ^ ^{a b c d} Simopoulos A.P. The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases. Exp Biol Med 2008;233(6):6746-88. doi:10.1016/S0753-3322(02)00253-6
34. ^{^} Smith W., Mukhopadhyay R. Essential fatty acids: the work of George and Mildred Burr. J Biol Chem 2012;287(42):35439-35441. doi:10.1074/jbc.O112.000005
35. ^ ^{a b} Stubbs C.D., Smith A.D. The modification of mammalian membrane polyunsaturated fatty acid composition in relation to membrane fluidity and function. Biochim Biophys Acta 1984;779(1):89-137. doi:10.1016/0304-4157(84)90005-4
36. ^ ^{a b c} Tocher D.R., Betancor M.B., Sprague M., Olsen R.E., Napier J.A. Omega-3 long-chain polyunsaturated fatty acids, EPA and DHA: bridging the gap between supply and demand. Nutrients 2019;11(1):89. doi:10.3390/nu11010089
37. ^{^} Yano K., MacLean C.J., Reed D.M., Shimizu Y., Sasaki H., Kodama K., Kato H., Kagan A. A comparison of the 12-year mortality and predictive factors of coronary heart disease among Japanese men in Japan and Hawaii. Am J Epidemiol 1988;127(3):476-87. doi:10.1093/oxfordjournals.aje.a114824
38. ^{^} Williams C.M., Burdge G. Long-chain n-3 PUFA: plant v. marine sources. Proc Nutr Soc 2006;65(1):42-50. doi:10.1079/pns2005473

Essential fatty acids or EFAs are unsaturated fatty acids that cannot be synthesized by animals, and, like other essential nutrients, must be obtained from the diet. They are linoleic acid and alpha-linolenic acid.
Animals cannot synthesize the two fatty acids due to the lack of delta-12 desaturase (E.C. 1.14.19.6) and delta-15 desaturase (EC 1.14.19.25).^[1][9][15] These enzymes introduce cis double bonds beyond carbon 9, and are present in plants and some microorganisms such as some bacteria, fungi and molds.^[10]

• Delta-12 desaturase catalyzes the synthesis of linoleic acid from oleic acid, by introducing a double bond at delta-12 position, namely, between carbons 6 and 7 from the methyl end of the fatty acid.^[15]
• Delta-15 desaturase or omega-3 desaturase or fatty acid desaturase 3 catalyzes the synthesis of alpha-linolenic acid from linoleic acid by introducing a double bond at delta-15 position, namely, between carbons 3 and 4 from the methyl end of the fatty acid. The enzyme is only present in the plastids and in the endoplasmic reticulum of phytoplankton and vascular terrestrial plants.^[1][9]

Linoleic acid and alpha-linolenic acid are the precursors to omega-6 polyunsaturated fatty acids and omega-3 polyunsaturated fatty acids. In the absence of dietary EFAs, a rather rare condition, the other omega-3 and omega-6 fatty acids become essential, too. For this reason, these lipids are defined by some as conditionally essential fatty acids.^[9]
It should be pointed out that all essential fatty acids are polyunsaturated fatty acids, but not all polyunsaturated fatty acids are essential, such as those belonging to the omega-7 and omega-9 families.

Contents

• The discovery
• Role
• Food sources
• References

The discovery

The first evidence of the existence of essential fatty acids dates back to 1918, when Hans Aron suggested that dietary fat could be essential for the healthy growth of animals and that, in addition to their caloric contribution, there was a inherent nutritive value due to the presence of certain lipid molecules.^[2]
In 1927, Herbert M. Evans and George Oswald Burr demonstrated that, despite the addition of vitamins A, D, and E to the diet, a deficiency of fat severely affected both growth and reproduction of experimental animals. Therefore, they suggested the presence of an essential substance in the fat that they called vitamin F.^[11]
Eleven years after Aron work, in 1929, Burr and his wife Mildred Lawson hypothesized that warm-blooded animals were not able to synthesize appreciable amounts of certain fatty acids.^[7] One year later, they discovered that linoleic acid was essential for animals, and it was they who coined the term essential fatty acid.^[8][19] However, EFA deficiency in humans was first described by Arild Hansen only in 1958, in infants fed a milk-based formula lacking them.^[14]
And in 1964, thanks to the research of Van Dorp and Bergstroem, one of their biological functions was discovered: being the precursors for the synthesis of prostaglandins.^[3][21]

Role

Essential fatty acids play important biological functions.

• Linoleic acid and alpha-linolenic acid can be used as an energy source through beta-oxidation.^[22]
• Linoleic acid and alpha-linolenic acid are the precursors to omega-6 polyunsaturated fatty acids and omega-3 polyunsaturated fatty acids. Indeed, many animals, including humans, can synthesize, although with variable efficiency, the other omega-3 and omega-6 polyunsaturated fatty acids, for example arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid.^[5][6][18]
• Essential fatty acids are structural components of cellular membranes, modulating, for example, their fluidity.^[20][22]
• They are essential in the skin, especially linoleic acid in sphingolipids of the stratum corneum, where they contribute to the formation of the barrier against water loss.^[23]
• They have a crucial role in the prevention of many diseases, particularly coronary heart disease.^[12][16]

Food sources

Linoleic acid is the most abundant polyunsaturated fatty acid in the Western diet, and accounts for 85-90 percent of dietary omega-6 polyunsaturated fatty acids.^[13] The richest dietary sources are vegetable oils and seeds of many plants.^[4]

Food sources	Linoleic acid (mg/g)
Safflower oil	∼ 740
Sunflower oil	∼ 600
Soybean oil	∼ 530
Corn oil	∼ 500
Cottonseed oil	∼ 480
Walnuts	∼ 340
Brazil nuts	∼ 250
Peanut oil	∼ 240
Rapeseed oil	∼ 190
Peanuts	∼ 140
Flaxseed oil	∼ 135

Linoleic acid is present in fair amounts also in animal products such as chicken eggs or lard, because it is present in their feed.^[9]
It should be noted that some of the major sources of linoleic acid, such as walnuts, flaxseed oil, soybean oil, and rapeseed oil are also high in alpha-linolenic acid.^[17]
Some of the richest dietary sources of alpha-linolenic acid are flaxseed oil, ~ 550 mg/g, rapeseed oil, ~ 85 mg/g, and soybean oil, ~ 75 mg/g. Other foods rich in alpha-linolenic acid include nuts, ~ 70 mg/g, and soybeans, ~ 10 mg/g.^[1]

References

1. ^ ^{a b c} Akoh C.C. and Min D.B. Food lipids: chemistry, nutrition, and biotechnology. 3th Edition. CRC Press, Taylor & Francis Group, 2008
2. ^{^} Aron H. Uber den Nahrwert. Biochem Z 1918; 92: 211-33
3. ^{^} Bergstroem S., Danielsson H., Klenberg D., Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-8. doi:10.1016/S0021-9258(18)91234-2
4. ^{^} Blasbalg T. L., Hibbeln J. R., Ramsden C. E., Majchrzak S. F. & Rawlings R. R. Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century. Am J Clin Nutr 2011;93(5):950-962. doi:10.3945/ajcn.110.006643
5. ^{^} Burdge G.C, Jones A.E, Wootton S.A. Eicosapentaenoic and docosapentaenoic acids are the principal products of α-linolenic acid metabolism in young men. Br J Nutr 2002;88(4):355-364. doi:10.1079/BJN2002662
6. ^{^} Burdge G.C., Wootton S.A. Conversion of α-linolenic acid to eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in young women. Br J Nutr 2002;88(4):411-420. doi:10.1079/BJN2002689
7. ^{^} Burr G. and Burr M. A new deficiency disease produced by the rigid exclusion of fat from the diet. J Biol Chem 1929;82:345-367. doi:10.1016/S0021-9258(20)78281-5
8. ^{^} Burr G.O., Burr M.M., Miller E.S. On the fatty acids essential in nutrition. III. J Biol Chem 1932;97(1):1-9. doi:10.1016/S0021-9258(18)76213-3
9. ^ ^{a b c d} Chow Ching K. Fatty acids in foods and their health implication. 3th Edition. CRC Press, Taylor &amp; Francis Group, 2008
10. ^{^} Das U.N. Essential fatty acids: biochemistry, physiology and pathology. Biotechnol J 2006;1;420-439. doi:10.1002/biot.200600012
11. ^{^} Evans H. M. and G. O. Burr. A new dietary deficiency with highly purified diets. III. The beneficial effect of fat in the diet. Proc Soc Exp Biol Med 1928;25:390-397. doi:10.3181/00379727-25-3867
12. ^{^} Farvid M.S., Ding M., Pan A., Sun Q., Chiuve S.E., Steffen L.M., Willett W.C., Hu F.B. Dietary linoleic acid and risk of coronary heart disease: a systematic review and meta-analysis of prospective cohort studies. Circulation 2014;130:1568-1578. doi:10.1161/CIRCULATIONAHA.114.010236
13. ^{^} Guyenet S.J., Carlson S.E. Increase in adipose tissue linoleic acid of US adults in the last half century. Adv Nutr 2015;6(6):660-664. doi:https://DOI.org/10.3945/an.115.009944
14. ^{^} Hansen A.E., Haggard M.E., Boelsche A.N., Adam D.J., Wiese H.F. Essential fatty acids in infant nutrition. III. Clinical manifestations of linoleic acid deficiency. J Nutr 1958;66(4):565-76. doi:10.1093/jn/66.4.565
15. ^ ^{a b} Malcicka M., Visser B., Ellers J. An evolutionary perspective on linoleic acid synthesis in animals. Evol Biol 2018;45:15-26. doi:10.1007/s11692-017-9436-5
16. ^{^} Mensink R.P., Zock P.L., Kester A.D., Katan M.B. Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials. Am J Clin Nutr 2003;77(5):1146-55. doi:10.1093/ajcn/77.5.1146
17. ^{^} Rajaram S. Health benefits of plant-derived α-linolenic acid. Am J Clin Nutr 2014;100 Suppl 1:443S-8S. doi:10.3945/ajcn.113.071514
18. ^{^} Rett B.S., Whelan J. Increasing dietary linoleic acid does not increase tissue arachidonic acid content in adults consuming Western-type diets: a systematic review. Nutr Metab (Lond) 2011;8:36. doi:10.1186/1743-7075-8-36
19. ^{^} Smith W., Mukhopadhyay R. Essential fatty acids: the work of George and Mildred Burr. J Biol Chem 2012;287(42):35439-35441. doi:10.1074/jbc.O112.000005
20. ^{^} Stubbs C.D., Smith A.D. The modification of mammalian membrane polyunsaturated fatty acid composition in relation to membrane fluidity and function. Biochim Biophys Acta 1984;779(1):89-137. doi:10.1016/0304-4157(84)90005-4
21. ^{^} Van Dorp D.A., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-841. doi:10.1038/203839a0
22. ^ ^{a b} Whelan J. and Fritsche K. Linoleic acid. Adv Nutr 2013;4(3): 311-312. doi:10.3945/an.113.003772
23. ^{^} Ziboh V.A. Prostaglandins, leukotrienes, and hydroxy fatty acids in epidermis. Semin Dermatol 1992;11(2):114-20

Invert sugar, also known as inverted sugar, is sucrose partially or totally cleaved into fructose and glucose and, apart from the chemical process used, the obtained solution has the same amount of the two carbohydrates.

Moreover, according to the product, not cleaved sucrose may also be present.

Invert sugar production

The breakdown of sucrose may happen in a reaction catalyzed by enzymes, such as:

• sucrase-isomaltase (EC 3.2.1.48), actives at our own intestinal level where it is involved in carbohydrate digestion;
• invertase, an enzyme secreted by honeybees into the honey and used industrially to obtain invert sugar.

Another process applies acid action, as it happens partly in our own stomach and as it happened in the old times, and still happens, at home-made and industrial level. Sulfuric and hydrochloric acids was used, heating the solution with caution for some time; in fact the reaction is as fast as the solution is acid, regardless of the type of acid used, and as higher the temperature is. The acidity is then reduced or neutralized with alkaline substances, as soda or sodium bicarbonate.
A chemical process as described occurs when acid foods are prepared; i.e. in the preparation of jams and marmalades, where both conditions of acidity, naturally, and high temperatures, by heating, are present. The situation is analogous when fruit juices are sweetened with sucrose.
The reaction develops at room temperature as well, obviously more slowly.
What is the practical outcome of that?
It means that, during storage, also sweets and acid foods, even those just seen, go towards a slow reaction of inversion of contained/residue sucrose, with consequent modification of the sweetness, since invert sugar at low temperatures is sweeter (due to the presence of fructose), and assumption of a different taste profile.

Properties and uses

It is principally utilized in confectionery and ice-cream industries thanks to some peculiar characteristics.

• It has an higher affinity for water (hydrophilicity) than sucrose (see fructose) therefore it keeps food more humid: e.g. cakes made with invert sugar dry up less easily.
• It avoids or slows down crystal formation (dextrose and fructose form less crystals than sucrose), property useful in confectionery industries for icings and coverage.
• It has a lower freezing point.
• It increases, just a bit, the sweetness of the product in which it has been added, as it is sweeter than an equal amount of sucrose (the sweetness of fructose depends on the temperature in which it is present).
• It may take part to Maillard reaction (sucrose can’t do it) thus contributing to the color and taste of several bakery products.

It should be noted that honey, lacking in sucrose, has a fructose and glucose composition almost equal to that of 100% invert sugar (fructose is slightly more abundant than glucose). So, diluted honey, better if not much aromatic, may replace industrial invert sugar.

References

1. Belitz .H.-D., Grosch W., Schieberle P. “Food Chemistry” 4th ed. Springer, 2009
2. Bender D.A. “Benders’ Dictionary of Nutrition and Food Technology”. 8th Edition. Woodhead Publishing. Oxford, 2006
3. Jordan S. Commercial invert sugar. Ind Eng Chem 1924;16(3):307-310. doi:10.1021/ie50171a037
4. Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012