The nutrition (Latin nurture: feed) is the set of processes by which a living being transforms food for its operation. Nutrition is also a science multidisciplinary, comprising two main axes. First, the physiology of nutrition deals with how the organization operates food processing, that is to say, metabolic processes. This includes the study of the role of macronutrients and micronutrients in cellular biochemical mechanisms and consequences on tissues from an imbalance between the energy absorbed and the energy expended by the body. On the other hand, the psychology of nutrition analyzes the food behavior of the individual or group. It focuses on issues such as “why do we eat? Or “How do we choose our food? “. In humans, these issues relate to environmental factors such as the built environment, media and health policies, as well as its peculiarities such as income or culture.

Definition of food 

Nutrition focusing on the relationships between living organisms and their food , it is necessary to define what is considered a food. There are two definitions, depending on the physiological or psychological approach. In the first, food encompasses everything that is nutritious, that is to say that “ingestion is necessary for survival, good health and growth of young people” 1 . The psychological approach is more restrictive, considering that “the most nutrient food can hardly count as if nobody eats it [because] people do not usually think about what they eat in terms of nutrient ‘ 1 . In other words, a food within the meaning of life depends on the cultural context: for example, the eyes of whales are nutritious because they contain protein and vitamin A 1 , but an individual may not consider this a food acceptable.


There are many nutrients different, divided into two categories: macronutrients and micronutrients.


The organization draws its energy from sugars (or carbohydrates ), fats (or lipids ) and proteins . These three energy nutrients form the class of macronutrients. The organization may also draw its energy from alcohol (or ethanol ) but it does not necessarily enter a recommended diet, unlike the previous three nutrients. Energy is measured in joules or calories , the symbols J and cal respectively. The conversion is 1 kcal to 4,186 kcal , or 1 kJ to 0,289 kcal . Energy is supplied to the body by reaction with oxygen, that is to say, by oxidation , resulting 9.44 kcal per gram fat, 5.6 kcal per ml of alcohol, and varies depending on the type of carbohydrate: 4.18 kcal per gram of starch , 3.94 kcal per gram of sucrose and 3.72 kcal per gram of glucose . The energy obtained by oxidation of a gram of protein is 5.6 kcal but that does not match what the body withdraws, which is 4.70 kcal for this particular case A 2 . All parts providing energy extraction is the digestive system , which transforms the sugars glucose (or galactose ), the protein amino acids and fats into fatty acids . Each of these transformations is divided into several functional blocks: for example, the glucose processing consists of digestion, hormonal regulation (that is to say the hormones present in the plasma ), the use and storage ( in the liver , the adipose tissue and muscles) A 3 .

Sugars (or carbohydrates)

The carbohydrates are found in foods such as rice (up to 79.95 g to 100 g of white rice uncooked long grain A 1 ) or bread (mostly present in the form of starch A 4 ). According to the British Nutrition Foundation, the three main sources are cereals 45%, potatoes and nibbling salted to 12% and drinks 10% A 5 . Carbohydrates are divided into categories according to the number of sugar units: monosaccharides (or dare ) to a unit, disaccharides (or disaccharide such as sucrose ) for two, and polysaccharides (or glycans ) beyond. A similar classification is obtained by considering the degree of polymerization DP: monosaccharide DP of 1 to 2, oligosaccharide DP 3 to 9 and polysaccharide beyond. The place of absorption of carbohydrates depends mainly on the category, but also other factors related to the subject or the quantity. For example, monosaccharides such as lactose are normally absorbed in the small intestine , but if the subject has a lactose intolerance (carbohydrate in milk) then the lactose will continue its run of the small intestine to the large intestine where it is fermented to produce volatile fatty acids  ; The gases released by this process cause, among other things, bloating, and serve for the diagnosis of lactose intolerance by testing the presence of hydrogen. About 75% of adults have lactose intolerance, such as in Asian populations, and other issues related to the absorption of sugars exist such as disability sucrose-isomaltase which affects 10% of the Inuit of Greenland A 6 .

Carbohydrates are necessary for the body, and the blood concentration (usually between 70 mg / dl and 100 mg / dl ) must be maintained at a fairly high level because the brain depends entirely A 3 . Based on average consumption needed by the brain, a daily intake of 130 g is recommended for adults. In practice, this contribution is greatly exceeded, with a median of 220 g to 330 g for men and 180 g to 230 g in women A 7 . One of the interests of the physiology of nutrition is to see how the body adapts according to the amount of nutrients provided. In the case of carbohydrate, if the quantity is low then the body tries to save it by drawing more energy from the fatty substances; otherwise, the power can be taken from carbohydrates, which then are transferred from the blood to the cells by insulin , and excess can be converted by the liver through a process called de novo lipogenesis (DNL). The surpluses are stored as fat and not as carbohydrates, an explanation by evolution being that the energy density of the fat is higher than that of the sugars, thus minimizing the gain in weight in order to retain the mobility of the fat, body A 3 .

Fats (or lipids)

Most fats are not soluble in water , which distinguishes the carbohydrates and proteins. The classification of lipids Bloor distinguishes four categories. Simple fatty substances are fatty acids linked by a bond ester to alcohols . For example, the triglyceride result of a molecule of glycerol esterified to three fatty acid molecules, and it is in the vegetable oil and animal fats . Complex fats have the same composition but with additional molecules: esterifying glycerol with two fatty acid molecules and a phosphate , is [? Who] obtained complex phosphoglyceride . The derivatives are obtained by hydrolysis of the previous two, and what does fall within any of the three categories is another (such as squalene ). The structure of fatty acids is also classified according to the length of their carbon chain ( short , medium, long, long) and the presence of at least one double carbon-carbon bond (the acid is so unsaturated and this class contains the trans fatty acid ).

It is considered acceptable that 20 to 35% of the energy comes from fat. The role of all fatty acids is not well understood, and it is therefore not possible to determine appropriate levels for their consumption in general . However, levels are considered adequate (which is not a recommendation) for certain essential fatty acid  : for α-linolenic acid , the group Omega-3 , it is 1.6 g per day for youth men and 1.1 g for young women, and the γ-linolenic acid , the group Omega-6 , the amounts are respectively 17 g and 12 g per day. Unlike previous, the group Omega-9 does not contain essential fatty acids, but more research is needed on the health benefits A 8 .

During digestion, fats are emulsified in the small intestine (duodenum). The emulsifier is the bile produced by the liver and stored in the gallbladder . The emulsion then passes into the small intestine where the lipids are degraded by a process called lipolysis and associated with other molecules in a lipoprotein to be transported in the blood A 9 . Lipoprotein is shown in the diagram against: lipids are inside, and the outside is formed of apolipoproteins of different types, denoted Apo and phospholipids .

There are five classes of lipoproteins, performing different functions. For example, high-density lipoprotein HDL avoid noted that the cholesterol accumulates in the blood vessels causing liver which eliminates; these lipoproteins are called good cholesterol , and those doing the opposite direction are the bad cholesterol , LDL noted. The concentration of LDL increases with the intake of saturated fatty acids or trans fatty acids, which also increases the risk of cardiovascular disease A 8 . Remove these acids plan may require profound changes that can lead to deficiencies, but it is possible to reduce the amount, for example in the Mediterranean diet .


The proteins of animal origin are the main source in North America and Western Europe. The animal includes both meat that is produced by animals, such as eggs ( 13.62 g of protein 100 g of fried egg A 1 ) or the cheese ( 19.80 g of protein in 100 g Camembert A 1 ). The vegetarians do not consume meat, and vegans rejecting all animal products, their proteins must come from vegetables and grains, which are the two main sources in Africa, Asia and Latin America 10 .

A protein consists of amino acids linked by peptide bonds . These amino acids are separated into two categories: indispensable , that is to say, those that can not be produced and which must be supplied in the diet, or non-essential. What is indispensable depends on the organism, eg, the arginine is necessary for the survival of a cat but not for man past the infant stage. These categories are refined by considering the essential amino acids with conditions . Indeed, some amino acids can be synthesized but with limited capabilities in general and variants under the conditions of the subject: for example, the synthesis of proline is limited in burn 10 .

Proteins are an essential component of the body and fulfill a very large number of roles. For example, a hair is composed of keratin , which is a protein; it is involved with another protein, the collagen in the strength and elasticity of the skin. Problems related proteins may also be associated with diseases, such as glutamate is involved in crisis convulsion epileptic A 11 . An inappropriate protein intake can therefore have strong and varied consequences on the body. The recommended amount of protein is empirically given to 0.80 g daily per kilogram of body weight . It is recommended that the diet does not exceed 35% protein A 12 .

The PDCAAS ( protein digestibility corrected amino acid scoring pattern ) summarizes the amounts per amino acid, but because of controversy and technical developments, the amounts recommended by the FAO and the WHO can differ greatly according to reports. In 1985, he was advised to take each day less than 10 mg / kg of threonine and almost 30 mg / kg in 1991 and about 15 mg / kg in 2001. A 13 . The Technical Report 935 FAO / WHO is compiling an inventory in 2007 A 14 . Protein digestion occurs primarily in the small intestine and releases the amino acids that continue to other organs. If undernourishment , some amino acids can be converted to glucose by gluconeogenesis .



The minerals are essential for proper body functioning. They are found in water in the form of chemical compounds. They can also be found in meat and vegetables. A deficiency can lead to diseases, for example, a lack of iron in the blood causes anemia. Minerals are elements of the periodic table.


A vitamin is a compound necessary for the metabolism of an organism, in the sense that “being a subject in private will develop diseases deficiency and signs of abnormal metabolism, and restore the missing compound will prevent or cure all diseases and make the normal metabolism ” B 1 . The fact that a vitamin is a compound distinguishes it from minerals, for example, and necessity means that it can not be synthesized. The organisms being capable of different syntheses, the denomination of vitamin is relative to the organism in question. Thus, vitamin C can be synthesized by most animals from glucose and is therefore considered that vitamin for species incapable of this synthesis such as humans and other species of the taxon haplorhini , the bald -souris or guinea pigs B 2 . The reason for this deficiency in humans comes from the inactivity of the gene for the enzyme L-gulonolactone oxidase, necessary for synthesis from glucose, on chromosome 8 .

Vitamin C is found in vegetables and fruits, such as kiwi ( 105.4 mg per 100 g ) or cantaloupe ( 36.7 mg per 100 g ) A 1 . Fresh vegetables and raw fruits and are the best source, since vitamin C content decreases dramatically with age or cutting, and cooking strongly B 1 .


An antioxidant is a molecule that decreases or prevents the oxidation of other chemicals. The best known antioxidants are the beta-carotene (provitamin A), the ascorbic acid (vitamin C), tocopherol (vitamin E), the polyphenols and lycopene . These include the flavonoids (widespread in plants), the tannins (in cocoa, coffee, tea, grapes,  etc. ), the anthocyanins (especially in red fruits) and phenolic acids (in Cereals, fruits and vegetables).


Phytochemicals are organic chemical compounds that can be found in foods of plant origin.


The malnutrition refers to inadequate or excessive intake of nutrients by an organism. In developed countries, malnutrition is often associated with inadequate or excessive consumption. Although there are diseases related to undernourishment, other organisms suffer from excessive nutrition. Research [ref. required] have shown that people who are physically active, not smoking, with moderate alcohol consumption and eating lots of fruits and vegetables have a risk of death that does not reach the quarter to which the risk of death People with systematic lifestyle habits that are harmful to health. Mortality increases abruptly as soon as individuals exceed the threshold of overweight. The life expectancy of an obese person is eight to ten years (for a BMI of 40-45) less than that of a person of normal weight, which corresponds to the loss of life expectancy, expose smokers 2 .



The sensation of thirst is understood by studying the mechanisms of red blood cells . Inside a red blood cell, called the cytoplasm is separated from the outside by a membrane which allows water to pass, and is therefore called semi-permeable . There is pressure between the interior of the cell and the outside, according to the respective concentrations of solutes . If the concentration outside is lower than in the cell, then it starts to swell due to the phenomenon of osmosis and can eventually explode because its membrane is not expandable. The exterior, that is to say the plasma, is then called hypotonic. Conversely, it can be hypertonic and the cell then tries to re-establish the pressure by letting water flow outwards. This second scenario arises when there is not enough water in the body: less water means a higher concentration of solutes outside the cells. This pressure is a very sensitive mechanism to trigger feelings of thirst: an increase of 2 to 3% is enough to feel a strong need to drink, both in humans and monkeys or rats. This effect is verifiable by injecting a solution with a high salt concentration, and the intensity of the thirst is proportional to the pressure 1 .

However, the body also has its regulation mechanisms: the base of the brain responds by secreting a hormone antidiuretic which acts on the kidneys retaining water by filtering urine. This pressure increases when an individual eats, and this action also acts on the kidneys that need more water for waste. Thus, drinking before or during the meal contributes to balance. On the other hand, another body response to the increase in pressure is a dry mouth due to a reduction in saliva. However, it can not be concluded that having a dry mouth is a mechanism of the body to ensure that the individual drinks: a simple counterexample is that individuals with failing salivary glands drink appropriate amounts, sensation they feel does not push them to drink 1 .

Understanding osmotic pressure provides insight into the basic mechanisms, but many problems remain open, such as knowing how an organism determines the amount of water to drink.


Nutrition is defined here as the science that analyzes the relationship between food and health  : study of the composition of foods , their properties, and their use by the body. These studies lead to the diet . It also takes into account, as part of nutrition, dietary behavior of individuals, especially during meals or when snacking .

However, from the clinical observation of diseases whose origin was a dietary deficiency (e.g., scurvy ), nutrition today also relates to diseases such as problems cardiovascular and cancer (with the method Kousmine for example), the osteoporosis and hypertension (excess salt in particular), the type 2 diabetes , the obesity , the autoimmune disease , Alzheimer’s disease 3 .

Nutrition plays an essential role in the prevention of many diseases. For example, more than 100 000 cases of cancer 4 could be prevented each year in France, by simply changing eating habits.

It should also be remembered that nutritional status is a prognostic factor in the evolution of cancers. An under-nourished person will be at greater risk of complications than a person receiving a diet that meets the needs of the body.

The complex processes to which nutrients are subjected – interactions between food, degradation, transformation and release of energy, transport and use of chemical compounds for construction (anabolism = construction, catabolism = waste disposal) of specialized fabrics and The maintenance of overall good health – are only partly elucidated. However, significant nutritional choices must be made to ensure the health of individuals, such as the very young and the elderly, and entire populations who suffer from malnutrition .

Adapting food practices to the satisfaction of physiological needs is dietetics .