Proteins make up one of the three major groups of macronutrients, together with carbohydrates and fats, i.e., one of the constituents of foods that contribute to energy intake.
Proteins can be considered schematically as long, linear or branched chains, more or less folded onto themselves, that may or may not be organised into a space-filling structure.
Amino acids are the basic unit making up the protein. There are a great many different amino acids but only twenty are used by the body to manufacture proteins (called “proteogenic” amino acids). Among these 20 amino acids, 11 can be manufactured by the human body and the other nine are called essential, because the body cannot synthesise them in sufficient quantitiesto meet its needs. Therefore, these amino acids must be supplied by food.
The amino acid composition of proteins is taken into account when assessing the protein quality of our food. Amino acids, and therefore the proteins of which they are made, are also high in nitrogen and are the main source of intake of this essential element for the body.
The role of protein
Proteins have a number of vital functions in the body:
- they have a structural role and are involved in the renewal of muscle tissue, integument (scalp hair, nails, body hair), bone matrix, and skin tissue, etc.;
- they take part in many physiological processes, such as in the formation of digestive enzymes, haemoglobin, hormones, receptors and immunoglobulins (antibodies).
They are also the body’s only source of nitrogen.
Food sources of protein
The quality of food sources of protein is almost exclusively defined by their ability to cover the body’s requirements for protein and essential amino acids. Animal protein, the major source of proteins in the diet of industrialised countries, comes particularly from milk, eggs, fish and meat.
Plant proteins come mainly from cereals and legumes. They can be found naturally in food or can be added for nutritional (special foods for infants or the elderly) or techno-functional (gelling property of egg white) purposes.
Proteins of animal origin
Animal proteins are relatively high in essential amino acids and generally higher than plant proteins. In general, animal proteins are slightly more digestible than plant proteins.
Foods of animal origin are characterised by a greater level of proteins that are high in nutritional value (as measured by composition of essential amino acids, digestibility, etc.).
Meat, fish, eggs, milk and dairy products are protein-rich foods.
Proteins of plant origin
The plant-based foods with the highest protein are oilseeds (peanuts, almonds, pistachios, etc.), legumes and their derivatives (tofu, chickpeas, beans, etc.) and then cereals.
No single plant protein contains all of the essential amino acids. Consequently, some plant proteins may contain a limited amount of certain essential amino acids, such as lysine for cereals, and sulphur-containing amino acids for legumes.
Different plant-based foods must be combined in order to achieve a balanced diet of amino acids from plant proteins. Therefore, it is useful to combine different plant foods: legumes (lentils, beans, peas, etc.) with cereals (rice, wheat, maize, etc.). For example, combining rice and soybeans helps to balance lysine intake, which is low in rice, but high in soy, and the intake of sulphur-containing amino acids, which are low in soy, but high in rice.
In addition to the issue of providing sufficient amino acid intakes, the source of the protein may affect coverage of requirements for other nutrients. Thus, a diet exclusively of plant origin can lead to a risk of vitamin B12 deficiency. A diet that is high in animal protein can lead to an inadequate intake of fibre and an excess of saturated fats.
In modern society, non-restrictive vegetarian diets (i.e. not excluding dairy products and eggs) ensure protein intake in satisfactory quantity and quality for both adults and children. In adult vegans, particular attention must be paid to covering protein-energy intake.
Recommendations of the Agency, to date
(A working group is currently investigating recommendations for the distribution of macronutrients in daily energy intake. The report of this working group is expected in 2014)
The Agency has established the recommended French population reference intake for proteins at 0.83 g/kg/d for adults in good health.
Given the lack of available data, it is difficult to define a safe upper limit for protein intake. In the current state of knowledge, intakes between 0.83 and 2.2 g/kg/d of protein (which is 10 to 27% of energy intake) can be considered to be satisfactory for an adult individual under 60 years of age.
The population reference intake is slightly increased for the elderly, of the order of 1 g/kg/d, as well as in pregnant and breastfeeding women, of the order of at least 70 g/d or 1.2 g/kg/d.
According to the second French Individual and national study on food consumption [INCA2], the average daily intakes of protein are 74g for adult women and 100g for adult men and represent for both genders approximately 17% of total energy intake. In children, the average daily intakes are at 63g in three to ten year olds and 74g in eleven to seventeen year olds, and account for15 to 16% of total energy intake.
Meat products (meats, poultry, delicatessen meats) provide 31% of protein intake for adults, followed by dairy products (17%), especially cheese (9%), and bread and bread products (11%). These same foods contribute similar shares of protein intake in children. Thus, meat products are the leading contributor to protein intake (28%), followed by dairy products (21%), especially milk (10%), and bread and bread products (7%).