PROTEINS
Proteins or peptides are complex substances found in animal and vegetable tissues. Proteins were the first substance to be recognised as vital for all organisms, which explains the Greek origin of its word "protos" which means "of prime importance".
Proteins are obtained from external sources through food, and thus the amounts we take depend on our diet habits. Proteins are the structural components of cells, and most are found in muscle and viscerae; other proteins are found in soft tissues (collagen), bone, teeth, blood of other body fluids. Enzymes, gammaglobulins and chromosomal structures are also proteinic structures.
Composition And Classification
Proteins contain carbon, hydrogen and oxygen as well as nitrogen, and occasionally some other elements such as iron, cobalt or phosphorous.
The basic structure of proteins is amino acids. Twenty amino acids have been identified to date, which constitute most proteins. A certain number of amino acids bind together in a specific manner to form different proteins. These proteins, in turn, adopt different very complex spatial shapes, which determine their characteristics and function.
These 20 amino acids are classified depending on their ability to be synthesised (manufactured) by the body. Essential amino acids are those that cannot be synthesised by the body and must necessarily come from the diet. Other amino acids that can be synthesised by the organism are known as non-essential amino acids. Special mention must be made to conditionally essential amino acids which the body is capable of synthesising under normal conditions but not in special circumstances (premature babies, malnutrition, and/or severe or serious diseases) in which the requirements are much greater and the body´s needs are not well satisfied.
Digestion, Absorption And Metabolism
The digestion of proteins starts in the stomach by action of pepsin and continues in the intestine by action of pancreatic juices. Smaller peptides can be absorbed or converted into amino acids that are then transported to the liver.
Amino acids are converted in the liver. Approximately 58% of absorbed amino acids and peptides are converted into glucose and fat, the rest is used to form complex proteins in body issues.
When the body´s proteins are degraded through the natural destruction and recycling process, ammonia is produced and transported to the liver to form urea. Urea is also a toxic product and must therefore be eliminated from the body through urine.
Certain hormones such as insulin, testosterone and the growth hormone stimulate the production of proteins from amino acids (anabolism) whilst corticoids have the opposite effect (catabolism).
Function Of Proteins
- Energy function: As an energy nutrient and as carbohydrates, proteins provide 4 Kcal per gram of protein. However, these energy products are more costly for the individual and for the body (as their assimilation and utilisation requires greater energy expenditure).
- Plastic function: This is the main role of proteins, as they are used to form tissues and to perform very specific functions. 80% of dry weight (excluding water) of cells corresponds to proteins.
- Genetic regulation: Hereditary characteristics are contained in the proteins inside the nucleus of cells.
- Immunological function: The antibodies that intervene in immune processes are proteins.
- Regulating function: Enzymes (substances responsible for degrading or forming other products), hormones, body fluids and secretions all contain proteins. They act as regulators in the transport of some vitamins, minerals and fats.
- Homeostasis and acid-base balance.
Sources And Quality Of Proteins
- Protein sources: Proteins are found in foods of animal origin (main source of proteins for consumption) as well as in food of vegetable origin. Proteins of animal origin are found mainly in meats, fish, eggs, viscerae, milk and derivates.
Vegetable foods with the highest amounts of protein are legumes, soy, peanuts and nuts.
Vegetables and fruits also contain protein. Developed countries consume large amounts of protein, with over 50% being of an animal origin. In developing countries, protein sources are scanty and are of worse quality.
- Quality of proteins: The nutritional value, or quality, of proteins is useful to establish the capacity they have to meet the body's requirements. The quality of a protein is determined by several protein characteristics, including:
Thus, the highest quality of a protein is yielded by a high biological value, high digestibility (which ensures high net utilisation) and a high amino acid score.
A diet rich in proteins provides all essential amino acids; however, vegetal proteins are usually deficient in some essential amino acids. The mixture of vegetable foods with animal foods in adequate amounts results in some foods complementing others. This complementarity of proteins is very appreciated in countries where due to cultural, religious or economical reasons; animal proteins are restricted (for example, mixing cereals with milk, or rice with a small portion of fish). In this way, a mixture containing all the essential amino acids is obtained.