The biological value (VB) is a parameter of evaluation of the plastic proteins introduced into the body with food. This index, which is expressed with a numerical value, refers to the quantity, quality and reciprocal relationship of the essential amino acids present in food peptides. Ultimately, the biological value is a nutritional aspect that describes the "protein quality and the plastic potential of the amino acids contained in foods".
Amino acids, essential and branched amino acids
Essential amino acids are molecules that the human organism is NOT able to synthesize from scratch and that, among the various functions, constitute the precursors of the other NON ESSENTIAL amino acids; therefore, it is necessary to introduce them regularly with the power supply.
As already explained, the content and the relationship between essential amino acids determines the biological value of proteins and foods; therefore, in order to obtain a balanced diet it is essential to recognize them and respect their recommended daily intake ration:
Among the essential amino acids some are distinguished by the high potential of neoglucogenesis (conversion to glucose to produce energy); is the case of the branched amino acids (BCAA): VALINA, LEUCINA and ISOLEUCINA. In addition to contributing to the increase in the biological value of proteins, they are particularly important in endurance athletes (whose oxidative energy needs increase significantly), in patients who are weak (hepatopathies and nephropathies) and in patients with a strict diet (slimming food therapy ). The optimal ratio for the three BCAAs in the diet and supplements is 2: 1: 1 (two parts of leucine, a part of isoleucine and a part of valine).
Impact of biological value on metabolism
The impact of the biological protein value on the metabolism is calculated by evaluating the nitrogen [N] introduced with foods or supplements, the unabsorbed one (expelled with feces) and the one eliminated with urine. Ultimately, it is possible to state that the biological value of dietary proteins can be estimated by comparing the nitrogen retained with that absorbed:
VB = (N food - N feces - N urine) / (N food - N urine)
Obviously, if it were so simple to calculate the biological value of proteins, the scientific community would not yet be engaged in evaluating the importance that it plays in the food and supplementary fields. In fact, there are other variables that affect the estimate of biological value; it is:
- Fecal metabolic nitrogen, deriving from: digestive enzymes, bile juices, mucous membranes of the digestive tract, catabolites and remains of the physiological bacterial flora
- Endogenous urinary nitrogen, deriving from the catabolism of tissue turnover
The biological value of dietary proteins is given by the similarity in the amino acid composition with respect to human proteins; it follows that the polymers of animal origin (especially eggs and milk) have a greater biological value than those of bacteria or of vegetable origin (medium or low VB). However, if it is true that this parameter takes into account the quantity, quality and ratio of the essential amino acids contained in the proteins of a food, it is equally true that MORE foods with medium and low biological value can complement each other mutually. In simple terms, it is not necessary to consume mainly proteins of animal origin to reach the share of essential amino acids, but it is possible (and in some cases advisable) to associate different proteins of medium or low biological value (cereals, legumes, vegetables, mushrooms, fruit ...) and get the same result. Obviously, it goes without saying that by combining foods characterized by medium and low biological value proteins such as "legumes and cereals", the nutritional picture undergoes a significant variation; the nutritional modifications of the substitution of products of animal origin with those of vegetable origin are:
- Increased carbohydrate intake
- Reduction in the intake of saturated fat and cholesterol
- Increased fiber intake
- Increased intake of polyunsaturated lipids
- Increased intake of other useful molecules such as lecithin
Cereals and legumes association
To compensate for the lack of high biological value proteins, in some cases (as in veganism), it is advisable to resort to the frequent consumption of dishes deriving from the association of several foods, in particular cereals and legumes. Cereals are characterized by a low biological value given by the low presence of tryptophan and lysine (1.5-2.5%); the latter essential amino acid is instead present to a greater extent in proteins with a medium biological value of legumes (4-5.5%); in parallel, the legumes are deficient in METHIONINE and CISTEIN, however these are present in good measure in cereals. Proteins of high biological value (eggs, milk, meat and fishery products) contain percentages of lysine that are around 7% of the amino acid pool.
Estimate the biological value
To give a reference to the biological value of dietary proteins it is first of all essential to verify whether these are defective in one or more amino acids, which in this case would be called "limiting amino acids"; secondly, it is necessary to estimate HOW the amino acid is limiting. This parameter, also called PROTEIN INDEX, is expressed as a percentage and refers to the nutritional requirements of each essential amino acid of the proteins in question; for example, a COMPLETE protein such as the egg has a protein index of 100, because all the essential amino acids are present in the right portion, while a cereal polypeptide could have a protein index of 75 due to lysine deficiency, because the latter it is present in quantities that reach only 75% of the requirement. Ultimately, it is possible to state that the protein index determines the biological value BUT its impact on nutrition is as important as the consumption portions; in fact, despite having a reduced protein index or biologist value, a more or less abundant portion of only legumes (medium biological value) is able to cover or almost the entire requirement of essential amino acids.