Biological value of proteins

Is the protein we drink as a supplement effective?

You have probably often wondered why some whey proteins cost more than others, given that they contain the same percentage of protein concentration. The answer is on the labels. Most often, the price-determining factor is PER.

To clarify the properties of proteins, we need to consider their evaluation parameters. To do this, we offer you an excerpt from the scientific work of prof. Pencho Dalev, “Belts – sources and problems”.

What is the biological value (value) of proteins?

This is an indicator that determines the degree of efficiency with which the body absorbs the protein it has taken in.

Efficiency, in turn, is characterised by two indicators:

  • degree of protein retention in the body;
  • the balance of essential amino acids it contains.

So far, no absolute quantitative values have been derived to express the biological value (BV) of proteins, so a comparative value, a standard, is used.

In sports nutrition, this equivalent is egg white. Taking it as one or 100%, scientists correlate it to every other known dietary protein.

On the other hand, the average nitrogen content of proteins was found to be 16%. This is a fundamental starting point for all methods that qualitatively and quantitatively determine proteins. Two main groups of methods are used to assess BS – biological and chemical.

Biological methods for protein estimation

These methods are based on the use of experimental animals or microorganisms and are considered to be the most accurate.

Nitrogen balance

The foundations for its definition have been laid with the concept of Biological Value (BV). It is expressed by the ratio BV = B/A, where B is the nitrogen retained in the body, and A – the nitrogen absorbed from food through the digestive system.

An additional characteristic of nitrogen balance is the “Net Protein Utilization(NPU) index. It represents the ratio of retained nitrogen (B) to total nitrogen in the ingested food (I), i.e. NPU= B/I.

Another indicator for the evaluation of proteins has been introduced – digestibility (Digestibility, D), i.e. the absorption of digested protein.

The degree of digestibility is expressed by the ratio D = A/I, and hence it is clear that NPU = BV.D

To assess the degree of nitrogen retention in the body, there are various methods developed, one of them is based on the change in body mass with the intake of a unit mass of protein. This correlation is called the Protein Efficiency Ratio (PER).

PER= (W-Wk )/P, where P is the amount of protein contained in the feed ingested for 24 h and W and Wk is the mass of the animal at the end and beginning of the experiment, respectively. This method has been accepted as the standard in bodybuilding because it is simple and easily applicable to fast weight gain professional bodybuilders.

It should be noted that the weight of non-steroid-using bodybuilders has a lower gain per day – then this change is very difficult to measure.

However, can it be argued that the PER of a particular protein is a constant quantity in every organism?

To obtain PER values for a particular protein, studies are done on rats, therefore generally relating the amino acid composition of the defined protein to the amino acid requirements of rats rather than humans. However, there are important differences between the amino acid needs of rats and humans.

For example, rats have a higher need for sulphur-containing amino acids to support growth than newborn and adolescent children.

Reliance on the amino acid requirements of rats or the use of rats in studies to assess PER in humans may lead to a relative overestimation of the quality of certain animal protein as human food and a relative underestimation of the quality of at least some plant protein as human food.

Keep this in mind when reading PER values on packaging!

Biochemical methods for protein estimation

They are based on the assessment of the quantitative and qualitative determination of certain substances (free amino acids, for example) in body fluids that have a clear relationship with the amount of protein ingested as food. To assess the nutritional quality of proteins, the ratio of essential to replaceable amino acids in blood plasma is most often used – this is the so-called “plasma amino acid’s ratio” .

PAA-r =(B-A)/R, where B is the concentration of a certain amino acid in the blood plasma after feeding, A – the concentration of the same amino acid determined “fasting” and R – the body’s needs for this amino acid (this value is measured in advance).

Measurement by determination of creatinine in urine

The amount of creatinine excreted in the urine over 24 hours has been found to be a relatively constant quantity, dependent on body mass.

The measurement is made by the hydroxyproline index (HP-index), which is the ratio of the hydroxyproline concentration and the product of the creatinine concentration and the body mass in kilograms.

Chemical methods for protein estimation

The ratio in concentration in milligrams of a particular amino acid between one gram of the protein under study and one gram of the reference protein multiplied by one hundred is used as an index in chemical methods.

This ratio will be called the amino acid number (or also chemical sign – score, SC).

But egg protein is not always taken as the benchmark. Often casein, ovalbumin, egg-milk mixture, muscle proteins, etc. are used instead, which, however, leads to inaccuracies because they do not have a constant amino acid composition and confusions occur.

Therefore, the FAO/WHO accepts (after long research with, as it can be seen, relatively temporary results) an “ideal” protein with a well-defined amino acid content (1957).

In 1973, however, the FAO/WHO radically changed these values. The results are probably refuted by new research, which (alas) we don’t have, but still – take heart.

Here is a scale of essential amino acids according to FAO/WHO and their content in some complete proteins (milligrams of amino acid per gram of protein).

Amino acid “Ideal “egg white
Egg white Casein Milk protein Beef
1957г. 1973г.
Isoleucine 42 40 63 66 51 68
Levtsin 48 70 88 101 77 90
Lysine 42 55 70 82 81 63
28 60 99 58 38 113
22 35 58 33 19 54
Treonin 28 40 51 45 67 50
Tryptophan 14 10 15 15 33 17
Valin 42 50 68 74 59 74

Human amino acid requirements

A normal, non-exercising body needs 8 essential amino acids (AA): methionine, threonine, tryptophan, leucine, isoleucine, lysine, phenylalvinine and valine.

However, young or athletic bodies are constantly deficient in 2 otherwise body-produced ACs – arginine, glutamine (60% of the amino acids in muscle protein are glutamic) and histadine. This is because of their accelerated growth (muscles, bones, ligaments, etc.), which requires an increased supply of amino acids.

In case of deficiency of one or more AK, depending on the duration of this period, it is possible to develop “cachex syndrome” (weight loss, muscle breakdown) – due to chronic overtraining.

Chronic lack of certain essential AK results in loss of appetite, fatigue, irritability and negative values in the body’s nitrogen balance due to reduced food intake.


Amino acid Daily requirement of essential AC – mg/kg
Engaged in fitness bodybuilding Working hard labour Working intellectual labour
Protein g/kg 1,85 – 2,2 0,8 – 1,0 0,57
Isoleucine 70 – 85 30 – 42 10 – 27
Levtsin 161 – 183 45 – 75 14 – 35
Lysine 103 – 118 60 -78 12 – 23
125 – 132 27 – 41 12 – 20
58 – 66 27 – 39 14 – 23
Treonin 87 – 101 35 – 42 7 – 18
Tryptophan 17 – 19 4 – 6 4
Valin 93 -100 33 – 35 10 -12

There is also an interesting correlation. To maintain nitrogen balance in a person who consumes a balanced mixture of amino acids, it is necessary to provide a relatively high amount of energy.

In a clinical trial, it was found that when a balanced mixture of (replaceable and irreplaceable) ACs was used as the nitrogen source, 45.5 kcal per 1 kg was used to maintain equilibrium.

If raw protein, for example casein, is used instead, the nitrogen balance is maintained by 35 kcal per 1 kg of consumption. This fact is explained by the different rate of amino acid uptake into the gastrointestinal tract.

Free ACs are more rapidly absorbed by the protein because of the need for proteolytic digestion of the latter and because of the likelihood of the involvement of additional (not yet fully understood) regulatory mechanisms accompanying protein synthesis.

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