[2][3] For accurate determination of BV:[4] These conditions mean the tests are typically carried out over the course of over one week with strict diet control.
Fasting prior to testing helps produce consistency between subjects (it removes recent diet as a variable).
[6] Faecal excretion of nitrogen must also be taken into account - this part of the ingested protein is not absorbed by the body and so not included in the calculation of BV.
Where: Note: This can take any value from 0 to 100, though reported BV could be out of this range if the estimates of nitrogen excretion from non-ingested sources are inaccurate, such as could happen if the endogenous secretion changes with protein intake.
Due to experimental limitations BV is often measured relative to an easily utilizable protein.
The principal advantage of measuring BV relative to another protein diet is accuracy; it helps account for some of the metabolic variability between individuals.
Three major properties of a protein source affect its BV: Amino acid composition is the principal effect.
In a related way if amino acids are missing from the protein source which are particularly slow or energy consuming to synthesise this can result in a low BV.
Some of food preparation may damage or destroy some EAAs, reducing the BV of the protein source.
If critical minerals or vitamins are missing from the protein source this can result in a massively lowered BV.
The principal effect on BV in everyday life is the organism's current diet, although many other factors such as age, health, weight, sex, etc.
In particular, whilst on a high protein diet the BV of all foods consumed is reduced — the limiting rate at which the amino acids may be incorporated into the body is not the availability of amino acids but the rate of protein synthesis possible in cells.
The exclusion of digestibility is a point of misunderstanding and leads to misrepresentation of the meaning of a high or low BV.
BV provides a good measure of the usability of proteins in a diet and also plays a valuable role in detection of some metabolic diseases.
[21] Critics have pointed to research that indicates that because whey protein isolate is digested so quickly it may in fact enter the bloodstream and be converted into carbohydrates through a process called gluconeogenesis much more rapidly than was previously thought possible, so while amino acid concentrations increased with whey it was discovered that oxidation rates also increased and a steady-state metabolism, a process where there is no change in overall protein balance, is created.
[22] They claim that when the human body consumes whey protein it is absorbed so rapidly that most of it is sent to the liver for oxidation.
[23] Also, the study by Poullain et al., which is often cited to demonstrate the superiority of whey protein hydrolysate by marketers, measured nitrogen balance in rats after three days of starvation, which corresponds to a longer period in humans.
This flaw is supported by the FAO/WHO/UNU, who state that BV and NPU are measured when the protein content of the diet is clearly below that of requirement, deliberately done to maximize existing differences in quality as inadequate energy intake lowers the efficiency of protein utilization and in most N balance studies, calorie adequacy is ensured.
This is because of the ability of organisms to conserve and recycle EAAs as an adaptation of inadequate intake of the amino acid.
Also, because of their fur, rats are assumed to have relatively high requirements of sulphur-containing amino acids (methionine and cysteine).
As a result, the analytical method that is universally recognized by the Food and Agriculture Organization (FAO), World Health Organization (WHO), the U.S. Food and Drug Administration (FDA), the United States Department of Agriculture (USDA), United Nations University (UNU) and the United States National Academy of Sciences when judging the quality of protein in the human is not PER or BV but the Protein Digestibility Corrected Amino Acid Score (PDCAAS), as it is viewed as accurately measuring the correct relative nutritional value of animal and vegetable sources of protein in the diet.