Unlike some measures of protein usability, biological value does not take into account how readily the protein can be digested and absorbed (largely by the small intestine). This is reflected in the experimental methods used to determine BV.
BV, confusingly, uses two similar scales:
These two values will be similar but not identical.
The BV of a food varies greatly, and depends on a wide variety of factors. In particular the BV value of a food varies depending on its preparation and the recent diet of the organism. This makes reliable determination of BV difficult and of limited use — fasting prior to testing is universally required in order to make the values reliable.
BV is commonly used in nutrition science in many mammalian organisms, and is a relevant measure in humans. It is a popular guideline in bodybuilding in protein choice.
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).
There are two scales on which BV is measured; percentage utilization and relative utilization. By convention percentage BV has a percent sign (%) suffix and relative BV has no unit.
However direct measurement of Nr is essentially impossible. It will typically be measured indirectly from nitrogen excretion in urine. Faecal excretion of nitrogen must also be taken into account - this protein is not absorbed by the body and so not included in the calculation of BV.
This can take any value of 100 or less, including negative. A BV of 100% indicates complete utilization of a dietary protein, ie. 100% of the protein ingested and absorbed is incorporated into proteins into the body. Negative values are possible if excretion of nitrogen exceeds intake in proteins. All non-nitrogen containing diets have negative BV. The value of 100% is an absolute maximum, no more than 100% of the protein ingested can be utilized (in the equation above Ne(u), Ne(f) and Nb cannot go negative, setting 100% as the maximum BV).
Due to experimental limitations BV is often measured relative to an easily utilizable protein. Normally egg protein is assumed to be the most readily utilizable protein and given a BV of 100. For example:
Two tests of BV are carried out on the same person; one with the test protein source and one with a reference protein (egg protein).
This is not restricted to values of less than 100. The percentage BV of egg protein is only 93.7% which allows other proteins with true percentage BV between 93.7% and 100% to take a relative BV of over 100. For example, whey protein takes a relative BV of 104, while its percentage BV is under 100%.
The principal advantage of measuring BV relative to another protein diet is accuracy; it helps account for some of the metabolic variability between individuals. In a simplistic sense the egg diet is testing the maximum efficiency the individual can take up protein, the BV is then provided as a percentage taking this as the maximum.
Providing it is known which protein measurements were made relative to it is simple to convert from relative BV to percentage BV:
While this conversion is simple it is not strictly valid due to the differences between the experimental methods. It is, however, suitable for use as a guideline.
Amino acid composition is the principal effect. All proteins are made up of combinations of the 21 biological amino acids. Some of these can be synthesised or converted in the body, whereas others cannot and must be ingested in the diet. These are known as essential amino acids (EAAs), of which there are 9 in humans. The number of EAAs varies according to species (see below).
EAAs missing from the diet prevent the synthesis of proteins that require them. If a protein source is missing critical EAAs, then its biological value will be low as the missing EAAs form a bottleneck in protein synthesis. For example, if a hypothetical muscle protein requires phenylalanine (an essential amino acid), then this must be provided in the diet for the muscle protein to be produced. If the current protein source in the diet has no phenylalanine in it the muscle protein cannot be produced, giving a low usability and BV of the protein source.
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.
Methods of food preparation also have an impact on availability of amino acids in a food source. Some of food preparation may damage or destroy some EAAs, reducing the BV of the protein source.
Many vitamins and minerals are vital for the correct function of cells in the test organism. If critical minerals or vitamins are missing from the protein source this can result in a massively lowered BV. Many BV tests artificially add vitamins and minerals (for example in yeast extract) to prevent this.
The fine dependence on the individual's metabolism makes measurement of BV a vital tool in diagnosing some metabolic diseases.
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. This is a major point of criticism of BV as a test; the test diet is artificially protein rich and may have unusual effects.
The exclusion of digestibility is a point of misunderstanding and leads to misrepresentation of the meaning of a high or low BV.
These all hold specific advantages and disadvantages over BV, although in the past BV has been held in high regard.
Common foodstuffs and their values: Note: These values use "whole egg" as a value of 100, so foodstuffs that provide even more nitrogen than whole eggs, can have a value of more that 100. 100, does not mean that 100% of the nitrogen in the food is incorporated into the body, and not excreted, as in in other charts.
A further critique published in the Journal of Sports Science and Medicine states that the BV of a protein does not take into consideration several key factors that influence the digestion and interaction of protein with other foods before absorption, and that it only measures a proteins maximal potential quality and not its estimate at requirement levels. 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. The study found that whey protein hydrolysate led to better nitrogen retention and growth than the other proteins studied. However the study's flaw is in the BV method used, as starvation affects how well the body will store incoming protein (as does a very high caloric intake), leading to falsely elevated BV measures.
So, the BV of a protein is related to the amount of protein given. BV is measured at levels below the maintenance level. This means that as protein intake goes up, the BV of that protein goes down. For example, milk protein shows a BV near 100 at intakes of 0.2 g/kg. As protein intake increases to roughly maintenance levels, 0.5 g/kg, BV drops only around 70., concluded that "biological measures of protein quality conducted at suboptimal levels in either experimental animals or human subjects may overestimate protein value at maintenance levels." As a result, while BV may be important for rating proteins where intake is below requirements, it has little bearing on individuals with protein intakes far above requirements.
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. And because no population derives all of its protein exclusively from a single food, the determination of BV of a single protein is of limited use for application to human protein requirements.
Another limitation of the use of Biological Value as a measure of protein quality is that proteins which are completely devoid of one essential amino acid (EAA) can still have a BV of up to 40. This is because of the ability of organisms to conserve and recycle EAAs as an adaptation of inadequate intake of the amino acid.
Lastly, the use of rats for the determination of protein quality is not ideal. Rats differ from humans in requirements of essential amino acids. This has led to a general criticism that experiments on rats lead to an over-estimation of the BV of high-quality proteins to man because human requirements of essential amino acids are much lower than those for rats (as rats grow at a much faster rate than humans). 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 FAO/WHO as well as the FDA, USDA, United Nations University (UNU) and the 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. However, scientific studies demonstrate that the PDCAAS scale has limitations in predicting protein quality of those protein sources which may contain naturally occurring growth-depressing factors or antinutritional factors formed during alkaline and/or heat processing. In short, PDCAAS makes no distinction of their performance relative to each other because after they pass a certain point their scores are all capped at the maximum 1.0 and receive an identical rating. This is because in 1990 at an FAO/WHO meeting it was decided that proteins having values higher than 1.0 would be rounded or "leveled down" to 1.0 as scores above 1.0 are considered to indicate that the protein contains essential amino acids in excess of the human requirements. This approach implies injustice to high-quality proteins which can compensate for low-quality ones by virtue of their high content of essential amino acids (egg has an actual PDCAA score of 1.19 compared to 0.91 for soy, however when leveled down, they appear much closer).
Regardless, the scientific community has raised critical questions about the validity of PDCAAS.