Biological value
Biological value (BV) is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body. It summarises how readily the broken down protein can be used in protein synthesis in the cells of the organism. Proteins are the major source of nitrogen food, unlike carbohydrates and fats. This method assumes protein is the only source of nitrogen and measures the proportion of this nitrogen absorbed by the body which is then excreted. The remainder must have been incorporated into the proteins of the organisms body. A ratio of nitrogen incorporated into the body over nitrogen absorbed gives a measure of protein 'usability' - the BV.
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:
- The true percentage utilization (usually shown with a percent symbol).
- The percentage utilization relative to a readily utilizable protein source, often egg (usually shown as unitless).
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 body building in protein choice.

# Determination of BV
For accurate determination of BV:
- the test organism must only consume the protein or mixture of proteins of interest (the test diet).
- the test diet must contain no non-protein sources of nitrogen.
- the test diet must be of suitable content and quantity to avoid use of the protein primarily as an energy source.
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.

## Percentage utilization
Biological value is determined based on this formula.
Where:
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.
Where:
Note:
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).

## Relative utilization
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).
Where:
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.

## Conversion
Providing it is known which protein measurements were made relative to it is simple to convert from relative BV to percentage BV:
Where:
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.

# Factors which affect BV
The determination of BV is carefully designed to accurately measure some aspects of protein usage whilst eliminating variation from other aspects. When using the test (or considering BV values) care must be taken to ensure the variable of interest is quantified by BV. Factors which affect BV can be grouped into properties of the protein source and properties of the species or individual consuming the protein.

## Properties of the protein source
Three major properties of a protein source affect its BV:
- Amino acid composition, and the limiting amino acid, which is usually lysine
- Preparation (cooking)
- Vitamin and mineral content
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.

## Properties of the test species or individual

### Under test conditions
Variations in BV under test conditions are dominated by the metabolism of the individuals or species being tested. In particular differences in the essential amino acids (EAAs) species to species has a significant impact, although even minor variations in amino acid metabolism individual to individual have a large effect.
The fine dependence on the individual's metabolism makes measurement of BV a vital tool in diagnosing some metabolic diseases.

### In everyday life
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. all have an effect. In short any condition which can affect the organism's metabolism will vary the BV of a protein source.
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.

## Factors with no effect
BV is designed to ignore variation in digestibility of a food - which in turn largely depends on the food preparation. For example compare raw soy beans and extracted soy bean protein. The raw soy beans, with tough cell walls protecting the protein, have a far lower digestibility than the purified, unprotected, soy bean protein extract. As a foodstuff far more protein can be absorbed from the extract than the raw beans, however the BV will be the same.
The exclusion of digestibility is a point of misunderstanding and leads to misrepresentation of the meaning of a high or low BV. and also it has to be about 25%.

# Advantages and disadvantages
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. BV is, however, a scientific variable determined under very strict and unnatural conditions. It is not a test designed to evaluate the usability of proteins whilst an organism is in everyday life - indeed the BV of a diet will vary greatly depending on age, weight, health, sex, recent diet, current metabolism, etc. of the organism. In addition BV of the same food varies significantly species to species. Given these limitations BV is still relevant to everyday diet to some extent. No matter the individual or their conditions a protein source with high BV, such as egg, will always be more easily used than a protein source with low BV.

## In comparison to other methods
There are many other major methods of determining how readily used a protein is, including:
- Net protein Utilization (NPU)
- Protein Efficiency Ratio (PER)
- Nitrogen Balance (NB)
- Protein digestibility (PD)
- Protein Digestibility Corrected Amino Acid Score (PDCAAS)
These all hold specific advantages and disadvantages over BV, although in the past BV has been held in high regard.

# In animals
The Biological Value method is also used for analysis in animals such as cattle, poultry, and various laboratory animals such as rats. It was used by the poultry industry to determine which mixtures of feed were utilized most efficiently by developing chicken.
Although the process remains the same, the biological values of particular proteins in humans differs from their biological values in animals due to physiological variations.

# Typical values
Common foodstuffs and their values: Note: this scale uses 100 as 100% of the nitrogen incorporated.
- Isolated Whey: 100
- Whole bean: 96
- Whole Soy Bean: 96 
- Human milk: 95
- Chicken egg: 94
- Soybean milk: 91
- Cow milk: 90
- Cheese: 84
- Rice: 83
- Defatted soy flour: 81
- Fish: 76
- Beef: 74.3
- Immature bean: 65
- Full-fat soy flour: 64
- Soybean curd (tofu): 64
- Whole wheat: 64
- White flour: 41
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.
- Whey protein concentrate: 104
- Whole egg: 100
- Cow milk: 91
- Beef: 80
- Casein: 77
- Soy: 74
- Wheat gluten: 64

# Criticism
Since the method measures only the amount that is retained in the body critics have pointed out what they perceive as a weakness of the biological value methodology. 
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.  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.  Hence they believe the reason so much is retained is that it is used for energy production not protein synthesis. This would bring into question whether the method defines which proteins are more biologically utilizable.
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.
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 a 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
-f 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.Template:Specify
On the other hand, BV makes a differentiation with respect to proteins with similar protein values such as egg whites at 88, cow's milk at 91, and whole eggs at 100.