Biological value is a measure of the proportion of protein absorbed from a food and its incorporation into the body's own Protein. It easily indicates how the digested protein can be used in protein synthesis in the body's cells. Protein is the main source of nitrogen in food. BV measures the proportion of this nitrogen absorbed by the body and that excreted. The absorbed part is incorporated into the body Protein that form the organism. The ratio of nitrogen incorporated in the body and nitrogen absorbed gives the measure of the "usability" of Protein: The BV.
Unlike other methods for determining the usability of a protein source, the BV Biological Value does not take into account how a protein can be digested and absorbed. The biological value uses two similar scales: 1- true use of the percentage (usually indicated with a percentage symbol) 2- percentage use related to an easily usable protein source, often eggs These two values will be similar but not identical The BV is a parameter commonly used in nutrition science.  Often popular in the bodybuilding world for choosing protein sources.  
Determination of the BV
Per the exact determination of the BV biological value : 
1- The test organism must consume only protein or a mixture of Protein of interest (test diet)
2- The test diet must contain non-protein sources of nitrogen
3- The test diet must be of appropriate contents and quantities in order to avoid the use of Protein as a source of energy
These test conditions are typically carried out over the course of more than a week with strict dietary controls. Furthermore, fasting before the test helps to obtain more consistent and reliable data.
There are two scales on which the biological value BV is measured: percentage of utilization and relative utilization. In the utilization percentage the BV has a percentage sign (%), and in the relative utilization the BV has no units.
Percentage of use
The biological value is determined by this basic formula  
BV = ( N r / N a ) * 100
N a = absorbed nitrogen of the Protein present in the test diet
N r = nitrogen incorporated in the body in the test diet
However, the direct measurement of N r is essentially impossible. It will typically be measured indirectly and accounted for urinary nitrogen excretion  and fecal nitrogen excretion. This part of ingested protein is not absorbed by the body and therefore is not included in the BV calculation.
BV = (( N i - N e (f) - N e (u) ) / ( N i - N e (f) )) * 100
N i = intake of nitrogen in Protein in the test diet
N e (f) = (nitrogen excreted with faeces, on test diet) - (nitrogen excreted with faeces not from ingested nitrogen)
N e (u) = (nitrogen excreted in urine, on test diet) - (nitrogen excreted in urine not from ingested nitrogen)
N r = N i - N e (f) - N e (u)
N a = N i - N e (f)
This can take any value from 0 to 100. A BV of 100% indicates full use of a protein. that is, 100% of the Protein taken in and absorbed are incorporated into the Protein of the body. The 100% value is an absolute maximum, no more than 100% of the Protein taken can be used (in the equation above N e (u) and N and (f) cannot go negative, setting 100% as maximum BV.
Due to experimental limitations, the BV Biological Value is often measured against an easily usable protein. Egg protein is normally considered the most readily usable protein and given a BV of 100. Per example:
Two BV tests are performed on the same person; one with the test protein source and one with the reference protein (egg protein)
relativo BV = ( BV(test) / BV(egg) ) * 100
BV (test) = BV percentage of the test diet for that individual
BV (egg) = percentage BV of the reference diet (egg) for that individual
This is not limited to values below 100. Egg protein BV percentage is only 93.7% which allows other Protein with BV percentage between 93.7% and 100% to take a relative BV of over 100 Per example whey protein takes a relative BV of 104, while its BV percentage is less than 100%.
Properties of protein sources
Three major properties of a protein source affect its BV:
1- Amino acid composition, and the limiting amino acid, which is usually lysine
2- Preparation (cooking)
3- Vitamin and mineral content
The amino acid composition is the main effect. All Protein are made up of combinations of the 21 biological amino acids. Some of these can be synthesized or converted in the body, while others can and should be ingested with the diet. These are known as essential amino acids (EAAs), of which there are 9 in humans. Missing EAAs in the diet prevents protein synthesis. If a protein source lacks critical EAAs, then its BV will be evaluated from the missing EAAs that form a bottleneck for protein synthesis. Per example, if a muscle protein requires phenylalanine (an essential amino acid), then it must be supplied with the diet to synthesize muscle protein. If the protein source in the diet does not have phenylalanine, the muscle protein cannot be produced,
Food preparation methods have an impact on the availability of amino acids in a food source. Some preparations and cooking can damage or destroy some EAAs, reducing the BV of a protein source.
Many vitamins and minerals are essential for the proper functioning of cells in the body. If minerals or vitamins are critically present in the protein source, the BV can be massively lowered. Many BV tests artificially add vitamins and minerals (e.g. in yeast extract) to avoid this.
Comparison with other methods
There are several other important methods for determining the usability of a protein source, including:
Net protein Utilization (NPU)
Protein Efficiency Ratio (PER)
Nitrogen Balance (NB)
Protein digestibility (PD)
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
Common foods and their values: (Note: this scale uses 100 as 100% of the incorporated nitrogen)
Whey protein: 96 
Soybeans: 96 
Human milk: 95 
Chicken egg: 94 
Soy milk: 91 
Buckwheat: 90 
Cow's milk: 90 
Cheese: 84 
Quinoa: 83 
Rice: 83 
Soybean meal: 81 
Fish: 76 
Meat: 74 
Bean: 65 
Common foods and their values:  (Note: these values use egg as a reference value (100). So foods that provide even more nitrogen than whole eggs can have a value of more than 100. This does not mean that 100% of the nitrogen in food is incorporated into the body, and not excreted)
Concentrated whey protein: 104
Whole egg: 100
Cow's milk: 91
Wheat gluten: 64
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- The Great Animal Versus Vegetable Protein Debate What Is The Best Protein Per Muscle Growth?
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