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Biological value is a term applied to specific proteins that describes the degree to which an organism can absorb and utilize a given protein. When proteins are absorbed from food, the body breaks them down and uses them to produce more proteins. The proteins in some foods can be broken down and used to a much greater degree than the protein in other foods. Eggs, for example, contain significantly more protein than white flour, so eggs have a higher biological value. People looking to build muscle require a significant amount of protein in their diets; they often consider the biological value of the food they choose to eat.
There are two different scales used in the measurement of biological value. The first is simply a percentage-based scale; a protein that is absorbed completely would be given a 100% value while a protein that is only half absorbed would be given a 50% value. The other scale sets some particular protein as the 100 mark (percent signs are usually omitted in this scale) and measures other proteins based on that one. Eggs are often assigned a value of 100 in this scale, as they have a high biological value. A protein that is absorbed to a greater degree, then, could have a value above 100.
Proteins are usually the main source of nitrogen in an individual's diet, so nitrogen concentration is used to measure biological value. The amount of nitrogen in the proteins of a given food is measured, as is the amount of nitrogen the organism eventually excretes. With this method of measurement, it is assumed that protein is, in fact, the only source of nitrogen in one's diet. It follows, then, that the amount of protein can be calculated from the difference between the amount of nitrogen in the initial protein and the amount of nitrogen excreted. The nitrogen not excreted belongs to proteins, which are broken down and used to construct other proteins in the body.
This method of measuring biological value has its weaknesses. Age, weight, gender, general fitness, and many other factors can affect the degree to which proteins are absorbed in the body. As such, studies involving biological value are usually quite rigorous; test subjects tend to be on strict diets and do not engage in strenuous activities that could result in the use of protein as an energy source. Such precautions generally result in relatively accurate results.