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:

1. The true percentage utilization (usually shown with a percent symbol).
2. The percentage utilization relative to a readily utilizable protein source, often egg (usually shown as unitless).

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. Thomas, K. Ueber die biologische Wertigkeit der stickstoff-substanzen in 1909 verschiedenen Nahrungsmitteln. Arch. Physiol., 219. It is a popular guideline in bodybuilding in protein choice. Optimum Sports Nutrition: Your Competitive Edge, A Complete Nutritional Guide For Optimizing Athletic Performance; Chapter 12. by Dr. Michael Colgan