Bioavailability refers to the quality of a protein by looking at its retention in the body. It basically measures the amount of protein that it absorbed and used by the body from any food source. The Biological Value is another term used to describe the Bioavailability of protein, and it can be calculated by measuring the amount of nitrogen used for tissue formation, divided by the nitrogen absorbed from the diet…so basically it measures the amount of nitrogen absorbed that is actually retained by the body.
Nitrogen is used to gauge this because it is only found in protein, and not carbs or fat. Therefore the level of nitrogen in the blood and urine is an ideal marker of protein breakdown and absorption. If no nitrogen is excreted then this indicates that 100% of the protein ingested was utilised by the body for tissue synthesis.
Llewellyn, W. (2009). Sport Supplement Reference Guide. FL : Molecular Nutrition LLC.