Biological Value (%) is calculated as retained nitrogen/ absorbed nitrogen X 100. To illustrate how this provides an evaluation of protein quality, consider a scenario in which all amino acids in a protein are used for protein synthesis. Here, retained and absorbed nitrogen would be equal and the biological value would equal 100%.
Conversely, if some indispensable amino acids are not available in the protein source, protein synthesis is impaired and some unused nitrogen is excreted from the body, retained nitrogen decreases relative to absorbed and the biological value drops below 100%. Consequently, some foods such as complete protein foods (e.g., milk) have high biological values (95%), while incomplete protein foods are much lower (Le., corn [60%1). (more…)