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, Le., 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). It is not possible to have a biological value > 1 00%. Net Protein Utilization is a similar measure but involves a direct measure of retained nitrogen, Le., destruction and analysis of the body, and therefore is only used with animal studies.
Protein Efficiency Ratio is determined as body mass gain/protein intake and, although a useful measure when used in growing animals, has little value for humans for two reasons. First, this method is not used in human experiments because, ethically, human subjects cannot be deprived of necessary nutrients. Second, the difference in species growth rates, Le., lower in humans versus rats, results in errors when animal data are used to estimate human needs. Protein Digestibility-Corrected Amino Acid Score is based on the amino acid content of a food corrected for digestibility. The score reflects the limiting indispensable amino acids (those amino acids that humans cannot synthesize) in a food. The reference value used is the requirement for a 2-5-year-old individual. This provides a built-in safety buffer because protein needs of this age group are greater than for adults.