A measure of the usefulness of a dietary protein for growth and maintenance of tissue, and, in animals, production of meat, eggs, wool, and milk. It is important only if the total intake of protein barely meets the requirement. The quality of individual proteins is unimportant in mixed diets, because of complementation between different proteins.
Two types of measurement are used to estimate protein quality: (a)biological assays and (b) chemical analysis.
(a) Biological Value (BV) is the proportion of absorbed protein retained in the body (i.e. taking no account of digestibility). A protein that is completely useable (e.g. egg and human milk) has BV = 0.9–1.0; meat and fish have BV = 0.75–0.8; wheat protein, 0.5; gelatine, 0.
Net Protein Utilization (NPU) is the proportion of dietary protein that is retained in the body under specified experimental conditions (i.e. it takes account of digestibility; NPU = BV × digestibility). By convention NPU is measured at 10% dietary protein (NPU10) at which level the protein synthetic mechanism of the animal can utilize all of the protein so long as the balance of essential amino acids is correct. When fed at 4% dietary protein, the result is NPU standardized. If the food or diet is fed as it is normally eaten, the result is NPU operative (NPUop).
Protein Efficiency Ratio (PER) is the gain in weight of growing animals per gram of protein eaten.
Net Protein Retention (NPR) is the weight gain of animals fed the test protein, minus the weight loss of a group fed a protein-free diet, divided by the protein consumed.
Protein Retention Efficiency (PRE) is the NPR converted into a percentage scale by multiplying by 16: it then becomes numerically the same as Net Protein Utilization.
Relative Protein Value (RPV) is the ability of a test protein, fed at various levels of intake, to support nitrogen balance, relative to a standard protein.
(b) Chemical Score is based on chemical analysis of the protein; it is the amount of the limiting amino acid compared with the amount of the same amino acid in egg protein.
Protein Score is similar to Chemical Score, but uses an amino acid mixture as the standard, also known as amino acid score. Protein digestibility-corrected amino acid score (PDCAAS) takes account of the digestibility of the protein as well as its amino acid composition.
Essential amino acid index is the sum of all the essential amino acids compared with those in egg protein or the amino acid target mixture.
Subjects: Medicine and Health.