Relation between protein efficiency and net protein utilization.

The simplest and most convenient method of measuring the nutritive value of proteins is by an assessment of the protein efficiency ratio (P.E.R.) (gain in weight in g/g protein eaten). This measurement has been frequently criticized and, in fact, Mitchell (1924) stated that simplicity was its only recommendation. There are two main drawbacks to the method. Firstly, P.E.R., as measured on any particular protein, increases with the quantity of protein consumed (Mitchell, 1924, 1944; Barnes, Maack, Knights & Burr, 1945 ; Stewart, Hensley & Peters, 1943). A certain amount of protein is required for maintenance of body-weight, and only the increment above this amount is available for growth. Consequently, when larger quantities are consumed more is available for growth and a higher P.E.R. results. Secondly, the method is based on the assumption that the increase in body-weight on the protein-containing diet is proportional to the protein retained. It has been repeatedly shown that the composition of the weight increase varies with the type of diet during the 6-8 weeks of the usual experiments (Mitchell & Carman, 1926; Hamilton, 1939; Kik, 1938). The more complex method of determining the nutritive value of protein, namely, measurement of biological value (B.v.) or net protein utilization (N.P.u.), which represents the percentage of the absorbed or fed protein retained in the body, is not subject to these drawbacks. The B.V. is constant irrespective of the quantity of protein fed as long as that quantity does not exceed the animal’s capacity to synthesize body protein. A number of proteins and amino-acid mixtures have been examined both for P.E.R. and N.P.U. and the results compared. A preliminary report of this work has been published by Bender (1955).