Evaluation of Protein in Foods

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Abstract
In studies on factors influencing the protein efficiency ratio (PER) of foods, groups of male and female weanling rats received otherwise adequate diets containing 7, 10 or 15% protein supplied by casein or by a mixture of plant proteins (whole wheat and soybean flours). PER values (grams of gain per gram of protein consumed) were calculated at weekly intervals for 10 weeks. Female rats tended to give maximal PER values at lower dietary protein levels than did males. PER values found with both sexes dropped as the experiment progressed, but the decline was much greater with the animals fed casein than with those fed the plant protein mixture. In both sexes, the protein level required for maximal PER tended to decline as the experiment progressed. Differences between casein and the plant protein mixture were greatest during the early stages of the experiment, in both sexes. In further studies, differences in PER values found with several strains of rats were largely eliminated by the use of casein as an internal standard. It is concluded that the factors studied must be standardized if comparable results are to be obtained in different laboratories.