The Paired-Feeding Method in Nutrition Experiments and Its Application to the Problem of Cystine Deficiencies in Food Proteins

Abstract

The paired-feeding method has been used effectively in a number of investigations in which the effect of a variable food intake must be eliminated to permit of a clear interpretation of the results with reference to the one deliberately imposed variable. Its results may be readily analyzed by simple statistical methods. The method is particularly well adapted to a study of the amino acid deficiencies of proteins and of the mixtures of proteins and non-protein nitrogenous compounds occurring in natural foods. In this experiment it has been used in the determination of the existence of cystine deficiencies among a number of food products. The results appear to justify the following conclusions: The proteins of lean beef and of white bread are not deficient in cystine since their growth-promoting value for rats is not enhanced by supplementing with cystine. The proteins of navy beans, potatoes, milk, and garden peas are deficient in cystine in supplying the growth requirements of the rat. A repetition of the experiments on milk and garden peas confirmed strongly the results of the preliminary experiments on only 4 pairs of rats. From the experiments on peas, in particular, it appears that the paired-feeding method is capable of giving clear-cut results under the adverse condition of inadequate food consumption by the experimental animals. The addition of cystine to the garden pea ration did not modify the completeness of digestion of the nitrogenous compounds contained in it. Further evidence is presented to show that the fat content of the gains of rats made on the same amounts of rations of nearly identical make-up may differ widely.