The determination of the nutritive value of a protein by its effect on liver nitrogen in rats.

Henry, Kosterlitz & Quenouille (1953) have described a method of determining the nutritive value of a protein by the effect it has on the quantity of protein in the liver of the rat. They found that the liver nitrogen, expressed as mg/Ioo g body-weight, was correlated with the biological value of the dietary protein and with the amount of protein eaten. When the proteins were tabulated according to the nutritive values obtained by this method, they were, on the whole, arranged in the order of the net utilization values estimated by the balance-sheet method of Mitchell (Mitchell, 1923-4; Mitchell & Carman, 1926). Soya-bean protein appeared to be an exception in that the liver-N method yielded for it rather low nutritive values. The liver-N method was based on the fact that, for relatively small protein intakes, the values of liver N (mg)/Ioo g initial body-weight varied linearly with the amount of a protein eaten, provided the nutritive value of the protein was not better than that of casein. Whole-egg protein, for instance, gave a non-linear regression line. It therefore appeared desirable to attempt to determine the shape of the dose-response curve over a wide range of protein intakes. Unfortunately, we have to report that experiments designed to achieve this aim had only limited success and that it was not possible to improve on the sevenand eight-point assays described in the earlier paper; these assays were used to assemble more information on nutritive values of various proteins and to assess the effects of supplementation of proteins by their limiting amino acids. Though inherent difficulties of the method might have been responsible for some of the discrepancies observed between the results obtained by the liver-N and the balance-sheet methods, other possibilities had to be explored. Since hormonal imbalances may affect liver protein and N balance in opposite directions, the results of a few preliminary experiments on the effects of cortisone on the nutritive values of proteins are included in this paper.