The Supplemental Value of Cystine and Methionine for Low Protein (Casein) Diets Fed the Young White Rat

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Abstract
Varying amounts of L-cystine, DL-methionine, and these two sulfur-containing amino acids in mixture, were added to the diets of young white rats as supplements to a 7.5% casein diet (basal diet) which contained approximately 0.2% methionine and 0.03% cystine. Choline (as the hydrochloride) was added to all diets to provide a source of non-protein methyl groups. A significant increase in the rate of growth was observed when cystine was added to the basal diet, despite the fact that the methionine content of this diet was considerably lower than the amount usually accepted as adequate. Increases in the rate of growth comparable to those obtained with the cystine supplement were observed when supplements of methionine or of mixtures of cystine and methionine in varying proportions were added to the basal diet. Essentially the same growth responses with diets of equal sulfur content were observed even though the cystine sulfur ranged from 4 to 75% of the total organic sulfur. The supplementary value of cystine for the basal casein diet would indicate that, under the conditions of these experiments, the cystine requirements may exceed those for methionine. These requirements may be filled by cystine present in the diet or by cystine synthesized from dietary methionine. The data do not indicate that greater physiological economy is achieved by inclusion in the diet of cystine itself rather than methionine from which cystine may be synthesized. Under the conditions of the present experiments, commerical lactalbumin was more effective in promoting the growth of young white rats than was casein (7.5%) in equivalent amounts (nitrogen content), despite the fact that the methionine content of the diet used in this study was less than that of any other diet in the series. In contrast to the results with the basal (7.5%) casein diet, it was not possible to increase significantly the rate of growth when lactalbumin was supplemented with DL-methionine or L-cystine.