The ineffectiveness of supplementary cystine in legume-based rat diets.

The effectiveness of cystine supplementation in improving the nutritional quality of diets deficient in sulfur amino acids was investigated by measuring the weight gain and protein efficiency ratio of rats fed basal 10% protein diets and basal diets supplemented with either 2.0 g/kg DL-methionine or 4.0 g/kg L-cystine. Whereas methionine improved the weight gain of rats and the protein quality of all sources of dietary protein tested, cystine gave a positive response with diets containing lupin tops, casein, yeast and leaf protein concentrate but was ineffective for sweet lupin seed, soybean meal, broad beans and lentils. The ineffectiveness was observed as a general depression of weight gain and protein efficiency ratio. The depression was statistically significant for some diets containing lentils as the sole source of protein. The endogenous methionine and cystine contents of the diets were determined. and the ratios of cystine to methionine were correlated with the responses to supplementary cystine. Generally supplementary cystine was effective where the ratio was less than one and was ineffective where greater than one. The data are discussed in relation to the generally accepted concepts of the dietary methionine-cystine interrelationship.