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.

Effects of pectin and cellulose on growth, feed efficiency, and protein utilization, and their contribution to energy requirement and cecal VFA in rats.

Graded levels of cellulose (0 to 20%) or of pectin (0 to 10%) were incorporated in purified diets with either 22 or 8.5% casein as the protein source. Weanling male rats were fed these diets for 4 weeks. Body weight gain, food consumption, feed efficiency, protein biological value, apparent digestion of food, fiber and protein, cecal weight and cecal volatile fatty acids (VFA) were recorded. Cellulose was largely and inert diet diluent causing increased weight gains when protein was limiting but not when protein was sufficient. The 25% disappearance in the gut was due, at least in part, to compaction in the ceca and in increased cecal VFA. Pectin caused progressively greater depressions in body weight gain, food intake and efficiency and protein digestion at both levels of dietary protein. At least part of the 75% disappearance of pectin during passage through the gut could be attributed to increased cecal VFA. However, a rat growth bioassay for energy showed that pectin as well as cellulose, agar and gum arabic contributed no measurable energy to young rats.