Effects of amino acid imbalance and protein content of diets on food intake and preference of young, adult, and diabetic rats

Effects of histidine or methionine imbalance and dietary levels (3-50%) of casein on food intake and preference of young, adult, and diabetic (2.5 month old) rats were examined. Depressions in food intake and growth caused by ingestion of the imbalanced diet were greatest in young rats and least or absent in diabetic rats. Alloxan diabetes induced hyperphagia and elevated concentrations of plasma branched-chain amino acids and decreased concentrations of tryptophan and tyrosine. The diabetic rats fed the imbalanced diet for 9 days had a higher concentration of the limiting amino acid in the plasma than the adult normal rats fed the same diet. The diabetic rats preferred the imbalanced diet over a protein-free diet when they were fed these diets concurrently. Ingestion of the imbalanced diet by normal rats caused greater changes in plasma and brain amino acid patterns than did the protein-free diet. Unlike the diabetic rats, the normal rats, especially the young rats, strongly preferred the protein-free diet over the imbalanced diet. The normal rats also preferred a 10% casein diet supplemented with L-methionine over a low or high casein diet. It seemed that young rats were able to select a protein diet that supported maximal growth when proportions of dietary amino acids were balanced. It also seemed that the susceptibility of the rats to amino acid imbalance varied directly with the status of overall protein synthesis of the animals.