Diabetes, dietary tryptophan, and protein intake regulation in weanling rats.

The effects of streptozotocin-induced diabetes and tryptophan content of the protein fed on protein intake regulation by weanling rats selecting from 10 and 60% casein diets were evaluated. In uncompensated diabetes the ratio of tryptophan to other selected neutral amino acids in plasma and brain tryptophan were reduced, protein intake per unit of body weight was increased, and serotonin, 5-hydroxyindoleacetic acid, and norepinephrine were unaffected. Enrichment of the tryptophan content of the ingested protein caused a decrease in protein, but not energy consumption of both diabetic and nondiabetic rats. The reduction in protein intake correlated inversely with increases in the tryptophan content relative to the neutral amino acids in plasma and with increases in brain tryptophan and serotonin levels in both diabetic and nondiabetic rats. The data suggest that protein-feeding behavior is regulated by a mechanism that includes brain serotonergic activity with insulin, through its influence on circulating amino acids, determining the quantity of protein consumed in relation to body weight.