Correlation of the plasma tyrosine to phenylalanine ratio with energy intake in self-selecting weanling rats

The relationship between changes in blood plasma amino acids and the quantity of protein and energy self-selected by the weanling rat, simultaneously offered two diets varying only in gluten (15 and 55%) concentration, was examined. Gluten and energy intakes were manipulated by additions of lysine, arginine or ammonia to gluten. In two experiments groups of ten weanling rats were fed the diets for a two week or four week period and food intake selection recorded. Blood samples were obtained between 0900–1100 hr at the end of the two week or four week period. Correlation coefficients of protein intake with the plasma TRP/NAA (Tyr+Phe+Leu+Val+Ile) ratios were ?0.97 and ?0.98, and of energy intake with TYR/PHE ratios were 0.77 and 0.70 in experiments 1 and 2, respectively. It is suggested that the plasma TRP/NAA and TYR/PHE ratios reflect the mechanisms regulating protein and energy intakes, respectively.