Metabolism of Essential Amino Acids in Growing Rats at Graded Levels of Soybean Protein Isolate

Abstract

The metabolic fates of the carbon skeletons of essential amino acids were investigated in growing rats fed on diets containing graded percentages of protein calories (5, 10, 15, 20, 30, and 40 PC%) using soybean protein isolate at 4, 100 kcal of metabolizable energy per kg of diet. The incorporation of ¹⁴C into the body protein 12h after the intraperitoneal injection of labeled essential amino acids was more than 60% of the dose in the 5 to 10 PC% groups, but it began to decline gradually in the higher PC% groups. The expired ¹⁴CO₂ production from labeled threonine, tryptophan, leucine, isoleucine, valine, phenylalanine, and lysine increased linearly with increasing levels of dietary soybean protein, but the rate of increase in the ¹⁴CO₂ production was lower in the higher PC% groups. In comparison, the ¹⁴CO₂ production from methio-nine, histidine, and arginine decreased in the 5 to 15 PC% groups, and it increased linearly in the higher PC% groups. The extent of ¹⁴CO₂ production varied among the essential amino acids in each dietary group. These results indicate that the carbon skeletons of essential amino acids are mainly utilized for body protein synthesis, but significant amounts of their carbons are oxidized to expired carbon dioxide for energy produc-tion, and that the metabolic responses of these amino acids to dietary protein level change at around 20 PC%, where the growth rate reached its approximate maximum. The utilization of individual essential amino acids in rats fed on the soybean protein diets changed markedly as compared to those on purified whole egg protein diets, the results of which have been reported elsewhere.