Amino Acid Balance and Food Intake: Effect of Different Dietary Amino Acid Patterns on the Plasma Amino Acid Pattern of Rats

The objective of this study was to test further the aminostatic hypothesis of food intake regulation, i.e., that elevated concentrations of plasma amino acids that cannot be channeled into protein synthesis may serve as a satiety signal for a food intake regulating mechanism and thereby result in depressed food intake. Groups of rats were fed one of a series of diets containing 6% casein and isonitrogenous amounts (0.7%) of incomplete mixtures of amino acids differing greatly in amino acid composition. Mixtures containing large amounts of indispensable amino acids depressed food intake much more than those containing large amounts of dispensable amino acids, as was anticipated, since activities of enzymes for the degradation of indispensable amino acids are ordinarily lower than those for the degradation of dispensable amino acids in animals fed a low protein diet. Relationships between food intake of rats fed ad libitum and plasma amino acid concentrations 5 hours after force-feeding a single meal of each diet were investigated before and after the animals had become adapted to the diets. The coefficient of correlation observed between food intake and plasma total indispensable amino acid concentration (-0.68) indicates that the hypothesis is valid only within certain limits. Concentrations of some amino acids appear to be more critical than others in giving rise to a stimulus that initiates food intake depression, and differences in plasma amino acid pattern cause deviations from the relationship. The slope of the straight line representing plasma total indispensable amino acids versus food intake became steeper as animals became adapted to the diets, presumably because increased capacity for amino acid degradation enabled them to consume more of the diets before the stimulus initiating food intake depression was activated.