Leucine-isoleucine antagonism in the rat

Abstract

Diets are described containing 5 and 7% of l-leucine which cause severe growth retardation of rats for a period of up to 14 days. These diets are more convenient for a study of the mechanism of the growth depression than the diet containing 3% of l-leucine described previously (i). Appetite was depressed in proportion to leucine concentration of the diet. Growth retardation of rats fed a diet containing 5% of l-leucine was almost completely overcome by the addition of isoleucine and valine, and as little as 0.16% of l-isoleucine + 0.15% of L-valine were as effective as much higher concentrations. The growth-retarding action of an excess of dietary l-leucine has been demonstrated with a diet in which the protein was replaced by an amino acid mixture, suggesting that the site of antagonism is not in some phase of protein digestion. No effect of excess of dietary leucine on the over-all nitrogen absorption, on nitrogen retention, or on the rate of gastric emptying was observed. Injections of insulin counteracted the appetite-depressing effect of excess leucine in some rats. When the food intake of these rats increased, they grew more rapidly indicating that a high intake of leucine per se is not toxic.