Influence of protein and amino acids on food intake in the rat

Abstract

The depression of food intake by high levels of dietary protein and by an excess of l-leucine in a low-protein diet was found to be independent of the presence of the hypothalamic mechanism regulating food intake. At extremely high levels of protein or amino acid intake, the amount of diet ingested was restricted to a similar level in normal and hyperphagic rats. The fact that hyperphagic rats consumed a relatively constant amount of casein in diets containing 60– 90% of protein suggested the possibility that a physiological "safety valve" operated to limit dietary intake so that protein intake did not exceed a certain threshold. An accumulation of amino acids that could not be metabolized or diverted into protein synthesis may have mediated the appetite effects. This suggestion was consistent with the finding that prefeeding high levels of protein (40 or 60% of casein), a procedure which is known to enhance amino acid catabolic activity, temporarily eliminated the appetite depression normally caused by subsequent ingestion of a high-leucine diet.