Effects of excess leucine on growth and tryptophan and niacin metabolism in rats.

The effects of an excess intake of leucine, for a short period (3 to 4 weeks) and a long period (12 weeks), on growth and excretion of tryptophan and niacin metabolites in rats were studied. Excess intake of leucine (4 or 5%) for a short period did not affect increase in body weight or organ weight (liver, kidney and heart) compared with those of rats fed the control diet. Excess intake of leucine for a long period resulted in a decrease in body weight, and an increase in the relative weight of the liver and kidney. Urinary excretions of N1 methyl nicotinamide (N-MNA), N1-methyl-2-pyridone-5-carboxamide (2 Py), niacin, quinolinic acid and 5-hydroxyindole acetic acid did not differ in rats fed an excess-leucine diet and rats fed a control diet. In niacin deficiency, the excretions of N-MNA, 2-Py and NiA decreased. Excess leucine intake increased the plasma levels of leucine and alanine and decreased those of valine, isoleucine, tryptophan and cystine. Excess leucine intake decreased the levels of valine, isoleucine, methionine and leucine in the liver, but these changes were not observed in niacin deficient rats. Changes in the levels of nicotinamide mononucleotide (NMN), nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP) in erythrocytes, suggested to occur in pellagra, were not observed. These findings do not support the idea that an excess intake of leucine results in changes in metabolism similar to those observed in pellagra.