Effect of dietary tryptophan levels on the urinary excretion of nicotinamide and its metabolites in rats fed a niacin-free diet or a constant total protein level.

We previously found that the sum total urinary excretion of nicotinamide and its metabolites in growing rats did not increase as the level of dietary casein increased. So, we investigated how the urinary excretion of nicotinamide and its metabolites changed when rats were fed nicotinic acid-free diets containing 10, 20 or 40% casein, or when they were fed diets containing nicotinic acid and the same level of protein but different levels of tryptophan. The latter diets were 10% casein-30% gelatin, 20% casein-20% gelatin or 40% casein. With the nicotinic acid-free diets, the sum total urinary excretion of nicotinamide and its metabolites was almost the same in the groups fed the 20 and 40% casein diets; however, it was significantly lower in the group fed the 10% casein diet. With diets containing the same level of protein, the sum total urinary excretion of these compounds in the groups fed the 10% casein-30% gelatin, 20% casein-20% gelatin and 40% casein diets were about 5, 7 and 11 mumol/d, respectively, values that were significantly different from each other. Therefore, it was found that only when the total protein intake was constant did the sum total urinary excretion of nicotinamide and its metabolites increase with increasing intake of tryptophan.