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Effect in Man of the Addition of Tryptophan or Niacin to the Diet on the Excretion of Their Metabolites

Department of Nutrition and Biochemistry, Institute of Public Health, Tokyo, Japan

Two experiments, each lasting 16 days, were carried out, the second experiment 1 month after the first one finished. Six healthy college women served as experimental subjects. The basal diet contained natural amino acid mixture, cornstarch, corn oil, minerals, and vitamin mixture, including 0.25 g of tryptophan and 0 mg of niacin. In experiment 1, tryptophan was added to the basal diet at different levels (0.50, 0.75, 1.25, and 1.75 g), keeping the amount of niacin at zero. In the second experiment, niacin was added to the basal diet at different levels (8.3, 12.5, 20.8, and 29.2 mg), keeping the amount of tryptophan at a constant level. Urinary excretion of total nitrogen, urea, creatinine, N-methyl-2-pyridone-5-carboxamide, N¹-methylnicotinamide, nicotinic acid, kynurenic acid, and xanthurenic acid was determined and, in the second experiment, the levels of NAD and NADP in blood plasma were also measured. All the subjects were in good health and their body weight was at a constant level throughout the experiment, nitrogen equilibrium being maintained. In experiment 1, excretion of niacin metabolites increased markedly on the addition of 1.25 g of tryptophan to the basal diet, and in the second experiment, excretion of these metabolites increased on the addition of 8.3 mg of niacin to the basal diet. The amount of niacin metabolites excreted on the addition of tryptophan or niacin was compared with that excreted during the basal diet period. The relationship between the amount of tryptophan added and three niacin metabolites seemed to be linear from 1.25 to 1.75 g of tryptophan added, and that between the amount of niacin added and niacin metabolites was also linear from 8.3 to 29.2 mg of niacin. The ratio of the amount (in mg) of tryptophan to that (in mg) of niacin added by which the same excretion of niacin metabolites appeared was calculated, i.e., how much tryptophan is convertible to niacin: this ratio decreased from about 122 to about 75 as the amounts of added tryptophan and niacin increased. The levels of NAD and NADP in blood plasma did not change significantly.