Some Aspects of Tryptophan and Niacin Metabolism in Young Women Consuming a Low Tryptophan Diet Supplemented with Niacin

Publisher Website
Google Scholar
Abstract
The metabolic use of supplements of niacin was studied in 6 college women transferred from a nutritionally adequate control diet of ordinary foods to a semipurified diet low in tryptophan and niacin. Criteria of evaluation were nitrogen balance, blood pyridine nucleotide levels, and urinary excretion of 6 tryptophan and 2 niacin metabolites. Nitrogen loss occurred with a tryptophan intake of 25 mg; nitrogen storage occurred on intakes of 160 or 185 mg. Niacin supplementation had no apparent effect on the amounts of nitrogen excreted. Conversely, the amount of nitrogen excreted did not correlate with metabolite excretion. Blood pyridine nucleotide levels decreased following the second period in which the diet was limiting in tryptophan and niacin, and 10 mg of niacin in the presence of 160 mg tryptophan re-established control levels. Additional niacin did not increase values further. The excretion of tryptophan metabolites tended to respond to the level of the tryptophan ingested, decreasing when tryptophan was limiting and increasing slightly when the tryptophan intake was increased. Niacin supplementation had no apparent effect on tryptophan metabolite excretion. Under these experimental conditions it appears that supplements of niacin added to a diet containing tryptophan at a level just sufficient to establish and maintain nitrogen balance are used first to establish and maintain the levels of the blood pyridine nucleotides and only then is there an increase in the excretion of urinary niacin metabolites.