Vitamin B6 Depletion in Man: Urinary Excretion of Quinolinic Acid and Niacin Metabolites

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Abstract
The effect of vitamin B6 depletion on the conversion of tryptophan to niacin by 6 male subjects was studied during a 55-day experiment. The urinary excretion of quinolinic acid, nicotinic acid, N1-methylnicotinamide and N-methyl-2-pyridone-5-carboxamide was measured (a) during a 5-day pre-depletion period when the subjects ate self-selected diets; (b) during depletion while they consumed a diet containing only 0.16 mg of vitamin B6 daily, (c) when they were fed diets supplemented with 0.6 or 0.9 mg of pyridoxine/day; and (d) when they received 50 mg of pyridoxine daily. The urinary levels of the 4 metabolites were determined before and after loading with 2 g of L-tryptophan during each of these periods. Post-tryptophan quinolinic acid excretion levels and yields (post-tryptophan values minus basal values) were significantly elevated during vitamin B6 depletion and these returned to pre-depletion levels upon repletion and saturation with pyridoxine. During the height of depletion, N1-methylnicotinamide values were not significantly different from values during repletion or saturation but differed from pre-depletion values probably because the subjects were eating self-selected diets during the pre-depletion period. The excretion levels of the 4 metabolites in other periods were not statistically different. The results of this experiment suggest a possible requirement for vitamin B6 coenzymes in the metabolism of quinolinic acid.