Effect of Protein Intake on the Excretion of Quinolinic Acid and Niacin Metabolites by Men during Vitamin B6 Depletion

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Abstract
The effect of the level of dietary protein on the conversion of tryptophan to niacin by man was studied. The urinary excretion of quinolinic acid, nicotinic acid, N1-methylnicotinamide and N-methyl-2-pyridine-5-carboxamide was determined before and after tryptophan loading both when subjects were adequately nourished with vitamin B6 and when they were depleted of the vitamin. The amount of quinolinic acid excreted in response to the tryptophan load dose was affected by the level of protein in the diet during vitamin B6 depletion. Post-tryptophan quinolinic acid excretion of subjects fed a diet containing 150 g of protein was significantly elevated after 6 days of vitamin B6 deprivation and the amount excreted continued to increase as the length of deficiency increased. Men fed the diet containing 54 g protein did not excrete significantly increased amounts of quinolinic acid in response to tryptophan loading until after 40 days of vitamin deprivation. The amount of nicotinic acid, N1-methylnicotinamide and N-methyl-2-pyridone-5-carboxamide excreted in response to the tryptophan load dose was not affected by either the vitamin B6 intake or the level of dietary protein.