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Effect of Protein Intake on the Excretion of Quinolinic Acid and Niacin Metabolites by Men during Vitamin B₆ Depletion¹

Department of Foods and Nutrition, School of Home Economics, University of Wisconsin, Madison, Wisconsin

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.

² Present address: Oregon State University, Corvallis, Oregon.

Manuscript received 4 August 1967.