Efficiency of Tryptophan as a Niacin Precursor in Man

Nineteen experiments were conducted with 14 adult subjects to determine the efficiency of conversion of tryptophan to niacin in man. The conversion ratio was calculated by comparison of excretion of niacin metabolites (N1-Me and pyridone) after administration of niacinamide with excretion after administration of tryptophan. The subjects were maintained with controlled diets furnishing low or moderate amounts of niacin and tryptophan. Calculations indicated that an average of 3.3% of the tryptophan which was administered was converted to niacin or, expressed in another way, that 55.8 mg of tryptophan was equivalent to 1 mg of niacin. The conversion ratio varied considerably among individual subjects: from 34 to 86 mg of tryptophan was found to be equivalent to 1 mg of niacin.