Relationship between Tryptophan — Niacin Metabolism and Changes in Nitrogen Balance

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Abstract
Two investigations of the conversion of tryptophan to niacin in adult women were made when nitrogen loss was induced by omission of lysine in one period. The normal food diet provided 10 g nitrogen, 680 mg tryptophan and 10 mg niacin. The semi-purified diet provided 10 g nitrogen, 255 mg tryptophan and 2.5 mg niacin in study 1 and 10 in study 2. A niacinamide test load indicated niacin tissue depletion in study 1 but not in study 2. A sharp increase in nitrogen retention was observed in both terminal normal food periods which followed semi-purified food periods in which there was small retention. N-Methylnicotinamide and 2-pyridone urinary excretions increased during nitrogen loss in study 2. An average of 1.7% of tryptophan was excreted as an increase in metabolites. Calculations showed that 59.2 mg tryptophan were equivalent to 1 mg niacin and efficiency of conversion was 3.0%. It is suggested that a specific percentage of tryptophan is always converted to niacin regardless of protein metabolism status.