Nicotinic Acid Metabolism in Humans I. The Urinary Excretion of Nicotinic Acid and its Metabolic Derivatives on Four Levels of Dietary Intake

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Abstract
The nicotinic acid metabolism of 6 freshman and sophomore college women on 4 levels of dietary intake of nicotinic acid and tryptophan was studied. On a control diet providing 11.3 mg of the vitamin and 885 mg of tryptophan, the average amount excreted by the 6 subjects as the vitamin and its two major metabolites, N1-Me and pyridone, balanced the intake. When the dietary nicotinic acid of the control diet was increased to 14.3 mg with a concomitant increase in tryptophan, there was a slight increase (1.0 mg) in the total amount of the vitamin and its metabolic derivatives found in the urine. Following the 28-day control periods of study, a basal diet containing 7.2 mg of nicotinic acid and 600 mg of tryptophan was fed for 11 days. The average decrease in total excretion during this regime as compared with the immediately preceding control level of excretion was 3.04 mg. On the basal diet the total average excretion (expressed as nicotinic acid) was 128% of the amount ingested. During the post-basal period with an increase to 16.3 mg intake, the excretion of nicotinic acid and N1-Me returned to control levels, while the amount of pyridone exceeded the pre-basal level. With each increment in dietary nicotinic acid approximately one-third of the increase was excreted as pyridon; on the other hand, a comparable decrease in intake of the vitamin was reflected in decreased amounts of the vitamin and N1Me excreted as well as the pyridone. There was considerable variation in the metabolic performance among the 6 experimental subjects. The significance of the findings in this study has been discussed in relation to “normal” nutrition with respect to the nutrients studied.