Biochemical markers for assessment of niacin status in young men: urinary and blood levels of niacin metabolites.

Biochemical markers of niacin status were studied in healthy young men fed 6.1 to 32 niacin equivalents (NE) per day over an 11-wk period while residing in a metabolic unit. Methylated metabolites of niacin, N1-methylnicotinamide (NMN) and N1-methyl-2-pyridone-5-carboxamide (2-pyr), in urine and plasma were determined during periods of low (6.1 or 10.1 NE per day), adequate (19 NE per day = 1 RDA) and high (25 or 32 NE per day) niacin intakes and after small test doses of nicotinamide. Urine excretion of less than 1.2 mg/d of either NMN or 2-pyr was a reliable indicator of subjects receiving the lowest intake of 6.1 NE/d, but the NMN metabolite was a better marker of subjects ingesting 10.1 NE/d. The ratio of 2-pyr/NMN in urine was not as good a measure of the 6.1 NE/d intake as the individual metabolite excretions and was not responsive to the 10.1 NE/d intake. Plasma niacin metabolites were generally not as reliable as urinary metabolites for identifying subjects receiving low niacin intakes, however, values for plasma 2-pyr dropped quickly and were eventually nondetectable. After a 1 RDA oral dose of nicotinamide, increases in urine and plasma 2-pyr levels above pre-dose baseline values were significantly decreased in subjects receiving low, as compared to adequate, niacin intake. A leucine supplement had no effect on the rate of repletion of niacin-deficient subjects nor on the level of methylated niacin metabolites in urine or plasma.