Urinary niacin
Niacin status can be assessed by analysis of the major niacin urinary metabolites, 1-methylnicotinamide (1-MN), and 1-methyl-2-pyridone-5-carboxamide (2-PYR). 1-methyl-4-pyridone-3-carboxide, a minor urinary metabolite, is sometimes included in the analysis. 2 and 24h urine collections have been used in the past, but spot urine samples have been suggested as an alternative as they are likely to provide information of status and recent intake.
For status measurements, fasting, early morning urine samples are recommended (5). However, it has also been stated that urinary niacin metabolites are not suitable to wide-scale screening and their relationship to niacin status is still not fully understood (8).
Method
Urinary niacin, see method in reference (5): Creeke & Seal. J Chromatogr B 2005;817:247-53.
Plasma niacin
Several approaches have been described for niacin metabolites, including nicotinic acid, nicotinamide, nicotinuric acid (6). However, only plasma 2-PYR and to a lesser extent 1-MN have been shown to be a useful plasma niacin biomarker for status (1, 9). The usefulness of plasma niacin analysis is therefore uncertain.
Method
Plasma niacin, see methods in references (6): Pfuhl et al. J Pharm Biomed Anal 2005;36:1045-52.
and (9): Jacob et al. J Nutr 1989;119:591-8.
Erythrocyte niacin
While whole blood NAD and NADP measurements have been indicated to provide erroneous estimates of niacin deficiency (10), erythrocyte NAD has been proposed to be a sensitive, reliable and convenient biomarker for niacin status assessment (11). Concentrations of erythrocyte NADP are fairly constant and therefore not affected by abnormal niacin status, which has been used to suggest a “niacin index”, calculated as the NAD:NADP-ratio (1, 11).
Method
Erythrocyte niacin, see method in reference (12): Nisselbaum & Green. Analytical Biochemistry 1969;27:212-7.