Whereas the measurement of B vitamin status has, in recent years, tended to focus on blood analysis, perhaps mainly because of the convenience of sample collection, the development of blood-based status analysis for niacin has lagged behind that of the other components of the B complex. Some studies have indeed suggested that the erythrocyte concentration of the niacin-derived coenzyme NAD may provide useful information about the niacin status of human subjects; that a reduction in the ratio of NAD to NADP to below 1.0 in red cells may provide evidence of niacin deficiency; and that a decline in plasma tryptophan levels may indicate a more severe deficiency than a decline in red cell NAD levels. These claims now need to be tested in naturally deficient human populations. The niacin coenzymes can be quantitated either by enzyme-linked reactions or by making use of their natural fluorescence in alkaline solution.
At present, niacin status is most commonly assessed by the assay of some of the breakdown products of niacin coenzymes in the urine. Of these, N1-methyl nicotinamide (NMN) is the easiest to measure, because of a convenient conversion in vitro to a fluorescent product, which can then be quantitated without the need for separation. However, more definitive and reliable information can be obtained by the measurement of urinary NMN in conjunction with one or more of the urinary pyridone turnover products (N1-methyl-2-pyridone-5-carboxamide and N1-methyl-4-pyridone-3-carbox-amide), which can be detected and quantitated by UV absorption following high-pressure liquid chromatography. The Interdepartmental Committee on Nutrition for National Defense (USA) selected the criterion of niacin deficiency in humans as an NMN excretion rate of <5.8 mmol (0.8 mg) NMN per day in 24 h urine samples.
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