Assessment of Niacin Nutritional Status

Although the nicotinamide nucleotide coenzymes function in a large number of oxidation and reduction reactions, thus carmot be exploited as a means of assessing the state of the body s niacin reserves, because the coenzymes are not firmly attached to their apoenzymes, as are coenzymes derived from thiamin (Section 6.5.3), riboflavin (Section 7.5.3), and vitamin Be (Section 9.5.3), but act as cosubstrates of the reactions, binding to and leaving the enzyme as 

The two methods of assessing niacin nutritional status are measurement of blood nicotinamide nucleotides and the urinary excretion of niacin metabolites, neither of which is wholly satisfactory. 

1 Tissue and Whole Blood Concentrations of Nicotinamide Nucieotides 

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Fu CS, Swendseid ME, Jacob RA, and McKee RW (1989) Biochemical markers for assessment of niacin nutritional status in young men levels of erythrocyte niacin coenzymes and plasma tryptophan./owraa/o/JVMtrifton 119,1945-9. 

The most widely used method for assessing niacin nutritional status is measurement of the urinary excretion of niacin metabolites. Table 8.1 shows the excretion of A( -methyl nicotinamide and methyl pyridone carboxamide in niacin adequacy and deficiency. 

Jacob, R. A., Swendseid, M.E., McKee, R.W., Fu, C.S., and Clemens, R.A., 1989. Biochemical markers for assessment of niacin status in young men urinary and blood levels of niacin metabolites. Journal of Nutrition. 119 591-598. 

K Shibata. The metabolism of niacin in each organ and the biological method for assessing the nutritional status of niacin in the rats. Vitamins (Japan) 61 39-56, 1987. 

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, -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 (N -methyl-2-pyridone-5-carboxamide and N -methyl-4-pyridone-3-carboxamide), 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 tmol (0.8 mg) NMN per day in 24h urine samples. 

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