Niacin, absorption metabolite excretion

That nongrowing animals require niacin implies that it is lost from the body either as intact niacin or as a modified or breakdown product of the vitamin. An amount of niacin equivalent to nearly 90% of our daily intake is excreted in the forms of N-methyl-2-p)nidone-5urinary metabolites can be used to assess niacin status. Loss of the normal quantity in the urine each day indicates that the supply in the diet is adequate. In humans, the healthy adult excretes 4 to 6 mg of N-methyl-nicoti-namide per day. An abnormally low level indicates that the dietary intake is not adequate. Measurement of urinary niacin metabolites has proven useful in determining the amoimt of niacin available in a variety of foods. The body s ability to use niacin in different foods may vary even if the foods contain identical quantities of the vitamin. One contributing factor to the low availability of niacin is the occurrence of the vitamin in the "bound form," as mentioned earlier. Excretion of normal levels of pyridone, for example, depends not only on normal absorption of the vitamin from the diet, but also on its conversion to NAD or NADP, followed by catabolism to the metabolite. 

Other urinary excretion products of niacin include nicotinuric acid (nicotinoyl glycine) nicotinamide N-oxide, and trigonelline (N -methyl nicotinic acid) the latter may arise from bacterial action in the gut or from the absorption of this substance from foods. The pattern of the different turnover metabolites varies between species, between diets (depending partly on the ratio of nicotinamide to nicotinic acid in the diet), and partly with niacin status thus there are complex regulatory mechanisms to be considered. 

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