Niacin describes a group of vitamins that are required for correct metabolic activity. Failure to obtain enough niacin in the diet results in the development of pellagra, which is characterised by dermatitis, diarrhoea and dementia, and if left untreated will lead to death. Treatment of pellagra is simply a case of returning adequate niacin to the diet. This can be done by addition of any of the compounds that share niacin activity which effectively means nicotinic acid or niacinamide. These are described as vitamers of niacin. Organic electron carriers such as nicotinic adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) also show niacin activity, but are much less bioavailable compared to nicotinic acid and niacinamide, and so these latter two compounds are the forms of niacin that are favoured nutritionally. One explanation for the lower bioavailability of NAD and NADP is that they are labile in acidic conditions as might be found in the stomach. One real world example of where this is important is in the maturation of corn, whereby bioactive forms of niacin are transformed into less bioavailable forms, reducing the nutritional value of the corn in terms of its niacin activity. Nicotinamide riboside is found in milk but its contribution to human nutrition is not fully understood.
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