Vitamin B3(Niacin)

Vitamin B3, also called niacin, is an essential water-soluble vitamin that participates as a cofactor in glucose, fatty acid and amino acid metabolism. The term niacin is used to describe a number of compounds that have biological activity associated with nicotinamide, including nicotinic acid, and a variety of pyridine nucleotide structures. Nicotinic acid and nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are metabolized to nicotinamide within the intestinal lumen and absorbed across the intestinal mucosa by either carrier-mediated transport or passive diffusion. Once in the enterocyte nicotinamide is either released as free nicotinamide or converted to NAD for use by the cell. Niacin derivatives are filtered by the renal tubules, with some active reabsorption during periods of low intake. Like most animals, dogs can synthesise a certain amount of niacin from the essential amino acid tryptophan. The tryptophan metabolite α-amino-β-carboxymuconic-ε-semialdahyde can be utilized in one of two pathways; it can be degraded by picolinic carboxylase to form acetyl-CoA and CO2 or it can be used to produce nicotinamide. Cats, unlike dogs, are unable to synthesise significant levels of niacin from tryptophan because they have very high activity of the enzyme picolinic carboxylase which results in rapid catabolism of trypophan to acetyl-CoA and CO2. As such, cats require preformed niacin in the diet. Niacin is used in oxidative-reduction reactions involving catabolism of glucose, fatty acids, ketone bodies, and amino acids. Chronic niacin deficiency results in a wide range of clinical signs from dermatitis and oral mucosa ulceration, to diarrhoea and death. This condition was originally called black-tongue in dogs and was used as a model to understand and prevent pellagra (i.e. niacin deficiency) in people.