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==What is Vitamin B3 (Niacin)?==

==What is Vitamin B3 (Niacin)?==

Vitamin B₃, also called niacin, is an essential '''water-soluble''' vitamin that participates as a [[Nutrition Glossary#Cofactor|cofactor]] in [[Sugars - Nutrition|glucose]], [[Fatty Acids Overview - Nutrition|fatty acid]] and [[Amino Acids Overview - Nutrition|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 - Physiology|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 [[Nephron Microscopic Anatomy#Proximal Tubule|renal tubules]], with some active reabsorption during periods of low intake<ref name="McCormick">McCormick DB. (2000) '''Niacin, Riboflavin, and Thiamin. In Biochemical and physiological aspects of human nutrition. '''2000 ''Philadelphia, PA: WB Saunders Company'' p. 459-468.</ref>.

Vitamin B₃, also called niacin, is an [[Nutrition Glossary#Essential Nutrients|essential]] '''water-soluble''' vitamin that participates as a [[Nutrition Glossary#Cofactor|cofactor]] in [[Sugars - Nutrition|glucose]], [[Fatty Acids Overview - Nutrition|fatty acid]] and [[Amino Acids Overview - Nutrition|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 - Physiology|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 [[Nephron Microscopic Anatomy#Proximal Tubule|renal tubules]], with some active reabsorption during periods of low intake<ref name="McCormick">McCormick DB. (2000) '''Niacin, Riboflavin, and Thiamin. In Biochemical and physiological aspects of human nutrition. '''2000 ''Philadelphia, PA: WB Saunders Company'' p. 459-468.</ref>.

Like most animals, '''dogs can synthesise a certain amount of niacin from the essential amino acid [[Tryptophan - Nutrition|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<ref name="NRC">National Research Council (NRC) (2006)''' Vitamins. In Nutrient Requirements for Dogs and Cats.''' 2006 ''Washington, DC: National Academies Press ''p.220-223.</ref>. As such, cats require preformed niacin in the diet<ref name="Morris">Morris, J.G. (2002) '''Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations.''''' Nutr Res Rev ''2002;15; 153-168.</ref>.

Like most animals, '''dogs can synthesise a certain amount of niacin from the essential amino acid [[Tryptophan - Nutrition|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<ref name="NRC">National Research Council (NRC) (2006)''' Vitamins. In Nutrient Requirements for Dogs and Cats.''' 2006 ''Washington, DC: National Academies Press ''p.220-223.</ref>. As such, cats require preformed niacin in the diet<ref name="Morris">Morris, J.G. (2002) '''Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations.''''' Nutr Res Rev ''2002;15; 153-168.</ref>.