Vitamin B₃ (Niacin and Metabolite)

Niacin (nicotinic acid) is a water-soluble vitamin that is also referred to as vitamin B₃.^1,2 Nicotinamide (nicotinic acid amide) is the derivative of niacin that is incorporated into the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP).^1-3 The nicotinamide moiety of NAD and NADP serves as an electron acceptor or donor in biological oxidation-reduction (redox) reactions catalyzed by several hundred different enzymes. NAD serves as the cofactor for enzymes that break down (catabolize) carbohydrates, fats, proteins, and alcohol while NADP supports biosynthetic (anabolic) reactions, including the synthesis of all macromolecules, such as fatty acids and cholesterol.^1,2 Nicotinamide has also been shown to serve as a cofactor in adenosine diphosphate (ADP)-ribose transfer reactions that play an integral part in deoxyribonucleic acid (DNA) repair calcium mobilization.^3,4 Studies of cultured cells and animal models suggest that ADP-ribose polymer-mediated DNA repair and cyclic ADP-ribose-mediated cell-signaling pathway may play a role in cancer prevention.^3,4

Both nicotinic acid and nicotinamide are absorbed from the normal diet. Nicotinamide is the form of vitamin B₃ that is commonly found in nutritional supplements and used to fortify foods.^1,5 Nicotinic acid is available both over the counter and with a prescription as a cholesterol-lowering agent.^6-8 Niacin deficiency can result from inadequate dietary intake of niacin and/or the amino acid tryptophan. Tryptophan, obtained from the breakdown of dietary protein, can be converted to nicotinamide by liver enzymes that require vitamin B₆, riboflavin, and iron. Deficiencies of these constituents can contribute to the development of niacin deficiency.^1,2 Hartnup's disease, a hereditary disorder that reduces tryptophan absorption, can lead to niacin deficiency.¹ Carcinoid syndrome diverts tryptophan to the increased production of serotonin and can reduce the production of nicotinamide.¹ Prolonged treatment with isoniazid has also been associated with niacin deficiency.¹

Niacin deficiency can affect the skin, digestive system, and the nervous system.^1,2 Severe niacin deficiency, referred to as pellagra, has been associated with the "four Ds": dermatitis, diarrhea, dementia, and ultimately death.² Pellagra dermatitis is characterized by a thick, scaly, darkly pigmented rash that develops symmetrically in areas exposed to sunlight.² Digestive system symptoms include vomiting, constipation or diarrhea, and a bright red tongue.^1,2 Pellagra can also cause neurologic symptoms, including apathy, fatigue, depression, headache, disorientation, and memory loss.^1,2