Vitamin B3
Vitamin B3, colloquially referred to as niacin, is a vitamin family that includes three forms, or vitamers: niacin (nicotinic acid), nicotinamide (niacinamide), and nicotinamide riboside.[1] All three forms of vitamin B3 are converted within the body to nicotinamide adenine dinucleotide (NAD).[1] NAD is required for human life and people are unable to make it within their bodies without either vitamin B3 or tryptophan.[1] Nicotinamide riboside was identified as a form of vitamin B3 in 2004.[2][1]
This article is about a class of chemically related vitamers sometimes called niacin. For other uses, see Niacin (disambiguation).
Niacin (the nutrient) can be manufactured by plants and animals from the amino acid tryptophan.[3] Niacin is obtained in the diet from a variety of whole and processed foods, with highest contents in fortified packaged foods, meat, poultry, red fish such as tuna and salmon, lesser amounts in nuts, legumes and seeds.[4][5] Niacin as a dietary supplement is used to treat pellagra, a disease caused by niacin deficiency. Signs and symptoms of pellagra include skin and mouth lesions, anemia, headaches, and tiredness.[6] Many countries mandate its addition to wheat flour or other food grains, thereby reducing the risk of pellagra.[4][7]
The amide nicotinamide (niacinamide) is a component of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP+). Although niacin and nicotinamide are identical in their vitamin activity, nicotinamide does not have the same pharmacological, lipid-modifying effects or side effects as niacin, i.e., when niacin takes on the -amide group, it does not reduce cholesterol nor cause flushing.[8][9] Nicotinamide is recommended as a treatment for niacin deficiency because it can be administered in remedial amounts without causing the flushing, considered an adverse effect.[10] In the past, the group was loosely referred to as vitamin B3 complex.[11]
Mechanism of action[edit]
Nicotinamide adenine dinucleotide (NAD), along with its phosphorylated variant nicotinamide adenine dinucleotide phosphate (NADP), are utilized in transfer reactions within DNA repair and calcium mobilization. NAD also plays a critical role in human metabolism, acting as a coenzyme in both glycolysis and the Krebs cycle.[12]
Toxicity[edit]
The daily limit for vitamin B3 has been set at 35 mg. At daily doses of as low as 30 mg, flushing has been reported, always starting in the face and sometimes accompanied by skin dryness, itching, paresthesia, and headache.[12] Liver toxicity is the most serious toxic reaction and it occurs at doses >2 grams/day.[36] Fulminant hepatitis has been reported at doses between 3-9 grams/day with needs for liver transplantation. Other reactions include glucose intolerance, hyperuricemia, macular edema, and macular cysts.[12]