Nicotinic acid

Niacin, also known as nicotinic acid or vitamin B3, is a water-soluble vitamin whose derivatives such as NADH, NAD, NAD+, and NADP play essential roles in energy metabolism in the living cell and DNA repair. The designation vitamin B3 also includes the corresponding amide nicotinamide, or niacinamide, whose chemical formula is C6H6N2O.

Deficiency and overdose
Severe lack of niacin causes the deficiency disease pellagra, whereas a mild deficiency slows down the metabolism decreasing cold tolerance. Extremely high doses of niacin can cause niacin maculopathy, a thickening of the macula and retina which leads to blurred vision and blindness. The recommended daily allowance of niacin is 2-12 mg a day for children, 14 mg a day for women, 16 mg a day for men, and 18 mg a day for pregnant or breast-feeding women.

Over 20 mg per day may produce hot flushes lasting 15-30 minutes in duration, along with a strange itching sensation and a reddening of the skin. In some cases it may cause a headache or even nausea. This is completely normal if a large dose of niacin hasn't been taken before and is caused by the cells of the body releasing histamine, which results in vasodilation. Once the body becomes used to the larger dose (usually over a period of days), these side effects will lessen or cease. Taking aspirin half an hour before the niacin will generally lessen the side effects.

Discovery
Nicotinic acid was first discovered from the oxidation of nicotine. When the properties of nicotinic acid were discovered, it was thought prudent to choose a name to dissociate it from nicotine and to avoid the idea that either smoking provided vitamins or that wholesome food contained a poison. The resulting name 'niacin' was derived from nicotinic acid + vitamin. Vitamin B3 is also referred to as "vitamin PP", a name derived from the obsolete term "pellagra-preventing factor."

Bioavailability
The liver can synthesize niacin from the essential amino acid tryptophan (see below), but the synthesis is extremely slow; 60 mg of tryptophan are required to make one milligram of niacin. Dietary niacin deficiency tends to occur only in areas where people eat corn, the only grain low in niacin, as a staple food, and that don't use lime during maize (corn) meal/flour production. Alkali lime releases the tryptophan from the corn so that it can be absorbed in the gut, and converted to niacin.

Biosynthesis
The 5-membered aromatic heterocycle of the essential amino acid, tryptophan, is cleaved and rearranged with the alpha amino group of tryptophan into the 6-membered aromatic heterocycle of niacin by the following reaction:


 * [[Image:Niacin biosynthesis.svg|left|thumb|600px|Biosynthesis: [[Tryptophan]] → kynurenine → niacin]]

Other uses
Niacin plays an important role in the production of several sex and stress-related hormones, particularly those made by the adrenal gland. It also plays a role in removing toxic and harmful chemicals from the body.

Niacin, when taken in large doses, increases the level of high density lipoprotein (HDL) or "good" cholesterol in blood, and is sometimes prescribed for patients with low HDL, and at high risk of heart attack. An extended release formulation of niacin for this indication is marketed by Abbott Laboratories under the trade name Niaspan. Niacin (but not niacinamide) is also used in the treatment of hyperlipidemia because it reduces very low density lipoprotein (VLDL), a precursor of low density lipoprotein (LDL) or "bad" cholesterol, secretion from the liver, and inhibits cholesterol synthesis. The main problem with the clinical use of niacin for dyslipidemia is the occurrence of skin flushing, even with moderate doses. There are current attempts to create a prolonged-release formulation of the substantance, to allow it to be used more often and more comfortably.

Niacin in itself is not toxic, but the chemicals converted by niacin are toxic to the skin and liver in overdose, and high doses of niacin should only be reached with gradual increase. Studies in laboratory animals have demonstrated behavioral changes when large doses of niacin are given.

Industrial use
Nicotinic acid reacts with hemoglobin and myoglobin in meat to form a brightly coloured complex, and thus has been used as a food additive, typically to improve the colour of minced (ground) meat. Niacin is licensed as a food colouring agent in some countries.