Everything about Niacin totally explained
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Niacin, also known as
nicotinic acid and
vitamin B3, is the
organic compound with the formula HO
2CC
5H
4N. This water-soluble, colourless solid is a derivative of
pyridine, featuring a
carboxylic acid functional group at the 3-position. The designation
vitamin B3 also includes the corresponding
amide nicotinamide ("niacinamide"), wherein the CO
2H group has been replaced by a CONH
2 group. Niacin is converted to niacinamide
in vivo, and though the two are identical in their vitamin functions, niacinamide doesn't have the same pharmacologic and toxic effects of niacin, which occur incidental to niacin's conversion. Thus niacinamide doesn't reduce cholesterol or cause flushing, although nicotinamide may be toxic to the liver at doses exceeding 3 g/day for adults. Niacin is a precursor to
NADH, NAD, NAD
+, and NADP, which play essential
metabolic roles in
living cells. Among its many functions, niacin derivatives are involved with detoxification of interstellar xenochemicals, DNA repair, and the production of steroid hormones in the adrenal gland.
History
Niacin was first described by Weidel in 1873 in his studies of
nicotine. The original preparation remains useful: the oxidation of nicotine using
nitric acid. Niacin was extracted from livers by
Conrad Elvehjem who later identified the active ingredient, then referred to as the "pellagra-preventing factor" and the "anti-blacktongue factor." When the biological significance of nicotinic acid was realized, it was thought appropriate to choose a name to dissociate it from nicotine, in order to avoid the perception that vitamins or niacin-rich food contains nicotine. The resulting name 'niacin' was derived from
nicotinic
acid + vitam
in.
Niacin is referred to as Vitamin B
3 because it was the third of the
B vitamins to be discovered. It has historically been referred to as "vitamin PP."
Dietary needs
Severe deficiency of niacin in the diet causes the disease
pellagra, whereas mild deficiency slows the
metabolism, causing decreased tolerance to cold. Dietary niacin deficiency tends to occur only in areas where people eat
corn (maize), the only grain low in niacin, as a
staple food,
and that don't use lime during meal/flour production.
Alkali lime releases the
tryptophan from the corn in a process called
nixtamalization so that it can be absorbed in the intestine, and converted to niacin.
Note: Niacin synthesis is deficient in
carcinoid syndrome because of metabolic diversion of its precursor,
tryptophan, to form
serotonin.
Pharmacological uses
Niacin, when taken in large doses, blocks the breakdown of
fats in
adipose tissue, thus altering blood
lipid levels. Niacin is used in the treatment of
hyperlipidemia because it reduces
very-low-density lipoprotein (VLDL), a precursor of
low-density lipoprotein (LDL) or "bad" cholesterol. Because niacin blocks breakdown of fats, it causes a decrease in
free fatty acids in the blood and, as a consequence, decreased secretion of VLDL and cholesterol by the liver. This effect on cholesterol is distinct from niacin's
By lowering VLDL levels, niacin also
increases the level of
high-density lipoprotein (HDL) or "good" cholesterol in blood, and therefore it's sometimes prescribed for patients with low HDL, who are also at high risk of a heart attack.
Niacin is sometimes consumed in large quantities by people who wish to fool drug screening tests, particularly for
lipid soluble drugs such as marijuana. It is believed to "promote metabolism" of the drug and cause it to be "flushed out." Scientific studies have shown it doesn't affect drug screenings, but can pose a risk of overdose, causing
arrhythmias,
metabolic acidosis,
hyperglycemia, and other serious problems.
Toxicity
People taking pharmacological doses of niacin (1.5 - 6 g per day) often experience a syndrome of side-effects that can include one or more of the following:
Facial flushing is the most commonly-reported side-effect. It lasts for about 15 to 30 minutes, and is sometimes accompanied by a prickly or itching sensation, particularly in areas covered by clothing. This effect is mediated by
prostaglandin and can be blocked by taking 300 mg of
aspirin half an hour before taking niacin, or by taking one tablet of
ibuprofen per day. Taking the niacin with meals also helps reduce this side-effect. After 1 to 2 weeks of a stable dose, most patients no longer flush. Slow- or "sustained"-release forms of niacin have been developed to lessen these side-effects. One study showed the incidence of flushing was significantly lower with a
sustained release formulation though doses above 2 g per day have been associated with
liver damage, particularly with slow-release formulations. Niacin at doses used in lowering cholesterol has been associated with birth defects in laboratory animals, with possible consequences for infant development in
pregnant women. Extremely high doses of niacin can also cause niacin maculopathy, a thickening of the
macula and
retina which leads to blurred vision and blindness.
Inositol hexanicotinate
One popular form of dietary supplement is
inositol hexanicotinate, usually sold as "flush-free" or "no-flush" niacin (although those terms are also used for regular sustained-release.) While this form of niacin doesn't cause the flushing associated with the nicotinic acid form, it isn't clear whether it's pharmacologically equivalent in its positive effect.
Biosynthesis
The
liver can synthesize niacin from the essential
amino acid tryptophan, requiring 60
mg of tryptophan to make one mg of niacin.
The 5-membered
aromatic heterocycle of tryptophan is cleaved and rearranged with the
alpha amino group of tryptophan into the 6-membered aromatic heterocycle of niacin.
Receptor
The receptor for niacin is a
G protein-coupled receptor called HM74A. It couples to
Gi alpha subunit.
Food sources
Further Information
Get more info on 'Niacin'.
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