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Niacin

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This article is about the organic compound and vitamin. For the band, see Niacin (band).

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Niacin, also known as vitamin B3, is a water-soluble vitamin which prevents the deficiency disease pellagra. It is an organic compound with the molecular formula C6H5NO2. It is a derivative of pyridine, with a carboxyl group (COOH) at the 3-position. Other forms of vitamin B3 include the corresponding amide, nicotinamide ("niacinamide"), where the carboxyl group has been replaced by an amide group (CONH2), as well as more complex amides and a variety of esters. The terms niacin, nicotinamide, and vitamin B3 are often used interchangeably to refer to any one of this family of molecules, since they have a common biochemical activity. Depending on the definition used, niacin is one of between 40 and 80 essential human nutrients. It is one of only four essential nutrients associated with a pandemic deficiency disease, namely Niacin (pellagra), vitamin C (scurvy), thiamin (beriberi), and vitamin D (rickets).

Niacin is converted to nicotinamide and then to NAD and NADP in vivo. Although the two are identical in their vitamin activity, nicotinamide does not have the same pharmacological effects of niacin, which occur as side-effects of niacin's conversion. Thus nicotinamide does not reduce cholesterol or cause flushing,[1] although nicotinamide may be toxic to the liver at doses exceeding 3 g/day for adults.[2] Niacin is a precursor to NADH, NAD, NAD+, NADP and NADPH, which play essential metabolic roles in living cells.[3] DNA repair, and the production of steroid hormones in the adrenal gland.

History

Niacin was first described by Hugo Weidel in 1873 in his studies of nicotine.[4] The original preparation remains useful: the oxidation of nicotine using nitric acid.[5] 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."[6] 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 + vitamin.

Niacin is referred to as Vitamin B3 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 do not 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.[7]

The recommended daily allowance of niacin is 2-12 mg/day for children, 14 mg/day for women, 16 mg/day for men, and 18 mg/day for pregnant or breast-feeding women.[8]

Note: Niacin synthesis is deficient in carcinoid syndrome because of metabolic diversion of its precursor, tryptophan, to form serotonin.

Pharmacological uses

The recommended daily allowance (RDA) for niacin is 20 mg/day. The argument that niacin does not provide any benefits at doses above the RDA has been decisively disproven. There are two decisively proven uses of pharmacological doses of niacin. These are for heart disease and skin conditions. Extensive evidence combining known biochemical functions of vitamin B3, controlled trials, and clinical observations indicate that doses of niacin above the RDA speed wound healing and help the immune system fight off viral infections. The available evidence supports the hypothesis that response is proportional to dose well past the side effect threshold. The side effect threshold for niacin varies dramatically from person to person, ranging from between 30 and 300 mg for the majority of the population.

Lipid modifying effects

Niacin, prescribed in doses between 1000 and 3000 mg/day, blocks the breakdown of fats in adipose tissue, a 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.[9]

By lowering VLDL levels, niacin also increases the level of high-density lipoprotein (HDL) or "good" cholesterol in blood, and therefore it is sometimes prescribed for patients with low HDL, who are also at high risk of a heart attack.[10][11]

As of August 2008, a combination of niacin with laropiprant is tested in a clinical trial. Laropiprant reduces facial flushes induced by niacin.

Topical use

A fat-soluble derivative of niacin has been developed that efficiently penetrates the skin. This form of niacin has been shown to improve a variety of skin conditions in a number of controlled clinical trials.

Niacin and drug screening tests

Niacin is sometimes consumed in large quantities by people who wish to fool drug screening tests, particularly for lipid soluble drugs such as marijuana.[12] It is believed to "promote metabolism" of the drug and cause it to be "flushed out." Scientific studies have shown it does not affect drug screenings, but can pose a risk of overdose, causing arrhythmias, metabolic acidosis, hyperglycemia, and other serious problems.[13]

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:[14]

Facial flushing is the most commonly-reported side-effect.[15] 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.[citation needed] Slow- or "sustained"-release forms of niacin have been developed to lessen these side-effects.[9][16] One study showed the incidence of flushing was significantly lower with a sustained release formulation[17] though doses above 2 g per day have been associated with liver damage, particularly with slow-release formulations.[14]

High-dose niacin may also elevate blood sugar, thereby worsening diabetes mellitus.[14] Hyperuricemia is another side-effect of taking high-dose niacin, and may exacerbate gout.[18] 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.[14]

Niacin at extremely high doses can have life-threatening acute toxic reactions.[19] Extremely high doses of niacin can also cause niacin maculopathy, a thickening of the macula and retina which leads to blurred vision and blindness.[20]

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 does not cause the flushing associated with the nicotinic acid form, it is not clear whether it is pharmacologically equivalent in its positive effect.[21]

Biosynthesis

File:Niacin biosynthesis.svg
Biosynthesis: Tryptophan → kynurenine → niacin
Biosynthesis

The liver can synthesize niacin from the essential amino acid tryptophan, requiring 60 mg of tryptophan to make one mg of niacin.[22]

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.[23] It couples to Gi alpha subunit.[24]

Food sources

Animal products: Fruits and vegetables: Seeds: Fungi:

References

  1. ^ Jaconello P (1992). "Niacin versus niacinamide". CMAJ. 147 (7): 990. PMC 1336277. PMID 1393911. {{cite journal}}: Unknown parameter |month= ignored (help)
  2. ^ Knip M, Douek IF, Moore WP; et al. (2000). "Safety of high-dose nicotinamide: a review". Diabetologia. 43 (11): 1337–45. doi:10.1007/s001250051536. PMID 11126400. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  3. ^ Cox, Michael; Lehninger, Albert L; Nelson, David R. (2000). Lehninger principles of biochemistry. New York: Worth Publishers. ISBN 1-57259-153-6.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. ^ Weidel, H (1873). "Zur Kenntniss des Nicotins". Justus Liebig's Annalen der Chemie und Pharmacie. 165: 330–349. doi:10.1002/jlac.18731650212.
  5. ^ Samuel M. McElvain (1941). "Nicotinic Acid" (PDF). Organic Syntheses; Collected Volumes, vol. 1, p. 385.
  6. ^ Elvehjem, C.A. "W. WOOLLEY 1938 The isolation and identification of the anti-blacktongue factor J". J. Biol. Chem. 123: 137. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ "Vitamin B3". University of Maryland Medical Center. 2002-01-04. Retrieved 2008-03-31.{{cite web}}: CS1 maint: year (link)
  8. ^ Jane Higdon, "Niacin", Micronutrient Information Center, Linus Pauling Institute, Oregon State University
  9. ^ a b Katzung, Bertram G. (2006). Basic and clinical pharmacology. New York: McGraw-Hill Medical Publishing Division. ISBN 0071451536.
  10. ^ McGovern ME (2005). "Taking aim at HDL-C. Raising levels to reduce cardiovascular risk". Postgrad Med. 117 (4): 29–30, 33–5, 39 passim. PMID 15842130.
  11. ^ Canner PL, Berge KG, Wenger NK; et al. (1986). "Fifteen year mortality in Coronary Drug Project patients: long-term benefit with niacin". J. Am. Coll. Cardiol. 8 (6): 1245–55. PMID 3782631. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  12. ^ "Niacin abuse in the attempt to alter urine drug tests". Pharmacy Technician’s Letter. 23 (6). 2007. Retrieved 2008-03-31..
  13. ^ Phend, C (2007-04-12). "Psychiatric Times - Overdoing Niacin Can't Thwart Drug Abuse Tests and Is Risky". Retrieved 2008-03-31.
  14. ^ a b c d Keith Parker; Laurence Brunton; Goodman, Louis Sanford; Lazo, John S.; Gilman, Alfred (2006). Goodman & Gilman's the pharmacological basis of therapeutics. New York: McGraw-Hill. ISBN 0071422803.{{cite book}}: CS1 maint: multiple names: authors list (link)
  15. ^ McGee, W (2007-02-01). "Medical Encyclopedia: Niacin". MedlinePlus. Retrieved 2008-03-31.
  16. ^ Barter, P (2006). "Options for therapeutic intervention: How effective are the different agents?". European Heart Journal Supplements. 8 (F): F47–F53. doi:10.1093/eurheartj/sul041. {{cite journal}}: |access-date= requires |url= (help)
  17. ^ Chapman MJ, Assmann G, Fruchart JC, Shepherd J, Sirtori C (2004). "Raising high-density lipoprotein cholesterol with reduction of cardiovascular risk: the role of nicotinic acid--a position paper developed by the European Consensus Panel on HDL-C". Curr Med Res Opin. 20 (8): 1253–68. doi:10.1185/030079904125004402. PMID 15324528.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  18. ^ Capuzzi DM, Morgan JM, Brusco OA, Intenzo CM (2000). "Niacin dosing: relationship to benefits and adverse effects". Curr Atheroscler Rep. 2 (1): 64–71. doi:10.1007/s11883-000-0096-y. PMID 11122726.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  19. ^ Mittal MK, Florin T, Perrone J, Delgado JH, Osterhoudt KC (2007). "Toxicity from the use of niacin to beat urine drug screening". Ann Emerg Med. 50 (5): 587–90. doi:10.1016/j.annemergmed.2007.01.014. PMID 17418450.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  20. ^ Gass JD (2003). "Nicotinic acid maculopathy. 1973". Retina (Philadelphia, Pa.). 23 (6 Suppl): 500–10. PMID 15035390.
  21. ^ Kittams, B (2003-01-15). "No-Flush Niacin for the Treatment of Hyperlipidemia". Medscape. Retrieved 2008-03-31.
  22. ^ Jacobson, EL (2007). "Niacin". Linus Pauling Institute. Retrieved 2008-03-31.
  23. ^ Zhang Y, Schmidt RJ, Foxworthy P; et al. (2005). "Niacin mediates lipolysis in adipose tissue through its G-protein coupled receptor HM74A". Biochem. Biophys. Res. Commun. 334 (2): 729–32. doi:10.1016/j.bbrc.2005.06.141. PMID 16018973. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  24. ^ Zellner C, Pullinger CR, Aouizerat BE; et al. (2005). "Variations in human HM74 (GPR109B) and HM74A (GPR109A) niacin receptors". Hum. Mutat. 25 (1): 18–21. doi:10.1002/humu.20121. PMID 15580557. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
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