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Niacin, also known as nicotinic acid, is an organic compound and a form of vitamin B3, an essential human nutrient. It has the formula C
6
H
5
NO
2
and belongs to the group of the pyridinecarboxylic acid.

Niacin
Kekulé, skeletal formula of niacin
Ball and stick model of niacin
Names
Pronunciation /ˈnəsɪn/
Preferred IUPAC name
Pyridine-3-carboxylic acid[1]
Other names
  • Nicotinic acid (INN)
  • Bionic
  • Vitamin B3
  • Vitamin PP
Identifiers
3D model (JSmol)
3DMet B00073
109591
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.401
EC Number 200-441-0
3340
KEGG
MeSH Niacin
RTECS number QT0525000
UNII
Properties
C
6
NH
5
O
2
Molar mass 123.1094 g mol−1
Appearance White, translucent crystals
Density 1.473 g cm−3
Melting point 237 °C; 458 °F; 510 K
18 g L−1
log P 0.219
Acidity (pKa) 2.0, 4.85
Isoelectric point 4.75
1.4936
0.1271305813 D
Thermochemistry
−344.9 kJ mol−1
−2.73083 MJ mol−1
Pharmacology
C04AC01 (WHO) C10AD02 (WHO)
License data
Intramuscular, by mouth
Pharmacokinetics:
20–45 min
Hazards
Irritant Xi
R-phrases (outdated) R36/37/38
S-phrases (outdated) S26, S36
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oilHealth code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
1
1
0
Flash point 193 °C (379 °F; 466 K)
365 °C (689 °F; 638 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☑Y verify (what is ☑Y☒N ?)
Infobox references

Niacin is obtained in the diet from a variety of whole and processed foods, with highest contents in fortified packaged foods, tuna, some vegetable and other animal sources. Some countries require its addition to grains.[2] Medication and supplemental niacin are primarily used to treat high blood cholesterol and pellagra (niacin deficiency). Insufficient niacin in the diet can cause nausea, skin and mouth lesions, anemia, headaches, and tiredness. The lack of niacin may also be observed in pandemic deficiency diseases, which are caused by a lack of five crucial vitamins (niacin, vitamin C, thiamin, vitamin D, and vitamin A) and are usually found in areas of widespread poverty and malnutrition.

This colorless, water-soluble solid 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 a carboxamide group (CONH
2
), as well as more complex amides and a variety of esters.

Niacin and nicotinamide are both precursors of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) in vivo.[3] NAD converts to NADP by phosphorylation in the presence of the enzyme NAD+ kinase. NADP and NAD are coenzymes for many dehydrogenases, participating in many hydrogen transfer processes.[4] NAD is important in catabolism of fat, carbohydrate, protein, and alcohol, as well as cell signaling and DNA repair, and NADP mostly in anabolism reactions such as fatty acid and cholesterol synthesis.[4] High energy requirements (brain) or high turnover rate (gut, skin) organs are usually the most susceptible to their deficiency.[5]

Niacin supplementation has not been found useful for decreasing the risk of cardiovascular disease in those already on a statin,[6] but appears to be effective in those not taking a statin.[7] Although niacin and nicotinamide are identical in their vitamin activity, nicotinamide does not have the same pharmacological effects (lipid-modifying effects) as niacin. Nicotinamide does not reduce cholesterol or cause flushing.[8] As the precursor for NAD and NADP, niacin is also involved in DNA repair.[9][10]

Contents

Medical usesEdit

Treatment of deficiencyEdit

Niacin and niacinamide are used for prevention and treatment of pellagra.[11]

Abnormal lipidsEdit

Niacin has sometimes been used in addition to other lipid-lowering medications.[12] Systematic reviews found no effect of niacin on cardiovascular disease or death, in spite of raising HDL cholesterol, and reported side effects including an increased risk of diabetes.[6][13][14]

ContraindicationsEdit

Niacin is contraindicated with active liver disease, persistent elevated serum transaminases, active peptic ulcer disease, or arterial bleeding.[15]

Side effectsEdit

The most common adverse effects are flushing (e.g., warmth, redness, itching or tingling), headache, pain, abdominal pain, diarrhea, dyspepsia, nausea, vomiting, rhinitis, pruritus and rash. These can be minimized by initiating therapy at low dosages, increasing dosage gradually, and avoiding administration on an empty stomach.[15] High doses of niacin often temporarily reduce blood pressure as a result of acute vasodilation.[16] In the longer term, high-dose niacin use may persistently lower blood pressure in individuals with hypertension, but more research is needed to determine the extent of this effect.[16]

Facial flushingEdit

Flushing usually lasts for about 15 to 30 minutes, though it can sometimes last up to two hours. It is sometimes accompanied by a prickly or itching sensation, in particular, in areas covered by clothing. Flushing can be blocked by taking 300 mg of aspirin half an hour before taking niacin, by taking one tablet of ibuprofen per day or by co-administering the prostaglandin receptor antagonist laropiprant. Taking niacin with meals also helps reduce this side effect. Acquired tolerance will also help reduce flushing; after several weeks of a consistent dose, most patients no longer experience flushing.[17] Reduction of flushing focuses on altering or blocking the prostaglandin-mediated pathway.[18] Slow- or "sustained"-release forms of niacin have been developed to lessen these side effects.[19][20] One study showed the incidence of flushing was significantly lower with a sustained-release formulation,[21] though doses above 2 g per day have been associated with liver damage, in particular, with slow-release formulations.[22]

Prostaglandin (PGD2) is the primary cause of the flushing reaction, with serotonin appearing to have a secondary role in this reaction.[23] The effect is mediated by prostaglandin E2 and D2 due to GPR109A activation of epidermal Langerhans cells and keratinocytes.[24][25] Langerhans cells use cyclooxygenase type 1 (COX-1) for PGE2 production and are more responsible for acute flushing, while keratinocytes are COX-2 dependent and are in active continued vasodilation.[26][27] Flushing was often thought to involve histamine, but histamine has been shown not to be involved in the reaction.[23]

Gastrointestinal and hepaticEdit

Gastrointestinal complaints, such as indigestion, nausea and liver failure, have also been reported. Hepatotoxicity is possibly related to metabolism via amidation resulting in NAD production.[28] The time-release form has a lower therapeutic index for lowering serum lipids relative to this form of toxicity.[29]

DiabetesEdit

The high doses of niacin used to improve the lipid profile have been shown to elevate blood sugar by 5-10%, thereby worsening existing diabetes mellitus.[22] In a meta-analysis of 11 trials with non-diabetic participants, niacin therapy increased the relative risk of new-onset diabetes by 34%.[30]

OtherEdit

Side effects of heart arrhythmias have also been reported.[22][page needed] Increased prothrombin time and decreased platelet count have been reported; therefore, these should be monitored closely in patients who are also taking anticoagulants.[15]

Particularly the time-release variety, at extremely high doses, can cause acute toxic reactions.[31] Extremely high doses of niacin can also cause niacin maculopathy, a thickening of the macula and retina, which leads to blurred vision and blindness. This maculopathy is reversible after niacin intake ceases.[32]

PregnancyEdit

Niacin in doses used to lower cholesterol levels has been associated with birth defects in laboratory animals, with possible consequences for infant development in pregnant women.[22]

DeficiencyEdit

 
A man with pellagra, which is caused by a chronic lack of vitamin B3 in the diet

Between 1906 and 1940 more than 3 million Americans were affected by pellagra, with more than 100,000 deaths. Joseph Goldberger was assigned to study pellagra by the Surgeon General of the United States and produced good results. In the late 1930s, studies by Tom Spies, Marion Blankenhorn, and Clark Cooper established that niacin cured pellagra in humans. The disease was greatly reduced as a result.

At present, niacin deficiency is sometimes seen in developed countries, and it is usually apparent in conditions of poverty, malnutrition, and chronic alcoholism.[33] It also tends to occur in less developed areas where people eat maize (corn) as a staple food, as maize is the only grain low in digestible niacin. A cooking technique called nixtamalization i.e., pretreating with alkali ingredients, increases the bioavailability of niacin during maize meal/flour production. For this reason, people who consume corn as tortillas or hominy are not at risk of niacin deficiency.

Mild niacin deficiency has been shown to slow metabolism, causing decreased tolerance to cold.

Severe deficiency of niacin in the diet causes the disease pellagra, which is characterized by diarrhea, dermatitis, and dementia, as well as Casal's necklace lesions on the lower neck, hyperpigmentation, thickening of the skin, inflammation of the mouth and tongue, digestive disturbances, amnesia, delirium, and eventually death, if left untreated.[34] Common psychiatric symptoms of niacin deficiency include irritability, poor concentration, anxiety, fatigue, restlessness, apathy, and depression.[34] Studies have indicated that, in patients with alcoholic pellagra, niacin deficiency may be an important factor influencing both the onset and severity of this condition. Patients with alcoholism typically experience increased intestinal permeability, leading to negative health outcomes.

Hartnup disease is a hereditary nutritional disorder resulting in niacin deficiency.[34] This condition was first identified in the 1950s by the Hartnup family in London. It is due to a deficit in the intestines and kidneys, making it difficult for the body to break down and absorb dietary tryptophan (an essential amino acid that is utilized to synthesize niacin). The resulting condition is similar to pellagra, including symptoms of red, scaly rash, and sensitivity to sunlight. Oral niacin is given as a treatment for this condition in doses ranging from 40–200 mg, with a good prognosis if identified and treated early.[34] Niacin synthesis is also deficient in carcinoid syndrome, because of metabolic diversion of its precursor tryptophan to form serotonin.

Dietary recommendationsEdit

Australia and New Zealand
Age group RDI for niacin (mg NE/day)[35] Upper level of intake[35]
Infants 0–6 months 2 mg/d preformed niacin* ND
Infants 7–12 months 4 mg/d NE*
1–3 6 10
4–8 8 15
9–13 12 20
14–18 - 30
19+ - 35
Females 14+ 14 -
Males 14+ 16
Pregnant females 14–50 18 -
Pregnant females 14–18 - 30
Pregnant females 19–50 - 35
Lactating females 14–50 17 -
Lactating females 14–18 - 30
Lactating females 19–50 - 35
* Adequate Intake for infants[36]
Canada
Age group (years) RDA of niacin (mg NE/d)[37] Tolerable upper intake level[37]
0–6 months 2 mg/d preformed niacin* ND
7–12 months 4 mg/d NE*
1–3 6 10
4–8 8 15
9–13 12 20
Females 14–18 14 30
Males 14–18 16
Females 19+ 14 35
Males 19+ 16
Pregnant females <18 18 30
Pregnant females 18–50 18 35
Lactating females <18 17 30
Lactating females 18–50 17 35
European Food Safety Authority
Gender Adequate Intake (mg NE/MJ)[38]
Females 1.3
Males 1.6
Age (years) Tolerable upper limit of Nicotinic acid (mg/day)[38] Tolerable upper limit of Nicotinamide (mg/day)[38]
1–3 2 150
4–6 3 220
7–10 4 350
11–14 6 500
15–17 8 700
United States
Age group RDA for niacin (mg NE/day) Tolerable upper intake level[36]
Infants 0–6 months 2* ND**
Infants 6–12 months 4*
1–3 years 6 10
4–8 years 8 15
9–13 years 12 20
Females 14–18 years 14 30
Males 14–18 years 16 30
Females 19+ years 14 35
Males 19+ years 16 35
Pregnant females 14–18 years 18 30
Pregnant females 19–50 years 18 35
Lactating females 14–18 years 17 30
Lactating females 19–50 years 17 35
* Adequate intake for infants, as an RDA has yet to be established
** Not possible to establish; source of intake should be formula and food only[36]

The U.S. National Academy of Medicine (then the Institute of Medicine [IOM]) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for B vitamins in 1998.

The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values (DRV), with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL defined the same as in United States. For women (including those pregnant or lactating), men and children the PRI is 1.6 mg niacin per megajoule (MJ) of energy consumed. As the conversion is 1 MJ = 238.8 kcal, an adult consuming 2388 calories should be consuming 16 mg niacin. This is comparable to U.S. RDAs.[39] The niacin UL is set at 10 mg/day, which is much less than the U.S. value. The UL applies to niacin as a supplement consumed as one dose, and in intended to avoid the skin flush reaction. This explains why the PRI can be higher than the UL.[40]

Both the DRI and DRV describe amounts needed as niacin equivalents (NE), calculated as 1 mg NE = 1 mg niacin or 60 mg of the essential amino acid tryptophan. This is because the amino acid is utilized to synthesize the vitamin.[36][39]

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For niacin labeling purposes 100% of the Daily Value was 20 mg, but as of May 27, 2016 it was revised to 16 mg to bring it into agreement with the RDA.[41] A table of the old and new adult Daily Values is provided at Reference Daily Intake. The original deadline to be in compliance was July 28, 2018, but on September 29, 2017 the FDA released a proposed rule that extended the deadline to January 1, 2020 for large companies and January 1, 2021 for small companies.[42]

Food sourcesEdit

Niacin is found in a variety of whole and processed foods, including fortified packaged foods, meat from various animal sources, seafoods, and spices.[43]

Among whole food sources with the highest niacin content per 100 grams:

Meats

Plant foods and spices

Fortified breakfast cereals have among the highest niacin contents (more than 20 mg per 100 grams).[43] Whole grain flours, such as from wheat, rice, barley or corn, and pasta have niacin contents in a range of 3–10 mg per 100 grams.[43]

PharmacologyEdit

PharmacodynamicsEdit

The therapeutic effects of niacin are partly mediated through the activation of G protein-coupled receptors, including niacin receptor 1 (NIACR1) and niacin receptor 2 (NIACR2) which are highly expressed in adipose tissue, spleen, immune cells, and keratinocytes, but not in other expected organs such as liver, kidney, heart or intestine.[44][45] NIACR1 and NIACR2 inhibit cyclic adenosine monophosphate (cAMP) production and thus fat breakdown in adipose tissue and free fatty acids available for liver to produce triglycerides and very-low-density lipoproteins (VLDL) and consequently low-density lipoprotein (LDL).[28][46] A decrease in free fatty acids also suppresses liver expression of apolipoprotein C3 and PPARg coactivator-1b, thus increasing VLDL turnover and reducing its production.[47]

The mechanism behind niacin increasing HDL is not totally understood, but seems to occur in various ways. Niacin increases apolipoprotein A1 levels due to anticatabolic effects resulting in higher reverse cholesterol transport. It also inhibits HDL hepatic uptake, down-regulating production of the cholesterol ester transfer protein (CETP) gene.[48] Finally, it stimulates the ABCA1 transporter in monocytes and macrophages and upregulates peroxisome proliferator-activated receptor gamma, resulting in reverse cholesterol transport.[49]

Niacin reduces secondary outcomes associated with atherosclerosis, such as low-density lipoprotein cholesterol (LDL), very low-density lipoprotein cholesterol (VLDL-C), and triglycerides (TG), but increases high-density lipoprotein cholesterol (HDL).[48] Despite the importance of other cardiovascular risk factors, high HDL was associated with fewer cardiovascular events independent of LDL reduction.[50][51] Other effects include anti-thrombotic and vascular inflammation, improving endothelial function, and plaque stability.[52] As mediators produced from adipocytes, adipokines, such as tumor necrosis factor (TNF)-a, interleukins and chemokines, have pro-inflammatory effects, while others, such as adiponectin, have anti-inflammatory effects that influence the onset of atherosclerosis.[53] Niacin also appears to upregulate brain-derived neurotrophic factor and tropomyosin receptor kinase B (TrkB) expression.[54]

Research has been able to show the function of niacin in the pathway lipid metabolism. It is seen that this vitamin can decrease the synthesis of apoB-containing lipoproteins such as VLDL, LDL, IDL and lipoprotein (a) via several mechanisms: (1) directly inhibiting the action of DGAT2, a key enzyme for triglyceride synthesis; (2) influencing binding to the receptor HCAR2 thereby decreasing lipolysis and FFA flux to the liver for triglyceride synthesis; and (3) increasing apoB catabolism.[citation needed] HDL cholesterol levels are increased by niacin through direct and indirect pathways, such as by decreasing cholesterylester transfer protein activity and triglyceride levels, while increasing HDL cholesterol levels.[55]

PharmacokineticsEdit

 
Niacin, serotonin (5-hydroxytryptamine), and melatonin biosynthesis from tryptophan

BiosynthesisEdit

The liver can synthesize niacin from the essential amino acid tryptophan, requiring 60 mg of tryptophan to make 1 mg of niacin. Riboflavin, vitamin B6 and iron are required for the process.[36]

Physical and chemical propertiesEdit

Laboratory synthesisEdit

Several thousand tons of niacin are manufactured each year, starting from 3-methylpyridine.

PreparationsEdit

Niacin is available as a prescription product, and in the United States as a dietary supplement. Prescription products can be immediate release (Niacor, 500 mg tablets) or extended release (Niaspan, 500 and 1000 mg tablets). Dietary supplement products can be immediate or slow release, the latter including inositol hexanicotinate.[56][57] The last has questionable clinical efficacy in reducing cholesterol levels.[58]

NicotinamideEdit

Nicotinamide may be obtained from the diet where it is present primarily as NAD+ and NADP+. These are hydrolysed in the intestine and the resulting nicotinamide is absorbed either as such, or following its hydrolysis to nicotinic acid. Nicotinamide is present in nature in only small amounts, however it is the main form of vitamin B3 in plasma. In unprepared foods, niacin is present mainly in the form of the cellular pyridine nucleotides NAD and NADP. Enzymatic hydrolysis of the co-enzymes can occur during the course of food preparation. Boiling releases most of the total niacin present in sweet corn as nicotinamide (up to 55 mg/kg).

Nicotinamide may be toxic to the liver at doses exceeding 3 g/day for adults.[59]

Extended releaseEdit

A prescription extended release niacin, Niaspan, has a film coating that delays release of the niacin, resulting in an absorption over a period of 8–12 hours. The extended release formulations generally reduce vasodilation and flushing side effects, but increase the risk of hepatotoxicity compared to the immediate release forms.[60][61]

A formulation of laropiprant (Merck & Co., Inc.) and niacin had previously been approved for use in Europe and marketed as Tredaptive. Laropiprant is a prostaglandin D2 binding drug shown to reduce vasodilatation and flushing up to 73%.[48][62][63][64] The HPS2-THRIVE study,[65] a study sponsored by Merck, showed no additional efficacy of Tredaptive in lowering cholesterol when used together with other statin drugs, but did show an increase in other side effects. The study resulted in the complete withdrawal of Tredaptive from the international market.[66][67][68]

Inositol hexanicotinateEdit

 
Inositol hexanicotinate

One form of dietary supplement is inositol hexanicotinate (IHN), which is inositol that has been esterified with niacin on all six of inositol's alcohol groups.[69] IHN is usually sold as "flush-free" or "no-flush" niacin in units of 250, 500, or 1000 mg/tablets or capsules. It is sold as an over-the-counter formulation, and often is marketed and labeled as niacin, thus misleading consumers into thinking they are getting the active form of the medication. While this form of niacin does not cause the flushing associated with the immediate-release products, the evidence that it has lipid-modifying functions is disputed. As the clinical trials date from the early 1960s (Dorner, Welsh) or the late 1970s (Ziliotto, Kruse, Agusti), it is difficult to assess them by today's standards.[70] One of the last of those studies affirmed the superiority of inositol and xantinol esters of nicotinic acid for reducing serum free fatty acid,[71] but other studies conducted during the same period found no benefit.[72] Studies explain that this is primarily because "flush-free" preparations do not contain any free nicotinic acid. A more recent placebo-controlled trial was small (n=11/group), but results after three months at 1500 mg/day showed no trend for improvements in total cholesterol, LDL-C, HDL-C or triglycerides.[73] Thus, so far there is not enough evidence to recommend IHN to treat dyslipidemia.

RenameEdit

In 1942, when flour enrichment with nicotinic acid began, a headline in the popular press said "Tobacco in Your Bread." So the Council on Foods and Nutrition of the American Medical Association approved of the Food and Nutrition Board's new names niacin and niacin amide for use primarily by non-scientists. It was thought appropriate to choose a name to dissociate it from nicotine, to avoid the perception that vitamins or niacin-rich food contains nicotine, or that cigarettes contain vitamins.[74] The resulting name niacin was derived from nicotinic acid + vitamin.[75][76]

HistoryEdit

Niacin was first described by chemist Hugo Weidel in 1873 in his studies of nicotine.[77] The original preparation remains useful: the oxidation of nicotine using nitric acid.[78] For the first time, niacin was extracted by Casimir Funk, but he thought that it was thiamine and due to the discovered amine group he coined the term "vitamine". Niacin was extracted from livers by biochemist Conrad Elvehjem in 1937, who later identified the active ingredient, then referred to as the "pellagra-preventing factor" and the "anti-blacktongue factor."[79] Soon after, in studies conducted in Alabama and Cincinnati, Dr. Tom Spies found that nicotinic acid cured the sufferers of pellagra.[80]

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", "vitamin P-P" and "PP-factor", that are derived from the term "pellagra-preventive factor".[75] Carpenter found in 1951 that niacin in corn is biologically unavailable, and can be released only in very alkaline lime water of pH 11.[81] In 1955, Altschul and colleagues described niacin as having a lipid-lowering property.[82] As such, niacin is the oldest lipid-lowering drug.

ResearchEdit

In animal models and in vitro, niacin produces marked anti-inflammatory effects in a variety of tissues – including the brain, gastrointestinal tract, skin, and vascular tissue – through the activation of NIACR1.[83][84][85][86] Niacin has been shown to attenuate neuroinflammation and may have efficacy in treating neuroimmune disorders such as multiple sclerosis and Parkinson's disease.[83][86] Unlike niacin, nicotinamide does not activate NIACR1; however, both niacin and nicotinamide activate the G protein-coupled estrogen receptor (GPER) in vitro.[87]

In 2014, concurring with earlier work in 2001 by Arizona State University, researchers from Pennsylvania State University working with NASA found niacin, pyridine carboxylic acids and pyridine dicarboxylic acids inside meteorites.[88]

ReferencesEdit

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