B vitamins

B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism. Though these vitamins share similar names, they are chemically distinct compounds that often coexist in the same foods. In general, dietary supplements containing all eight are referred to as a vitamin B complex. Individual B vitamin supplements are referred to by the specific number or name of each vitamin: B₁ = thiamine, B₂ = riboflavin, B₃ = niacin, etc. Some are better known by name than number: niacin, pantothenic acid, biotin and folate.

Each B vitamin is either a cofactor (generally a coenzyme) for key metabolic processes or is a precursor needed to make one.

Sources

B vitamins are found in highest abundance in meat. Most are also found in small quantities in whole unprocessed carbohydrate based foods. Processed carbohydrates such as sugar and white flour tend to have lower B vitamin than their unprocessed counterparts. For this reason, it is required by law in many countries (including the United States) that the B vitamins thiamine, riboflavin, niacin, and folic acid be added back to white flour after processing. This is sometimes called "Enriched Flour" on food labels. B vitamins are particularly concentrated in meat such as turkey, tuna and liver.[1] Good sources for B vitamins include legumes (pulses or beans), whole grains, potatoes, bananas, chili peppers, tempeh, nutritional yeast, brewer's yeast, and molasses. Although the yeast used to make beer results in beers being a source of B vitamins,[2] their bioavailability ranges from poor to negative as drinking ethanol inhibits absorption of thiamine (B₁),[3][4] riboflavin (B₂),[5] niacin (B₃),[6] biotin (B₇),[7] and folic acid (B₉).[8][9] In addition, each of the preceding studies further emphasizes that elevated consumption of beer and other alcoholic beverages results in a net deficit of those B vitamins and the health risks associated with such deficiencies.

The B₁₂ vitamin is not abundantly available from plant products,[10] making B₁₂ deficiency a legitimate concern for vegans. Manufacturers of plant-based foods will sometimes report B₁₂ content, leading to confusion about what sources yield B₁₂. The confusion arises because the standard US Pharmacopeia (USP) method for measuring the B₁₂ content does not measure the B₁₂ directly. Instead, it measures a bacterial response to the food. Chemical variants of the B₁₂ vitamin found in plant sources are active for bacteria, but cannot be used by the human body. This same phenomenon can cause significant over-reporting of B₁₂ content in other types of foods as well.[11]

A popular way of increasing one's vitamin B intake is through the use of dietary supplements. B vitamins are commonly added to energy drinks, many of which have been marketed with large amounts of B vitamins[12] with claims that this will cause the consumer to "sail through your day without feeling jittery or tense."[12] Some nutritionists have been critical of these claims, pointing out for instance that while B vitamins do "help unlock the energy in foods," most Americans acquire the necessary amounts easily in their diets.[12]

Because they are soluble in water, excess B vitamins are generally readily excreted, although individual absorption, use and metabolism may vary.[12] The elderly and athletes may need to supplement their intake of B₁₂ and other B vitamins due to problems in absorption and increased needs for energy production. In cases of severe deficiency, B vitamins, especially B₁₂, may also be delivered by injection to reverse deficiencies.[13] Both type 1 and type 2 diabetics may also be advised to supplement thiamine based on high prevalence of low plasma thiamine concentration and increased thiamine clearance associated with diabetes.[14] Also, Vitamin B₉ (folic acid) deficiency in early embryo development has been linked to neural tube defects. Thus, women planning to become pregnant are usually encouraged to increase daily dietary folic acid intake and/or take a supplement.[15]

List of B vitamins

B number	Name	Description
Vitamin B1	Thiamine	A coenzyme in the catabolism of sugars and amino acids.
Vitamin B2	Riboflavin	A precursor of coenzymes called FAD and FMN, which are needed for flavoprotein enzyme reactions, including activation of other vitamins
Vitamin B3	Niacin (nicotinic acid), nicotinamide, nicotinamide riboside	A precursor of coenzymes called NAD and NADP, which are needed in many metabolic processes.
Vitamin B5	Pantothenic acid	A precursor of coenzyme A and therefore needed to metabolize many molecules.
Vitamin B6	Pyridoxine, pyridoxal, pyridoxamine	A coenzyme in many enzymatic reactions in metabolism.
Vitamin B7	Biotin	A coenzyme for carboxylase enzymes, needed for synthesis of fatty acids and in gluconeogenesis.
Vitamin B9	Folate	A precursor needed to make, repair, and methylate DNA; a cofactor in various reactions; especially important in aiding rapid cell division and growth, such as in infancy and pregnancy.
Vitamin B12	Cobalamins	Commonly cyanocobalamin or methylcobalamin in vitamin supplements. A coenzyme involved in the metabolism of every cell of the human body, especially affecting DNA synthesis and regulation, but also fatty acid metabolism and amino acid metabolism.

Note: other substances once thought to be vitamins were given numbers in the B-vitamin numbering scheme, but were subsequently discovered to be either not essential for life or manufactured by the body, thus not meeting the two essential qualifiers for a vitamin. See section #Related compounds for numbers 4, 8, 10, 11, and others.

Molecular functions

Vitamin	Name	Structure	Molecular Function
Vitamin B1	Thiamine		Thiamine plays a central role in the release of energy from carbohydrates. It is involved in RNA and DNA production, as well as nerve function. Its active form is a coenzyme called thiamine pyrophosphate (TPP), which takes part in the conversion of pyruvate to acetyl coenzyme A in metabolism.[16]
Vitamin B2	Riboflavin		Riboflavin is involved in release of energy in the electron transport chain, the citric acid cycle, as well as the catabolism of fatty acids (beta oxidation).[17]
Vitamin B3	Niacin		Niacin is composed of two structures: nicotinic acid and nicotinamide. There are two co-enzyme forms of niacin: nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). Both play an important role in energy transfer reactions in the metabolism of glucose, fat and alcohol.[18] NAD carries hydrogens and their electrons during metabolic reactions, including the pathway from the citric acid cycle to the electron transport chain. NADP is a coenzyme in lipid and nucleic acid synthesis.[19]
Vitamin B5	Pantothenic acid		Pantothenic acid is involved in the oxidation of fatty acids and carbohydrates. Coenzyme A, which can be synthesised from pantothenic acid, is involved in the synthesis of amino acids, fatty acids, ketone bodies, cholesterol,[20] phospholipids, steroid hormones, neurotransmitters (such as acetylcholine), and antibodies.[21]
Vitamin B6	Pyridoxine, pyridoxal, pyridoxamine		The active form pyridoxal 5'-phosphate (PLP) (depicted) serves as a cofactor in many enzyme reactions mainly in amino acid metabolism including biosynthesis of neurotransmitters.
Vitamin B7	biotin		Biotin plays a key role in the metabolism of lipids, proteins and carbohydrates. It is a critical co-enzyme of four carboxylases: acetyl CoA carboxylase, which is involved in the synthesis of fatty acids from acetate; pyruvate CoA carboxylase, involved in gluconeogenesis; β-methylcrotonyl CoA carboxylase, involved in the metabolism of leucine; and propionyl CoA carboxylase, which is involved in the metabolism of energy, amino acids and cholesterol.[22]
Vitamin B9	Folate		Folate acts as a co-enzyme in the form of tetrahydrofolate (THF), which is involved in the transfer of single-carbon units in the metabolism of nucleic acids and amino acids. THF is involved in purine and pyrimidine nucleotide synthesis, so is needed for normal cell division, especially during pregnancy and infancy, which are times of rapid growth. Folate also aids in erythropoiesis, the production of red blood cells.[23]
Vitamin B12	Cobalamin		Vitamin B12 is involved in the cellular metabolism of carbohydrates, proteins and lipids. It is essential in the production of blood cells in bone marrow, and for nerve sheaths and proteins.[24] Vitamin B12 functions as a co-enzyme in intermediary metabolism for the methionine synthase reaction with methylcobalamin, and the methylmalonyl CoA mutase reaction with adenosylcobalamin.[25]

Deficiencies

See also: Vitamin deficiency § Water-soluble vitamins

Several named vitamin deficiency diseases may result from the lack of sufficient B vitamins. Deficiencies of other B vitamins result in symptoms that are not part of a named deficiency disease.

Vitamin	Name	Deficiency effects
B1	Thiamine	Deficiency causes beriberi. Symptoms of this disease of the nervous system include weight loss, emotional disturbances, Wernicke encephalopathy (impaired sensory perception), weakness and pain in the limbs, periods of irregular heartbeat, and edema (swelling of bodily tissues). Heart failure and death may occur in advanced cases. Chronic thiamin deficiency can also cause alcoholic Korsakoff syndrome, an irreversible dementia characterized by amnesia and compensatory confabulation.
B2	Riboflavin	Riboflavin deficiency can cause ariboflavinosis, which may result in cheilosis (cracks in the lips), high sensitivity to sunlight, angular cheilitis, glossitis (inflammation of the tongue), seborrheic dermatitis or pseudo-syphilis (particularly affecting the scrotum or labia majora and the mouth), pharyngitis (sore throat), hyperemia, and edema of the pharyngeal and oral mucosa.
B3	Niacin	Deficiency, along with a deficiency of tryptophan, causes pellagra. Symptoms include aggression, dermatitis, insomnia, weakness, mental confusion, and diarrhea. In advanced cases, pellagra may lead to dementia and death (the 3(+1) D's: dermatitis, diarrhea, dementia, and death).
B5	Pantothenic acid	Deficiency can result in acne and paresthesia, although it is uncommon.
B6	Pyridoxine, pyridoxal, pyridoxamine	Vitamin B6 deficiency causes seborrhoeic dermatitis-like eruptions, pink eye and neurological symptoms (e.g. epilepsy).
B7	Biotin	Deficiency does not typically cause symptoms in adults, other than cosmetic issues such as decreased hair and nail growth,[26] but may lead to impaired growth and neurological disorders in infants. Multiple carboxylase deficiency, an inborn error of metabolism, can lead to biotin deficiency even when dietary biotin intake is normal.
B9	Folic acid	Deficiency results in a macrocytic anemia, and elevated levels of homocysteine. Deficiency in pregnant women can lead to birth defects.
B12	Cobalamins	Deficiency results in a macrocytic anemia, elevated methylmalonic acid and homocysteine, peripheral neuropathy, memory loss and other cognitive deficits. It is most likely to occur among elderly people, as absorption through the gut declines with age; the autoimmune disease pernicious anemia is another common cause. It can also cause symptoms of mania and psychosis. In rare extreme cases, paralysis can result.

Side effects

Because water-soluble B vitamins are eliminated in the urine, taking large doses of certain B vitamins usually only produces transient side effects (only exception is pyridoxine). General side effects may include restlessness, nausea and insomnia. These side effects are almost always caused by dietary supplements and not foodstuffs.

Vitamin	Tolerable Upper Intake Level (UL)	Harmful effects
B1	None[27]	No known toxicity from oral intake. There are some reports of anaphylaxis caused by high-dose thiamin injections into the vein or muscle. However, the doses were greater than the quantity humans can physically absorb from oral intake.[27]
B2	None[28]	No evidence of toxicity based on limited human and animal studies. The only evidence of adverse effects associated with riboflavin comes from in vitro studies showing the production of reactive oxygen species (free radicals) when riboflavin was exposed to intense visible and UV light.[28]
B3	US UL = 35 mg as a dietary supplement[29]	Intake of 3000 mg/day of nicotinamide and 1500 mg/day of nicotinic acid are associated with nausea, vomiting, and signs and symptoms of liver toxicity. Other effects may include glucose intolerance, and (reversible) ocular effects. Additionally, the nicotinic acid form may cause vasodilatory effects, also known as flushing, including redness of the skin, often accompanied by an itching, tingling, or mild burning sensation, which is also often accompanied by pruritus, headaches, and increased intracranial blood flow, and occasionally accompanied by pain.[29] Medical practitioners prescribe recommended doses up to 2000 mg per day of niacin in either immediate-release or slow-release formats, to lower plasma triglycerides and low-density lipiprotein cholesterol.[30]
B5	None	No toxicity known.
B6	US UL = 100 mg/day; EU UL = 25 mg/day	See Vitamin B6 § Toxicity for more information.
B7	None	No toxicity known.
B9	1 mg/day[31]	Masks B12 deficiency, which can lead to permanent neurological damage.[31]
B12	None established[32]	Skin and spinal lesions. Acne-like rash [causality is not conclusively established].[32][33]

Discovery

Vitamin	Name	Discoverer	Date	Notes
B1	Thiamine	Umetaro Suzuki	1910	Failed to gain publicity.
		Casimir Funk	1912
B2	Riboflavin	D.T. Smith and E.G. Hendrick	1926	Max Tishler invented methods for synthesizing it.
B3	Niacin	Conrad Elvehjem	1937
B5	Pantothenic acid	Roger J. Williams	1933
B6	Pyridoxine etc	Paul Gyorgy	1934
B7	Biotin	Research by multiple independent groups in the early 1900s; credits for discovery include Margaret Averil Boas (1927),[34] Paul Gyorgy (1939, as Vitamin H),[35] and Dean Burk.[36]
B9	Folic acid	Lucy Wills	1933
B12	Cobalamins	Five people have been awarded Nobel Prizes for direct and indirect studies of vitamin B12: George Whipple, George Minot and William Murphy (1934), Alexander R. Todd (1957), and Dorothy Hodgkin (1964).[37]

Related compounds

Many of the following substances have been referred to as vitamins as they were once believed to be vitamins. They are no longer considered as such, and the numbers that were assigned to them now form the "gaps" in the true series of B-complex vitamins described above (for example, there is no vitamin B₄). Some of them, though not essential to humans, are essential in the diets of other organisms; others have no known nutritional value and may even be toxic under certain conditions.

• Vitamin B₄: can refer to the distinct chemicals choline, adenine, or carnitine.[38][39] Choline is synthesized by the human body, but not sufficiently to maintain good health, and is now considered an essential dietary nutrient.[40] Adenine is a nucleobase synthesized by the human body.[41] Carnitine is an essential dietary nutrient for certain worms, but not for humans.[42]
• Vitamin B₈: adenosine monophosphate (AMP), also known as adenylic acid.[43] Vitamin B₈ may also refer to inositol.[44]
• Vitamin B₁₀: para-aminobenzoic acid (pABA or PABA), a chemical component of the folate molecule produced by plants and bacteria, and found in many foods.[45][46] It is best known as a UV-blocking sunscreen applied to the skin, and is sometimes taken orally for certain medical conditions.[45][47]
• Vitamin B₁₁: pteryl-hepta-glutamic acid (PHGA; chick growth factor). Vitamin Bc-conjugate was also found to be identical to PHGA.
• Vitamin B₁₃: orotic acid.[48]
• Vitamin B₁₄: cell proliferant, anti-anemia, rat growth factor, and antitumor pterin phosphate, named by Earl R. Norris. Isolated from human urine at 0.33ppm (later in blood), but later abandoned by him as further evidence did not confirm this. He also claimed this was not xanthopterin.
• Vitamin B₁₅: pangamic acid,[48] also known as pangamate. Promoted in various forms as a dietary supplement and drug; considered unsafe and subject to seizure by the US Food and Drug Administration.[49]
• Vitamin B₁₆: dimethylglycine (DMG)[50] is synthesized by the human body from choline.
• Vitamin B₁₇: pseudoscientific name for the poisonous compound amygdalin, also known as the equally pseudoscientific name "nitrilosides" despite the fact that it is a single compound. Amygdalin can be found in various plants, but is most commonly extracted from apricot pits and other similar fruit kernels. Amygdalin is hydrolyzed by various intestinal enzymes to form, among other things, hydrogen cyanide, which is toxic to human beings when exposed to a high enough dosage. Some proponents claim that amygdalin is effective in cancer treatment and prevention, despite its toxicity and a severe lack of scientific evidence.[51]
• Vitamin B₂₀: L-carnitine.[50]
• Vitamin B_f: carnitine.[43]
• Vitamin B_m: myo-inositol, also called "mouse antialopaecia factor".[52]
• Vitamin B_p: "antiperosis factor", which prevents perosis, a leg disorder, in chicks; can be replaced by choline and manganese salts.[42][43][53]
• Vitamin B_T: carnitine.[54][42]
• Vitamin B_v: a type of B₆ other than pyridoxine.
• Vitamin B_W: a type of biotin other than d-biotin.
• Vitamin B_x: an alternative name for both pABA (see vitamin B₁₀) and pantothenic acid.[42][47]

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