B vitamins

B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism and synthesis of red blood cells. They are a chemically diverse class of compounds.

Sources for B vitamins also include spinach, legumes (pulses or beans), whole grains, asparagus, potatoes, bananas, chili peppers, breakfast cereals. (although it has been found in moderate abundance in fermented vegetable products, certain seaweeds, and in certain mushrooms, with the bioavailability of the vitamin in these cases remaining uncertain), making B₁₂ deficiency a legitimate concern for those maintaining a vegan diet. 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.

A common way to increase vitamin B intake is by using dietary supplements. B vitamins are commonly added to energy drinks, many of which have been marketed with large amounts of B vitamins.

Because they are soluble in water, excess B vitamins are generally readily excreted, although individual absorption, use and metabolism may vary. 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. Also, folate 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 folate intake or take a supplement.

Molecular functions

{| class="wikitable"

! scope="col" | Vitamin

! scope="col" | Name

! scope="col" | Structure

! scope="col" | Molecular function

|-

! scope="row" | Vitamin B₁

| Thiamine

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| 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.

|-

! scope="row" | Vitamin B₂

| Riboflavin

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| 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).

|-

!Vitamin B₃

| Niacin

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| 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. 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.

|-

! scope="row" | Vitamin B₅

| Pantothenic acid

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| 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, phospholipids, steroid hormones, neurotransmitters (such as acetylcholine), and antibodies.

|-

! scope="row" | Vitamin B₆

| Pyridoxine, pyridoxal, pyridoxamine

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| 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.

|-

! scope="row" | Vitamin B₇

| Biotin

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| 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.

|-

! scope="row" | Vitamin B₉

| Folate

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| 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.

|-

! scope="row" | Vitamin B₁₂

| Cobalamin

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| 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. 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.

|}

thumb|330px|A diagram of the chemical structure of 5 classes of vitamin B (B2, B3, B5, B9, and B12) and the essential biochemical reactants that they are precursors to.

To the right, a diagram of some of the major B vitamins (2, 3, 5, 9, and 12) are shown as precursors for certain essential biochemical reactants (FAD, NAD+, coenzyme A, and heme B respectively). The structural similarities between them are highlighted, which illustrates the precursor nature of many B vitamins while also showing the functionality of the end product used by essential reactions to support human, animal, or cellular life.

FAD, NAD+, and coenzyme A are all essential for the catabolic release of free energy (dG) to power the activity of the cell and more complex life forms. See the article on Catabolism for more details on how these three essential biochemical reactants help support life.

Tetrahydrofolate is a necessary co-reactant for synthesizing some amino acids, such as glycine. Heme B is the porphyrin derivative macrocycle molecule that holds the iron atom in place in hemoglobin, allowing for the transportation of oxygen through blood.

Deficiencies

Several named vitamin deficiency diseases may result from the lack of sufficient B vitamins.

| 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.

| 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.

| 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.

|-

! B₅

| None

| No toxicity known.

|-

! B₆

| US&nbsp;UL: 100&nbsp;mg/day;<br/>EU&nbsp;UL: 12.5&nbsp;mg/day

| Causes megavitamin-B₆ syndrome.

|-

! B₇

| None

| No toxicity known.

|-

! B₉<br/>(Folate)

| 1&nbsp;mg/day

| Masks vitamin B₁₂ deficiency.

| Skin and spinal lesions. Acne-like rash (causality is not conclusively established).

|}

Discovery

{| class="wikitable"

! Vitamin!! Name !! Discoverer!!Date!!Notes

|-

! rowspan="2" |Vitamin&nbsp;B₁

| rowspan="2" |Thiamine || Umetaro Suzuki||1910||Failed to gain publicity.

|-

| Casimir Funk || 1912||

|-

! Vitamin&nbsp;B₂

| Riboflavin || D. T. Smith and E. G. Hendrick

| 1926|| Max Tishler invented methods for synthesizing it.

|-

! Vitamin&nbsp;B₃

| Niacin || Conrad Elvehjem|| 1937||

|-

! Vitamin&nbsp;B₅

| Pantothenic acid || Roger J. Williams||1933||

|-

! Vitamin&nbsp;B₆

| Pyridoxine etc. || Paul Gyorgy||1934||

|-

! Vitamin&nbsp;B₇

| Biotin || colspan=3|Research by multiple independent groups in the early 1900s; credits for discovery include Margaret Averil Boas (1927), Paul Gyorgy (1939, as Vitamin H), and Dean Burk.

|-

! Vitamin&nbsp;B₉

| Folic acid || Lucy Wills||1933||

|-

!Vitamin&nbsp;B₁₂

| Cobalamins || colspan=3|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).

|}

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.
• Choline is synthesized by the human body, but not sufficiently to maintain good health, and is now considered an essential dietary nutrient.
• Adenine is a nucleobase synthesized by the human body.
• Carnitine is an essential dietary nutrient for certain worms, but not for humans. Vitamin B₈ may also refer to inositol.
• 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. It is best known as a UV-blocking sunscreen applied to the skin, and is sometimes taken orally for certain medical conditions.
• Vitamin B₁₃: orotic acid. 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.
• Vitamin B₁₆: dimethylglycine (DMG)
• Vitamin B₂₀: <small>L</small>-carnitine.
• Vitamin B_f: carnitine.
• Vitamin B_p: "antiperosis factor", which prevents perosis, a leg disorder, in chicks; can be replaced by choline and manganese salts.
• Vitamin B_T: carnitine.

References