B vitamins

B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism and synthesis of red blood cells.^[1][2] They are a chemically diverse class of compounds.^[1]

Dietary supplements containing all eight are referred to as a vitamin B complex. Individual B vitamins are referred to by B-number or by chemical name, such as B₁ for thiamine, B₂ for riboflavin, and B₃ for niacin,^[1][2] while some are more commonly recognized by name than by number, such as pantothenic acid (B₅), biotin (B₇), and folate (B₉).^[1] B vitamins are present in protein-rich foods, such as fish, poultry, meat, dairy products, and eggs; they are also found in leafy green vegetables, beans, and peas.^[1] Fortified foods, such as breakfast cereals, baked products, and infant formulas, may contain B vitamins.^[1]

Each B vitamin is either a cofactor (generally a coenzyme) for key metabolic processes or is a precursor needed to make one.^[1][2]

Note: Other substances once thought to be vitamins were given B-numbers, but were disqualified once discovered to be either manufactured by the body or not essential for life. See #Related compounds for numbers 4, 8, 10, 11, and others.

B vitamins are found in abundance in meat, eggs, and dairy products.^[2] Processed carbohydrates such as sugar and white flour tend to have lower B vitamin content than their unprocessed counterparts. For this reason, it is common in many countries (including the United States) that the B vitamins thiamine, riboflavin, niacin, and folic acid are added back to white flour after processing. This is referred to as "enriched flour" on food labels. B vitamins are particularly concentrated in meat such as turkey, tuna and liver.^[3]

Sources for B vitamins also include spinach, legumes (pulses or beans), whole grains, asparagus, potatoes, bananas, chili peppers, breakfast cereals.^[2] The B₁₂ vitamin is not abundantly available from plant products^[4] (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),^[5] 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.^[6]

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.^[7]

Because they are soluble in water, excess B vitamins are generally readily excreted, although individual absorption, use and metabolism may vary.^[7] 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.^{[medical citation needed]} In cases of severe deficiency, B vitamins, especially B₁₂, may also be delivered by injection to reverse deficiencies.^{[8][unreliable medical source?]} 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.^[9] 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.^[10]

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.

Several named vitamin deficiency diseases may result from the lack of sufficient B vitamins.^[2] Deficiencies of other B vitamins result in symptoms that are not part of a named deficiency disease.

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.

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.^[33][34]
  • Choline is synthesized by the human body, but not sufficiently to maintain good health, and is now considered an essential dietary nutrient.^[35]
  • Adenine is a nucleobase synthesized by the human body.^[36]
  • Carnitine is an essential dietary nutrient for certain worms, but not for humans.^[37]
• Vitamin B₈: adenosine monophosphate (AMP), also known as adenylic acid.^[38] Vitamin B₈ may also refer to inositol.^[39]
• 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.^[40][41] It is best known as a UV-blocking sunscreen applied to the skin, and is sometimes taken orally for certain medical conditions.^[40][42]
• Vitamin B₁₁: pteroylheptaglutamic acid (PHGA; chick growth factor). Vitamin Bc-conjugate was also found to be identical to PHGA. Derivative of folate ("pteroylmonoglutamic acid" in this nomenclature).^[43]
• Vitamin B₁₃: orotic acid.^[44]
• 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,^[44] 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.^[45]
• Vitamin B₁₆: dimethylglycine (DMG)^[46] 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 lack of scientific evidence.^[47]
• Vitamin B₂₀: L-carnitine.^[46]
• Vitamin B_f: carnitine.^[38]
• Vitamin B_m: myo-inositol, also called "mouse antialopaecia factor".^[48]
• Vitamin B_p: "antiperosis factor", which prevents perosis, a leg disorder, in chicks; can be replaced by choline and manganese salts.^[37][38][49]
• Vitamin B_T: carnitine.^[50][37]
• 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.^[37][42]