B vitamins are a class of water-soluble vitamins that play important roles in cellmetabolism and synthesis of red blood cells.[1] They are a chemically diverse class of compounds; some contain sulfur and B12 contains cobalt.[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] while some are more commonly recognized by name than by number, such as pantothenic acid (B5), biotin (B7), and folate (B9).
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.
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.
Sources
B vitamins are found in abundance in meat, eggs, and dairy products.[1] Processed carbohydrates such as sugar and white flour tend to have lower B vitamin 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.[2]
Sources for B vitamins also include spinach, legumes (pulses or beans), whole grains, asparagus, potatoes, bananas, chili peppers, breakfast cereals.[1]
The B12 vitamin is not abundantly available from plant products[3] (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),[4] 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.[5]
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.[6]
Because they are soluble in water, excess B vitamins are generally readily excreted, although individual absorption, use and metabolism may vary.[6] 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.[7][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.[8] 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.[9]
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.[10]
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.[12] 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.[13]
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,[14][better source needed] phospholipids, steroid hormones, neurotransmitters (such as acetylcholine), and antibodies.[15]
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.[16]
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.[17][better source needed]
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.[18]
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.[19][better source needed] 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.[20]
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.
Several named vitamin deficiency diseases may result from the lack of sufficient B vitamins.[1] Deficiencies of other B vitamins result in symptoms that are not part of a named deficiency disease.
Biotin deficiency does not typically cause symptoms in adults, other than cosmetic issues such as decreased hair and nail growth, 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.
Vitamin B12 deficiency results in a macrocytic anemia, elevated methylmalonic acid and homocysteine, peripheral neuropathy, sense loss, change in mobility, 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. Untreated, it is possible to cause irreversible damage to the brain and nerve system — In rare extreme cases, paralysis can result.
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.
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.[21]
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.[22]
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.[23] 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.[24]
Research by multiple independent groups in the early 1900s; credits for discovery include Margaret Averil Boas (1927),[28] Paul Gyorgy (1939, as Vitamin H),[29] and Dean Burk.[30]
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 B10: para-aminobenzoic acid (pABA or PABA), a chemical component of the folate molecule produced by plants and bacteria, and found in many foods.[39][40] It is best known as a UV-blocking sunscreen applied to the skin, and is sometimes taken orally for certain medical conditions.[39][41]
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).[42]
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,[43] 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.[44]
Vitamin B16: dimethylglycine (DMG)[45] 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.[46]
^Whitney N, Rolfes S, Crowe T, Cameron-Smith D, Walsh A (2011). Understanding Nutrition. Melbourne: Cengage Learning.
^National Academy of Sciences. Institute of Medicine. Food and Nutrition Board, ed. (1998). "Chapter 6 - Niacin". Dietary Reference Intakes for Tjiamine, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin and Choline. Washington, DC: National Academy Press.
^Schnepp, Zoe (2002). "Pantothenic Acid". University of Bristol. Retrieved 16 September 2012 – via bris.ac.uk.
^Gropper S, Smith J (2009). Advanced nutrition and human metabolism. Belmont, California: Cengage Learning.
^"Vitamin B6". Micronutrient Information Center, Linus Pauling Institute, Oregon State University, Corvallis, OR. May 2014. Archived from the original on 14 March 2018. Retrieved 7 March 2017.
^Schnepp, Zoe (2002). "Biotin". University of Bristol. Retrieved 17 September 2012 – via bris.ac.uk.
^National Academy of Sciences. Institute of Medicine. Food and Nutrition Board, ed. (1998). "Chapter 8 - Folate". Dietary Reference Intakes for Thiamine, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin and Choline. Washington, DC: National Academy Press.
^Schnepp, Zoe (2002). "Vitamin B12". University of Bristol. Retrieved 16 September 2012 – via bris.ac.uk.
^Food and Nutrition Board, Institute of Medicine (1998). "Biotin". Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press. pp. 374–389.
^SPIES, TD; GARCIA LOPEZ, G (1947). "A comparative study of the effectiveness of synthetic folic acid, pteroyldiglutamic acid, pteroyltriglutamic acid and pteroylheptaglutamic acid (Bc conjugate)". Internationale Zeitschrift fur Vitaminforschung. International journal of vitamin research. Journal international de vitaminologie. 19 (1–2): 1–12. PMID18905002.
^Carter HE, Bhattacharyya PK, Weidman KR, Fraenkel G (July 1952). "Chemical studies on vitamin BT isolation and characterization as carnitine". Archives of Biochemistry and Biophysics. 38 (1): 405–16. doi:10.1016/0003-9861(52)90047-7. PMID12997117.
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