A vitamin ( /ˈvaɪtəmɪn/ or /ˈvɪtəmɪn/) is an organic compound required by an organism as a vital nutrient in limited amounts. An organic chemical compound (or related set of compounds) is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on the circumstances and on the particular organism. For example, ascorbic acid (vitamin C) is a vitamin for humans, but not for most other animals, and biotin and vitamin D are required in the human diet only in certain circumstances. By convention, the term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids (which are needed in larger amounts than vitamins), nor does it encompass the large number of other nutrients that promote health but are otherwise required less often. Thirteen vitamins are universally recognized at present.

Vitamins are classified by their biological and chemical activity, not their structure. Thus, each "vitamin" refers to a number of vitamer compounds that all show the biological activity associated with a particular vitamin. Such a set of chemicals is grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A", which includes the compounds retinal, retinol, and four known carotenoids. Vitamers by definition are convertible to the active form of the vitamin in the body, and are sometimes inter-convertible to one another, as well.

Vitamins have diverse biochemical functions. Some have hormone-like functions as regulators of mineral metabolism (e.g., vitamin D), or regulators of cell and tissue growth and differentiation (e.g., some forms of vitamin A). Others function as antioxidants (e.g., vitamin E and sometimes vitamin C). The largest number of vitamins (e.g., B complex vitamins) function as precursors for enzyme cofactors, that help enzymes in their work as catalysts in metabolism. In this role, vitamins may be tightly bound to enzymes as part of prosthetic groups: For example, biotin is part of enzymes involved in making fatty acids. Vitamins may also be less tightly bound to enzyme catalysts as coenzymes, detachable molecules that function to carry chemical groups or electrons between molecules. For example, folic acid carries various forms of carbon group – methyl, formyl, and methylene – in the cell. Although these roles in assisting enzyme-substrate reactions are vitamins' best-known function, the other vitamin functions are equally important.

Until the mid-1930s, when the first commercial yeast-extract vitamin B complex and semi-synthetic vitamin C supplement tablets were sold, vitamins were obtained solely through food intake, and changes in diet (which, for example, could occur during a particular growing season) usually greatly altered the types and amounts of vitamins ingested. However, vitamins have been produced as commodity chemicals and made widely available as inexpensive semisynthetic and synthetic-source multivitamin dietary and food supplements and additives, since the middle of the 20th century.

The term vitamin was derived from "vitamine," a compound word coined in 1912 by the Polish biochemist Kazimierz Funk when working at the Lister Institute of Preventive Medicine. The name is from vital and amine, meaning amine of life, because it was suggested in 1912 that the organic micronutrient food factors that prevent beriberi and perhaps other similar dietary-deficiency diseases might be chemical amines. This proved incorrect for the micronutrient class, and the word was shortened to vitamin.

Read more about Vitamin:  History, In Humans, In Nutrition and Diseases, Supplements, Names in Current and Previous Nomenclatures, Anti-vitamins

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Alan Clemetson - Medical Hypotheses
... conducted and published the first studies concerning ascorbic acid (vitamin C) metabolism and depletion in pre-eclampsia ... be high histamine levels associated with low serum vitamin C, the latter deficiency arising before birth due to factors such as the pregnant mother's malnutrition, and ... Hemorrhage in Infants and Elevated Blood Histamine Caused by Vaccinations and Vitamin C Deficiency May Mimic the Shaken Baby Syndrome ...
Vitamin - Anti-vitamins
... Anti-vitamins are chemical compounds that inhibit the absorption or actions of vitamins ... Pyrithiamine is similar to thiamine, vitamin B1, and inhibits the enzymes that use thiamine ...
... Lumisterol is a compound that is part of the vitamin D family of steroid compounds ... of ergosterol and was produced as a photochemical by-product in the preparation of vitamin D1, which was a mixture of vitamin D2 and lumisterol ... Vitamin D2 can be formed from lumisterol by a electrocyclic ring opening and subsequent sigmatropic hydride shift ...
Calcitriol Receptor
... The calcitriol receptor, also known as the vitamin D receptor (VDR) and also known as NR1I1 (nuclear receptor subfamily 1, group I, member 1), is a member of the nuclear receptor family of transcription factors ... Upon activation by vitamin D, the VDR forms a heterodimer with the retinoid-X receptor and binds to hormone response elements on DNA resulting in expression or transrepression of specific ... In humans, the vitamin D receptor is encoded by the VDR gene ...
Alcohol Consumption - Effects of Alcohol On Health - Alcoholism
... common in alcoholism due to deficiency of folate, riboflavin, vitamin B6, and selenium this can lead to Korsakoff's syndrome ... Wernicke-Korsakoff syndrome, which is caused by a deficiency in thiamine (vitamin B1) ... Osteoporosis and bone fractures may occur due to deficiency of vitamin D ...