Fat-soluble vitamins

Vitamin
Drug class
Photograph of an opened bottle of B-complex vitamin pills
Pronunciation	UK: /ˈvɪtəmɪn,ˈvaɪ-/ US: /ˈvaɪtəmɪn/

A vitamin is an organic compound and an essential nutrient, or micronutrient, that an organism needs in small amounts. An organic chemical compound (or related set of compounds) is called a vitamin when the organism cannot make the compound itself, (either at all, or in sufficient quantities) and it must be obtained through the diet. Different organisms have different vitamin needs. For example, vitamin C is needed in the diet by humans and other primates, but other animals can synthesize it. Vitamin D is a needed micronutrient when its synthesis, initiated by skin exposure to ultraviolet light, is insufficient due to a lack of adequate skin exposure to sunlight.

By convention the term vitamin does not include other essential nutrients, such as dietary minerals, essential fatty acids or essential amino acids. Thirteen vitamins are universally recognized at present. Vitamins are classified by both biological and chemical activity, and 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 interconvertible to one another, as well.

Vitamins have diverse biochemical functions. Some, such as vitamin D, have hormone-like functions as regulators of mineral metabolism, or regulators of cell and tissue growth and differentiation (such as some forms of vitamin A). Others function as antioxidants (e.g., vitamin E and sometimes vitamin C). The largest number of vitamins, the B complex vitamins, function as enzyme cofactors (coenzymes) or the precursors for them; coenzymes 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. They 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 may carry methyl, formyl, and methylene groups in the cell. Although these roles in assisting enzyme-substrate reactions are vitamins' best-known function, the other vitamin functions are equally important.