A vitamin is an organic compound and an essential nutrient that an organism requires in limited amounts. An organic chemical compound (or related set of compounds) is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and it must be obtained through the diet; thus, the term vitamin is conditional upon the circumstances and the particular organism. For example, ascorbic acid (one form of vitamin C) is a vitamin for humans, but not for most other animals. Vitamin D is essential only for people who do not have adequate skin exposure to sunlight, as ultraviolet light promotes synthesis in skin cells. Supplementation is important for the treatment of certain health problems, but there is little evidence of nutritional benefit when used by otherwise healthy people. By convention the term vitamin does not include other essential nutrients, such as dietary minerals, essential fatty acids and 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 inter-convertible to one another, as well.
