A vitamin is an organic compound and an essential nutrient that an organism requires in limited amounts. An organic chemical compound is called a vitamin when the organism cannot make 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, vitamin C is a vitamin for humans, but not most other animals which make enough internally. Vitamin D is essential only for people who do not have adequate skin exposure to sunlight, because the ultraviolet light in sunlight normally promotes synthesis of vitamin D. While vitamin supplements are important for the treatment of certain health problems and pregnant women, evidence is limited on the benefits of vitamin supplementation in reducing events of cardiovascular disease and cancer.
The term was coined in 1912 by biochemist Casimir Funk, who isolated a complex of micronutrients and proposed the complex be named vitamine. By convention the word 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. Each vitamin name (the word vitamin followed by a letter of the alphabet ) refers to a number of vitamer compounds that all show the same biological activity. For example, vitamin A refers to the compounds retinal, retinol, and four known carotenoids.
