Any of a group of organic compounds that, in very small amounts, are essential for normal growth, development, and metabolism. They cannot be synthesized in the body (with a few exceptions) and must be supplied by the diet. Lack of sufficient quantities of any of the vitamins produces a specific deficiency disease. Vitamins are generally classified as water-soluble or fat-soluble. The water-soluble vitamins are the vitamin-B complex and vitamin C; the fat-soluble vitamins are vitamins A, D, E, and K.