Vitamin, any of the organic compounds required by the body in small amounts for metabolism, to protect health, and for proper growth in children. Vitamins also assist in the formation of hormones, blood cells, nervous-system chemicals, and genetic material. The various vitamins are not chemically related, and most differ in their physiological actions. They generally act as catalysts, combining with proteins to create metabolically active enzymes that in turn produce hundreds of important chemical reactions throughout the body. Without vitamins, many of these reactions would slow down or cease. The intricate ways in which vitamins act on the body, however, are still far from clear.The 13 well-identified vitamins are classified according to their ability to be absorbed in fat or water. The fat-soluble vitamins-A, D, E, and K-are generally consumed along with fat-containing foods, and because they can be stored in the body's fat, they do not have to be consumed every day. The water-soluble vitamins-the eight B vitamins and vitamin C-cannot be stored and must be consumed frequently, preferably every day (with the exception of some B vitamins, as noted below).The body can manufacture only vitamin D; all others must be derived from the diet. Lack of them causes a wide range of metabolic and other dysfunctions. In the U.S., since 1940, the Food and Nutrition Board of the National Research Council has published recommended dietary allowances (RDA) for vitamins, minerals, and other nutrients. Expressed in milligrams or international units (IU) for adults and children of normal health, these recommendations are useful guidelines not only for professionals in nutrition but also for the growing number of families and individuals who eat irregular meals and rely on prepared foods, many of which are now required to carry nutritional labeling.A well-balanced diet contains all the necessary vitamins, and most individuals who follow such a diet can c...
