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.

There are 15 well-identified vitamins. 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 11 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 1941, the Food and Nutrition Board of the National Research Council has published recommended dietary allowances (RDA) for vitamins, minerals, and other nutrients. For adults and children of normal health, the recommendations set in 1989 have been 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. In August 1997 the Institute of Medicine reviewed and expanded the dietary requirements for calcium and other nutrients related to the health of bones; an additional report in 1998 set new recommendations for the B vitamins. Until new RDAs on other nutrients are issued, the 1989 recommendations remain in use.

A well-balanced diet contains all the necessary vitamins, and most individuals who follow such a diet can correct any previous vitamin deficiencies. However, persons who are on special diets, who are suffering from intestinal disorders that prevent normal absorption of nutrients, or who are pregnant or lactating may need particular vitamin supplements to bolster their metabolism. Beyond such real needs, vitamin supplements are also often popularly believed to offer "cures" for many diseases, from colds to cancer; but in fact the body quickly eliminates most of these preparations without absorbing them. In addition, the fat-soluble vitamins can block the effect of other vitamins and even cause severe poisoning when taken in excess.

Much attention has been given to the so-called antioxidants, vitamins C, E, and beta-carotene (from which vitamin A is formed), found in fruits and vegetables. Consumption of foods high in antioxidants appears to neutralize the cell-damaging byproducts, called free radicals, which are formed in cells as the body metabolizes oxygen. Research is under way to determine if it is the antioxidant vitamins themselves or something else in these foods that protects against free radicals, and experts warn against the over consumption of vitamin supplements.

Vitamin A The B Vitamins Vitamin C Ascorbic Acid
Vitamin D Vitamin E Vitamin K