Organic compounds nutritionally essential in small amounts to control metabolic processes in the body. They cannot be synthesized internally. Vitamins are usually classified by their solubility, their occurrence in foodstuffs, their distribution in body fluids, and tissue storage capacity. Each of the fat-soluble vitamins A, D, E and K has a distinct and separate physiologic role. Several have antioxidant properties to depress the effects of metabolic byproducts called free radicals, which are thought to cause degenerative changes related to aging. Most of the water-soluble vitamins are components of essential enzyme systems. Many are involved in the reactions supporting energy metabolism. These vitamins are not normally stored in the body in appreciable amounts and are normally excreted in the urine. Thus, a daily supply is desirable to avoid depletion and interruption of normal physiologic functions.