Vitamins are organic compounds that are nutritionally essential in small amounts to control metabolic processes and cannot be synthesized by the body. Vitamins are usually classified by their solubility, which to some degree determines their stability; occurrence in foodstuffs; 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.