Vitamins are compounds that you need in small amounts to remain healthy and function normally. They help regulate the chemical reactions the body utilizes to convert food into energy and tissue. They’re important for such diverse functions as forming red blood cells to helping your body fight and avoid infections. Vitamin doses are usually measured in milligrams, micrograms, or international units. Scientists usually group vitamins into two general categories: those soluble in fat and those soluble in water. The fat-soluble vitamins, A, D, E, and K, are stored in the body’s fatty tissue and aren’t regularly excreted in the urine. The water-soluble vitamins, Vitamin C and the B-complex vitamins, dissolve easily in water. They’re excreted in the urine, so they must frequently be replaced. Vitamins and all other dietary supplements are classified as ‘foods’ by the U-S Food and Drug Administration, which regulates their safety and manufacturers’ claims. Those sold in supplement forms are labeled either ‘natural’ or ‘synthetic.’ Synthetic vitamins are copies of the natural vitamins isolated from food. They’re usually cheaper in price, and their potency can be controlled. There’s a difference of opinion as to which is better, and whether or not the body can tell the difference. The amounts of various vitamins that the body needs daily used to be referred to as the ‘recommended dietary allowances,’ or ‘R-D-A’s.’ Today, they’re classified by the percentage of the daily value for each nutrient, or ‘percent D-V.