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Vitamins and minerals are essential nutrients. That is, they cannot be synthesized by the body and so must be consumed in the diet.

Vitamins are classified as water soluble—vitamin C and the eight members of the vitamin B complex—or fat soluble—vitamins A, D, E, and K (see Vitamins: Introduction). Only vitamins A, E, and B₁₂ are stored to any large extent in the body.

Some minerals are required in fairly large quantities (about 1 or 2 grams a day) and are considered macronutrients (see Minerals and Electrolytes: Introduction). They include calcium, chloride, magnesium, phosphorus (occurring mainly as phosphate in the body), potassium, and sodium. Minerals required in small amounts (trace minerals) are considered micronutrients. They include chromium, copper, fluoride, iodine, iron, manganese, molybdenum, selenium, and zinc. Except for chromium, all of these minerals are incorporated into enzymes or hormones required in metabolism. Chromium helps the body keep blood sugar levels normal. Trace minerals such as arsenic, cobalt, fluoride, nickel, silicon, and vanadium, which may be essential in animal nutrition, have not been established as requirements in human nutrition. Fluoride helps stabilize the mineral content of bones and teeth by forming a stable compound with calcium and thus helps prevent tooth decay. All trace minerals are toxic at high levels, and some (arsenic, nickel, and chromium) can cause cancer.

Some vitamins (such as vitamins C and E) and minerals (such as selenium) act as antioxidants, as do other substances in fruits and vegetables (such as beta-carotene). Antioxidants protect cells against damage by free radicals, which are by-products of the normal activity of cells. Free radicals readily participate in chemical reactions—some useful to the body and some not—and are thought to contribute to such disorders as heart and blood vessel disorders and cancer. People who eat enough fruits and vegetables, which are rich in antioxidants, are less likely to develop heart and blood vessel disorders and certain cancers. However, whether these benefits are due to antioxidants, other substances in the fruits and vegetables, or other factors is not known.

Getting enough vitamins and minerals from foods is usually preferable to getting them from supplements. Foods, unlike supplements, contain other substances necessary for good health. However, always eating a healthy, well-balanced diet may be difficult. So taking a multivitamin that contains the recommended daily allowances for vitamins and minerals is a good idea, particularly when a healthy diet may not be possible.