Vitamins, though they are available from a variety of sources, are unevenly distributed in natural sources. For example, some vitamins, such as vitamin D, are produced only by animals, whereas other vitamins are found only in plants. (For natural sources of vitamins, see table.) All vitamins can be synthesized, or produced commercially, from foods and other sources, and there is no evidence that natural vitamins are superior to those that are synthetically derived.
Some foods are fortified with vitamins--that is, vitamins that are not normally present in the food, or that have been removed during processing, are added to the food before it is sold. Milk, for example, is fortified with vitamin D, and vitamins that have been lost from flour during processing are often replaced.
Although vitamin supplementation is generally unnecessary for otherwise well-nourished persons, there are times when the body's vitamin requirements may increase and when vitamin supplementation may be essential. Those likely to require such supplements include pregnant women, the elderly, and the chronically ill. Excessive intakes of supplemental vitamins should be avoided, however, because of the possibility of toxicity.