Vitamins are carbon-containing substances are required for normal metabolism but are not synthesized in the body. They are obtained, therefore, from such outside sources as food and water or are administered orally or intravenously. Exceptions to this definition include vitamin D, synthesized in the body to limited extent, and vitamin B12 and K, which are synthesized by bacterial flora in the intestinal tract. Minerals also must be obtained from outside sources.
Vitamins and minerals function as “cofactors” in the metabolism of product in the body. Most aspects of bodily metabolism proceed with the aid of specific enzymes, but if additional catalysts were not present, for example, the cofactor vitamins and minerals, the reactions would proceed so slowly that they would be ineffective.
RDA, in the United States a food and nutrition board was established to determine vitamin and mineral requirements. Since 1940 the board has periodically prepared a brochure listing are “Recommended Dietary Allowances (RDA) of vitamins and other nutrients, based on existing knowledge. The RDA figures are estimations of the needs of most human beings; particular requirement will be less or more, depending on numerous individual factors such as genetics, environmental-influences, and presence or absence of disease.
A diet containing generous amount of fresh vegetables, fruits, dairy products, and meat assures and adequate intake of vitamins and minerals. Extra vitamin intake is not necessary. Dietary supplements of vitamin are often recommended by physician when any of the following condition is present; diet obviously deficiency in vitamins; conditions or disease causing poor intestinal absorption; and increased needs that occur in relatively healthy individuals during periods of growth, hard physical work, pregnancy, lactation, and menstruation. Some disorders, including hyperthyroidism, infectious diseases, and tissue wasting diseases, also cause increased tissue requirements.
Most of the water-soluble vitamins ingested in excessive amounts are rapidly excreted in the urine and thus rarely cause toxicity. The fat-soluble vitamins, on the other hand, are store in body fat and are capable of causing severe toxicity when taken in excessive amounts.