With a few exceptions, the body is unable to make vitamins; they must be supplied in the daily diet or through supplements. One exception is vitamin D, which can be produced in the skin when the skin is exposed to sunlight. Another vitamin, vitamin K, is not made by the human body but is formed by microorganisms that normally flourish in the intestinal tract only when green, leafy vegetables and vegetable oils are ingested.
The body's vitamin requirements are expressed in terms of recommended dietary allowances, or RDA. These allowances are the amount of essential nutrients that, if acquired daily, are considered to be sufficient to meet the known nutritional needs of most healthy persons. In the United States, the RDA values are established by the Food and Nutrition Board of the National Academy of Sciences/National Research Council (NAS/NRC). In addition, two agencies of the United Nations--the Food and Agriculture Organization and the World Health Organization--develop RDA for different, worldwide population groups
In the past, the strength of a vitamin or the amount of the vitamin necessary to produce a certain effect in the body was often expressed in terms of international units, abbreviated IU. The unit corresponds to a weight of the purified vitamin, and its value differs from one vitamin to another. Today, the strength of a vitamin is generally expressed directly in metric weights--micrograms or milligrams.