It is known that a substantial proportion of the U.S. population is exposed to suboptimal levels of sunlight. Under these conditions, vitamin D becomes a true vitamin which dictates that it must be supplied in the diet on a regular basis. Since vitamin D3 can be produced by the body and since it is retained for long periods of time by animal tissues, it has been difficult to determine with precision the minimum daily requirements for this fat soluble vitamin. However, in some parts of the world where food is not routinely fortified and sunlight is often limited during some periods of the year, obtaining adequate amounts of vitamin D becomes more of a problem. As a result, the incidence of rickets in these countries is higher than in the United States.
Animal products constitute the bulk source of vitamin D that occurs naturally in unfortified foods.
Salt water fish such as herring, salmon, sardines, and fish liver oils are good sources of vitamin D3. Since the 1960 s vitamin D3 has been used also for the supplementation of farm animal and poultry food. International Units of vitamin D3 was manufactured and sold for over 3 million dollars. This vitamin D3 is then purified and crystallized further before it is formulated for use in dairy milk and animal feed supplementation. Milk from all lactating animals, including humans, contains vitamin D3 that has been produced photochemically from 7-dehydrocholesterol present in the skin.