الفهرس 
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Abstract
While we are all familiar with the important role of vitamin D in calcium absorption and bone metabolism, many doctors and patients are not aware of the recent research on vitamin D and the widening range of therapeutic applications available for cholecalciferol, which can be classified as both a vitamin and a proohormone. Additionally, we also now realize that the Food and Nutrition Board’s previously defined Upper Limit for safe intake at 2,000 IU/day was set far too low and that the physiologic requirement for vitamin D in adults may be as high as 5,000 ill/day, which is less than half of the> 10,000 IU that can be produced endogenously with full-body sun exposure (Vieth, et al., 1999; Heaney, et al., 2003).
With the discovery of vitamin D receptors in tissues other than the gut and bone especially the brain, breast, prostate, and lymphocytes and the research suggesting that higher vitamin D levels provide protection from diabetes mellitus, osteoporosis, osteoarthritis, hypertension, cardiovascular disease, metabolic syndrome, depression, several autoimmune diseases, and cancers of the breast, prostate, and colon, we can now utilize vitamin D for a wider range of preventive and therapeutic applications to maintain and improve our patients’ health (Holick, 2004).
Vitamin D is obtained from two sources; synthesis in the skin (major source) and dietary sources such as fortified milk, milk products, oily fish, egg yolks, and fortified foods (Olson, 1998).
Vitamin D3 is formed after exposure of 7 -dehydocholesterol in the skin to ultraviolet light Solar UV -B (wavelengths of 290-315 nm) then it goes to circulation and bound to vitamin D-binding protein. This complex is transported to the liver where the vitamin D is hydroxylated to form 25-hydroxyvitamin D (calcidiol)which is transported to the kidney where it is hydroxylated again to form 1,25 hydroxyvitamin