Vitamin D (where D refers to D2 or D3) is a fat-soluble pro-hormone that when metabolized into its active form targets less than 200 human genes in a wide variety of tissues in the body. In addition to the major physiological function of vitamin D metabolites to maintain calcium and phosphate homeostasis, vitamin D status has been associated with a variety of disease states including cancer, cardiovascular disease, diabetes, multiple sclerosis, osteoporosis, rheumatoid arthritis, and chronic pain.

Vitamin D3 (cholecalciferol) is formed in the skin upon exposure to sunlight and vitamin D2(ergocalciferol) is obtained from the ultraviolet irradiation of plant materials (Figure 1). Natural sources of vitamin D3 include oily fish (such as salmon or mackerel), cod liver oil, and fortified food (such as milk, orange juice, butter, cheeses, and breakfast cereals). Prescription vitamin D preparations contain ergocalciferol (50,000 IU/ capsule) while over-the-counter supplements contain cholecalciferol (400, 800, 1000, and 2000 IU/capsule).

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