Vitamin D is found in many dietary sources, such as fish, eggs, fortified milk, and fish oil. The sun contributes significantly to daily production of vitamin D; as little as 10 minutes of exposure may be enough to prevent deficiencies. The term "vitamin D" refers to several different forms of this vitamin. Two forms are important in humans: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D2 is synthesized by plants. Vitamin D3 is synthesized by humans in the skin when it is exposed to ultraviolet B (UVB) rays from sunlight.
The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. It is used, alone or in combination with calcium, to increase bone mineral density and decrease fractures.
Because Crohn's disease limits the absorption of vitamins into the blood stream it can be difficult to maintain proper levels of Vitamin D and therefore to maintain healthy bones. Vitamin D is often prescribed to Crohn's patients to prevent poor bone density, increased fracturing, and bone loss among other symptoms.
Information from Mayo Clinic
Current Users' Health
Compared to people not currently on Vitamin D
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