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Vitamin D is a fat soluble vitamin primarily created by the action of UVB sunlight on the skin. Vitamin D's journey through the body begins when its precursor is converted to cholecalciferol (or D3) by sunlight. From there, D3 travels to the liver and becomes five times more potent, then travels to the kidneys where it's converted to it's most potent form, 1, 25-dihydroxycholecalciferol. This form is ten times as potent as D3, and more than 2,000 genes and tissues throughout the body are regulated by this form.
Vitamin D has long been known to be responsible for calcium balance in the body, helping the body to absorb calcium and maintain strong healthy bones. But recent studies have shown that Vitamin D's influence on the body reaches far beyond the bones. It has been linked to cell differentiation, allowing the body to recognize when to allow cells to proliferate (such as when a wound requires healing) and when to stop excessive cell growth, which can lead to diseases such as cancer. A powerful immune system modulator, D3 deficiencies have been linked to many autoimmune diseases as well as impaired insulin secretion in Type II Diabetes (Higdon, 2008).
The best way to maintain adequate levels of Vitamin D is through short daily periods of as much skin being exposed to the sun as possible (being very careful not to burn). Alternately, you may supplement 5,000 IU of Vitamin D per day for 2-3 months, then monitor blood levels with a 25-hydroxyvitamind blood test, adjusting your dose as necessary to maintain 50-80 ng/mL. However, make sure your supplement is D3, as D2 is a synthetic version and does not produce the same health benefits.