The form of vitamin D that we get from food or supplements is not fully active.  It requires conversion by the liver, and then by the kidneys, before it becomes fully active. This is why people with liver or kidney disorders are at a higher risk of osteoporosis. 

When the skin is exposed to the sun's ultraviolet rays, a cholesterol compound  in the skin is transformed into a precursor of vitamin D. Exposing the face and arms to the sun for fifteen minutes three times a week is an effective way to ensure adequate  amounts of vitamin D in the body. 

Vitamin D has been  the ignored vitamin until recently. Studies have shown  that at least 40% of people have less-than-optimal levels of the vitamin in their blood. As much as 70 to 80 percent of Hispanic-Americans and African-Americans may be deficient in vitamin D. Those with more coloring in the skin have a harder time absorbing vitamin D from sunlight. In addition, those who live above the 37th latitude obtain virtually no vitamin D from sunlight between November and March. 

Not getting enough vitamin D in the diet or from direct sunlight has been linked to the development of several diseases including heart disease, osteoporosis, diabetes and cancers such as breast and colon. As baby boomers age, the risk of osteoporosis increases. Taking more than 400 IU of vitamin D has been shown to reduce the risk of fractures by 20 percent in those over sixty-five years of age.