Studies increasingly suggest that vitamin D, often known as the sunshine vitamin, is important for a range of body functions including bone metabolism, muscle strength, immune function, reducing hypertension and preventing cancer and autoimmune disease.

Research has also suggested that as many as a billion people around the world may now be vitamin D deficient, posing possibly serious threats to their health.

Doctors are increasingly starting to test their patients for vitamin D deficiency, especially above 40 degrees latitude -- an area that in the U.S. includes Philadelphia, Denver, New York City, Boston, Chicago, Detroit, and Seattle.

The current official U.S. government recommendation for vitamin D is 200 I.U. per day, but many now think this level is far too low. Even moderate sun exposure can provide about 100 times that much. Some studies under way with pregnant women are giving them 4,000 I.U. per day in supplements. At least one recent study indicates that 2,000 I.U. per day could prevent 30 percent of the cases of breast cancer and 50 percent of the cases of colon cancer in the United States, at extremely low cost and with few or no adverse effects.