BOSTON (Reuters Health) - While a lot of sun exposure increases the risk of skin cancer, a little bit of sunlight is good for you, according to experts who spoke here Friday at the annual meeting of the American Association for the Advancement of Science.

Researchers spoke at a press briefing on the effects of solar radiation on human health, particularly ultraviolet-B (UV-B)--the component of sunlight that triggers the body's production of vitamin D.

Vitamin D doesn't just help keep bones strong, noted Dr. Michael Holick, a professor at Boston University School of Medicine who has studied the effect of sun on the body for more than 10 years. It also activates several genes responsible for regulating cell growth, thus helping to prevent the out-of-control cell division that can lead to cancer, he explained.

Sunlight exposure, Holick said, is a more effective way to get vitamin D than taking the vitamin orally, and vitamin D manufactured by the body from the sun's energy also lasts longer in the body.

"Vitamin D deficiency is a major unrecognized epidemic in the adult population," Holick said. As many as 40% to 50% of adults older than 50 may be deficient in the vitamin, according to the researcher. He noted that some people who appear to have fibromyalgia, a syndrome marked by chronic muscle and joint pain, may in fact have vitamin D deficiency.

Holick also said that African Americans may be "chronically vitamin D deficient," largely because their darker skin allows less UV-B radiation to penetrate.

For whites, who absorb UV-B radiation more easily, Holick said that exposing the hands, face and arms to sunlight for 5 to 10 minutes two to three times a week will be enough to build the body's vitamin D stores.

Also, he added, vitamin D is fat soluble, so if a person gets enough sun in the spring, summer and fall, he or she can rely on stores of the nutrient in their body fat during the winter months when sun is scarce.

But darker skinned people require 10 to 20 times the sun exposure length that whites do to build up the same amount of vitamin D, Holick noted.

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