Tired of spending your entire summer in the shadows? Some scientists are now saying that a moderate amount of sunshine is good for you, and shunning it altogether could actually increase your risk of cancer.

The human body has a love-hate relationship with the sun's ultraviolet rays, Celeste Biever reports in New Scientist (Aug. 9, 2003). Though it's widely known that UV radiation is linked to skin cancer, it's also one of our two main sources of vitamin D. (The other is a diet rich in dairy products and certain oily fish.) It's long been understood that vitamin D helps us absorb calcium, and in recent decades researchers have begun to think it may also block or at least hinder the runaway cell growth associated with cancer.

The evidence intrigues some cancer researchers, who go so far as to suggest that vitamin D-based treatments might be used someday to slow many common forms of the disease, including cancer of the colon, prostate, breast, lung, and skin. And if more vitamin D somehow helps to control cancer growth, does less vitamin D leave us vulnerable to it? At least one scientist thinks that may be the case.

During the 1990s, William Grant, a NASA physicist studying atmospheric ozone, noticed that cancer death rates were higher in the Northeastern United States than in some other regions of the country. Many assumed that a fatty diet was the culprit, but Grant's work pointed to another factor. While fat intake appeared to vary 10 to 20 percent between the Northeast and the Southwest, he saw that cancer incidence between these regions varied by a staggering 150 percent. Diet alone seemed unlikely to account for so big a difference. The real cause? Grant suggests that it may be a lack of vitamin D, tied to a regional underexposure to UV rays.

Grant based his case on a compelling correlation. He found that death rates for 13 common cancers appeared to be higher in places where people tend to get less UV exposure. In other words, people in the sunny Southwest actually seemed less likely to die from these forms of cancer than people in the often overcast Northeast. He went on to estimate that at least 23,600 Americans die annually from cancer tied to a lack of sunshine, compared to 9,800 from skin cancer. When his study was attacked on several grounds, Grant went back and revised his figures -- upward. His new analysis suggested that as many as 40,000 die each year from the apparent cancer-causing effects of sunlight deprivation.