The research team -- led by Esther John, Ph.D., of the Northern California Cancer Center and including Sue Ingles, Ph.D., of the Keck School of Medicine of the University of Southern California and Gary G. Schwartz, Ph.D., of the Comprehensive Cancer Center of Wake Forest University -- found that men with high sun exposure had half the risk of prostate cancer than did men with low sun exposure.

In men with certain gene variants, risk was reduced even further -- to as much as 65 percent, they said.

Sunlight Provides Active Vitamin D

"We believe that sunlight helps to reduce the risk of prostate cancer because the body manufactures the active form of vitamin D from exposure to sunlight," John said.

The prostate uses vitamin D to promote the normal growth of prostate cells and to inhibit the invasiveness and spread of prostate cancer cells to other parts of the body, previous research by Schwartz and his colleagues had shown.

"The genes involved are those that determine the type of vitamin D receptors a person has," said Schwartz. "These receptors, which function with vitamin D like a lock and key, vary in their ability to bind vitamin D and thus to influence cell behavior."

The researchers stressed that sunlight is not the only source of vitamin D, and that men should not try to reduce their risk of prostate cancer by sunbathing because that increases the risk of sun-induced skin cancer, especially melanoma.

"If future studies continue to show reductions in prostate cancer risk associated with sun exposure, increasing vitamin D intake from diet and supplements may be the safest solution to achieve adequate levels of vitamin D," they said.

Sun Exposure Measured

The researchers compared 450 non-Hispanic white patients in the San Francisco Bay area who had advanced prostate cancer with a matched control group of 455 men who did not have prostate cancer. They defined advanced prostate cancer as cancer that had penetrated through the prostate capsule either to the same region of the body or spread to distant sites.

The scientists measured sun exposure by comparing pigmentation of underarm skin, which is usually not exposed to sunlight, with forehead pigmentation, which is, using a portable reflectometer.

Because it is hard for the sun to reach the underarm area, there was no difference in the underarm measurement between the prostate cancer cases and the control group. But when the forehead color was compared to the underarm color, the control group had significantly darker pigmentation than the cancer patients.

More Sun, Less Cancer

"Increasing darkness was associated with a trend of decreasing risk of prostate cancer," they said. The scientists also obtained a sun exposure history from each participant so they could track outdoor activity.

"Reduced risk of advanced prostate cancer was associated with high sun exposure determined by reflectometry and high occupational outdoor activity," they said. "Further studies in large populations, including non-whites, are warranted to confirm the combined effects of sun exposure and genotype and define the exposure period that is important in influencing prostate cancer risk."

David Van Den Berg, Ph.D., of the University of Southern California also participated in the study. The work was supported by the Cancer Research Fund of the California Department of Health Services.