Geographic patterns of prostate cancer mortality. Evidence for a protective effect of ultraviolet radiation.

BACKGROUND. Prostate cancer is the most prevalent nonskin cancer among men in the United States and is the second leading cause of cancer deaths in men. The cause of prostate cancer remains obscure. Recently it was hypothesized that low levels of vitamin D, a hormone with potent antitumor properties, may increase the risk for clinical prostate cancer. METHODS. Because the major source of vitamin D is casual exposure to ultraviolet (UV) radiation, the authors examined the geographic distributions of UV radiation and prostate cancer mortality in 3073 counties of the contiguous United States using linear regression and trend surface analyses. RESULTS. The geographic distributions of UV radiation and prostate cancer mortality are correlated inversely (P < 0.0001). Prostate cancer mortality exhibits a significant north-south trend, with lower rates in the South. These geographic patterns are not readily explicable by other known risk factors for prostate cancer. CONCLUSIONS. These data lend support to the hypothesis that UV radiation may protect against clinical prostate cancer. Viewed in conjunction with other recent data, including those demonstrating a differentiating effect of vitamin D on human prostate cancer cells, these findings suggest that vitamin D may have an important role in the natural history of prostate cancer.