Vitamin D, Sunlight, and the Epidemiology of Prostate Cancer

Abstract

The hypothesis that vitamin D deficiency increases the risk of clinical prostate cancer has stimulated an extensive body of research. Ecologic studies have shown that mortality rates from prostate cancer are inversely correlated with levels of ultraviolet radiation, the principal source of vitamin D. Human prostate cells express receptors for 1,25-Dihydroxyvitamin D which exerts pleitropic anticancer effects on these cells in vitro and in animal models. Moreover, normal prostate cells synthesize 1,25-Dihydroxyvitamin D from circulating levels of 25-OHD, whose levels are dependent on exposure to ultraviolet light. Analytic epidemiologic studies of vitamin D and prostate cancer have focused on polymorphisms in the vitamin D receptor (VDR), on serum vitamin D levels, and on solar exposure. A role for VDR polymorphisms in prostate cancer risk and progression is established. Prospective studies of serum 25(OH)D do not support a protective role for higher levels of 25(OH)D on prostate cancer risk overall, but a role for vitamin D deficiency is supported by several studies. Conversely, a growing body of evidence implicates low levels of 25-OHD with an increased risk of fatal prostate cancer. The results of most epidemiologic studies of sunlight exposure are consistent with a protective effect of exposure to ultraviolet radiation. The discrepancy between the results of studies of solar exposure and studies of serum 25-OHD may be related to methodological differences and to uncertainties regarding the critical period for vitamin D exposure. Additionally, both high dietary intake of calcium and high levels of calcium in serum are positively associated with prostate cancer risk. The relationship between serum 25(OH)D levels and risk of prostate cancer may differ by calcium intake.