Vitamin D, Season, and Risk of Prostate Cancer: a Nested Case-control Study Within Norwegian Health Studies

Background: It is biologically plausible that vitamin D might prevent prostate cancer. However, recent meta-analyses concluded that there is no consistent relation between circulating 25-hydroxyvitamin D [25(OH)D] concentrations and prostate cancer, and several large studies have actually reported an increased risk of prostate cancer associated with high 25(OH)D.

Objective: We aimed to assess the prospective relations between serum 25(OH)D, serum retinol, and risk of prostate cancer.

Design: In this nested case-control study, Norwegian men who participated in population-based health studies between 1981 and 1991 were followed with respect to prostate cancer throughout 2006. For each case (n = 2106), a matched control was selected. Stored serum was analyzed for 25(OH)D by using HPLC atmospheric pressure chemical ionization mass spectrometry.

Results: We showed a positive relation between an increasing 25(OH)D concentration and prostate cancer risk [rate ratio (RR): 1.15 (95% CI: 1.04, 1.27) per 30-nmol/L increase in 25(OH)D concentration]. Predefined analyses stratified by season showed no relation for subjects with serum collected during winter and spring (RR: 1.00 per 30-nmol/L increase), whereas a strengthened positive association [RR: 1.27 (95% CI: 1.09, 1.47) per 30-nmol/L increase] was observed in men with serum collected during the summer and autumn. There was no relation between serum retinol and prostate cancer.

Conclusions: The cause for increased risk of prostate cancer related to a high 25(OH)D concentration only during the summer and autumn is not obvious. The effect may be related to vitamin D itself or to other factors associated with sun exposure.