Baseline selenium status and effects of selenium and vitamin e supplementation on prostate cancer risk.

BACKGROUND The Selenium and Vitamin E Cancer Prevention Trial found no effect of selenium supplementation on prostate cancer (PCa) risk but a 17% increased risk from vitamin E supplementation. This case-cohort study investigates effects of selenium and vitamin E supplementation conditional upon baseline selenium status. METHODS There were 1739 total and 489 high-grade (Gleason 7-10) PCa cases and 3117 men in the randomly selected cohort. Proportional hazards models estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for effects of supplementation within quintiles of baseline toenail selenium. Cox proportional hazards models were used to estimate hazard ratios, and all statistical tests are two-sided. RESULTS Toenail selenium, in the absence of supplementation, was not associated with PCa risk. Selenium supplementation (combined selenium only and selenium + vitamin E arms) had no effect among men with low selenium status (<60th percentile of toenail selenium) but increased the risk of high-grade PCa among men with higher selenium status by 91% (P = .007). Vitamin E supplementation (alone) had no effect among men with high selenium status (≥40th percentile of toenail selenium) but increased the risks of total, low-grade, and high-grade PCa among men with lower selenium status (63%, P = .02; 46%, P = .09; 111%, P = .008, respectively). CONCLUSIONS Selenium supplementation did not benefit men with low selenium status but increased the risk of high-grade PCa among men with high selenium status. Vitamin E increased the risk of PCa among men with low selenium status. Men should avoid selenium or vitamin E supplementation at doses that exceed recommended dietary intakes.