In a clinical study, one hundred forty men with localized nonmetastatic prostate cancer, who had elected to forgo active treatment, and instead to be followed by active surveillance, were randomly assigned to receive, in double-blind fashion, selenium (200 or 800 mcg per day as high-selenium yeast) or placebo for up to 5 years. Prostate-specific antigen (PSA) velocity was used as a marker of cancer progression. The median PSA doubling time was 6.24 years in the placebo group, 6.98 years in the group receiving 200 mcg.per day of selenium, and 8.45 years in the group receiving 800 mcg per day of selenium (differences not statistically significant).
In secondary subgroup analysis, among men in the highest quartile of baseline plasma selenium, mean PSA velocity was significantly higher in those receiving 800 mcg per day of selenium than in those receiving placebo (p = 0.018). The authors concluded that selenium supplementation did not slow the rise in PSA levels in men with localized prostate cancer, and that high-dose selenium was associated with increased PSA velocity in men with high baseline plasma selenium concentrations.
Selenium has demonstrated anticancer activity in animals, and there is some evidence (though conflicting) that it can help prevent some types of cancer in humans.
In addition, selenium supplementation has been reported to decrease the adverse effects of certain chemotherapy regimens. In the present study, there was a nonsignificant trend toward slower progression of early prostate cancer in men given selenium supplements. However, high-dose selenium treatment may have had an adverse effect in men with high baseline plasma selenium levels. This latter effect must be interpreted with caution, since it was derived from a secondary subgroup analysis.