Use caution with selenium supplements

Early studies of selenium supplementation indicated that the mineral might reduce prostate cancer risk. However, a National Cancer Institute trial designed to specifically test that theory was halted in 2008, almost three years early, after researchers learned that selenium had no effect on prostate cancer risk. But that trial didn’t address the impact of selenium on men who already have prostate cancer.

So researchers in Boston and San Francisco analyzed blood samples, DNA, and medical records of 489 patients who had been diagnosed with prostate cancer between 1994 and 2001. They measured the level of selenium in the blood and, using stored DNA, determined the specific form of a gene — SOD2 — carried by each patient. Having a high level of selenium was associated with a slightly higher risk of aggressive prostate cancer.

Having a certain variant of the SOD2 gene, however, seemed to more strongly affect risk: men with the highest selenium levels and the “AA” form of the SOD2 gene were 40% less likely to have been diagnosed with aggressive prostate cancer than men with the same form of the gene and low levels of selenium. For men carrying the “V” form of the gene, selenium had the opposite effect: those with the highest level of selenium were twice as likely to have an aggressive type of cancer as their counterparts with low selenium levels.

The interaction between the SOD2 gene and selenium may help explain the apparently conflicting results about the benefits of the mineral in previous studies.

SOURCE: Chan JM, Oh WK, Xie W, et al. Plasma Selenium, Manganese Superoxide Dismutase, and Intermediate- or High-Risk Prostate Cancer. Journal of Clinical Oncology 2009;27:3577–83. PMID: 19528373.

Originally published November 2009; last reviewed February 24, 2011.