Selenium Intake May Worsen Prostate Cancer in Some

Higher selenium levels in the blood may worsen prostate cancer in some men who already have the disease, according to a study by researchers at Dana-Farber Cancer Institute and the University of California, San Francisco.

A higher risk of more-aggressive prostate cancer was seen in men with a certain genetic variant found in about 75 percent of the prostate cancer patients in the study. In those subjects, having a high level of selenium in the blood was associated with a two-fold greater risk of poorer outcomes than men with the lowest amounts of selenium. By contrast, the 25 percent of men with a different variant of the same gene and who had high selenium levels were at 40 percent lower risk of aggressive disease. The variants are slightly different forms of a gene that instructs cells to make manganese superoxide dismutase (SOD2), an enzyme that protects the body against harmful oxygen compounds.

The research findings suggest that “if you already have prostate cancer, it may be a bad thing to take selenium,” says Philip Kantoff, MD, director of Dana-Farber’s Lank Center for Genitourinary Oncology and senior author of the study that is published by the Journal of Clinical Oncology on its website now and later will be in a print journal. The lead author is June Chan, ScD, of the University of California, San Francisco.

The unexpected results are the first to raise concern about this potentially harmful consequence of taking supplemental selenium. Kantoff says, “These findings are interesting particularly in light of the recent negative results from the SELECT prevention study, which asked if selenium could protect against prostate cancer.”
The new study reveals the strong interaction between selenium and SOD2 to influence the biology of prostate cancer, a finding that these investigators had shown in a previous study. The authors say the current research demonstrated that variations in the make up of the SOD2 gene dramatically alter the effects of selenium on the risk of aggressive prostate cancer.

Selenium is a mineral found widely in rocks and dirt. Small amounts of selenium are essential for health: 40 to 70 micrograms is the recommended daily intake. In recent years, supplemental selenium has been sold and promoted as a means of preventing prostate cancer, largely based on observational studies that found higher risk of prostate cancer incidence and mortality in areas of the country that are naturally low in selenium.

However, research aimed at confirming the benefits of selenium supplementation have been mixed. Recently, the SELECT study, which involved 35,000 men, was halted early when, after more than five years, it showed that the supplements didn’t affect the incidence of prostate cancer.

Previous studies had found that the risk of developing prostate cancer was modified by a strong interaction between SOD2 and selenium. The new research was designed to look at the effect of this interaction on men already diagnosed with prostate cancer.