Over the years many men have taken the mineral selenium and/or vitamin E in the belief that these supplements may help prevent prostate cancer (especially when taken together), based largely on some early studies that suggested protective effects. More recent research has made it clear that this was a bad idea indeed.
To put the supplements to the test, the National Cancer Institute started the Selenium and Vitamin E Cancer Prevention Trial (SELECT) in 2001. This large and important study gave men 200 micrograms of selenium and/or 400 IU of synthetic vitamin E a day—or a placebo. It used forms of the nutrients that previous research suggested were beneficial. However, after an average of 5.5 years, the study found that the supplements, taken individually or together, did not reduce the risk of prostate cancer— nor did they reduce the risk of lung or colorectal cancer or cardiovascular disease (these were other proposed benefits of the supplements).
That was disappointing, but in a follow-up done 18 months after the study’s end and after the men had stopped taking the supplements, researchers found that those who had taken vitamin E had a 17 percent higher risk of prostate cancer than those who had taken the placebo. This indicates that long-term effects of supplements can sometimes show up years after the last pills are taken.
The news gets worse
Now comes further analysis of SELECT data, published in the Journal of the National Cancer Institute in February. Researchers wanted to test the hypothesis that selenium supplements might have reduced cancer risk in those men who had relatively low levels of the mineral in their body at baseline (as determined by standard analysis of toenail clippings). This proved not to be the case. What the researchers did find was that in men with high baseline levels of selenium, the supplement almost doubled the risk of high-grade prostate cancer.
What’s more, while the vitamin E supplement did not affect cancer risk in men with high selenium status at baseline, it increased the risk of all prostate cancers by 63 percent (and more than doubled the risk of high-grade cancers) in men with lower initial selenium status.
These unexpected—and somewhat confusing—findings drive home the point that high-dose supplements can have complex and unpredictable effects and interactions, depending in part on how much of the nutrients people normally consume and have in their bodies.
Bottom line: The study’s conclusion makes sense: “Given the risks and lacking evidence of benefit for other diseases of equal or greater public health importance than prostate cancer, men aged 55 and older should avoid supplementation with either vitamin E or selenium at doses that exceed recommended dietary intakes.”