Q. I've been taking selenium in the hope that it would prevent me from getting prostate cancer. But I heard on the radio that the National Cancer Institute is advising men to stop taking selenium. Should I stop it?
A. Many of us shared the hope that selenium might reduce the risk of prostate cancer. The optimism stemmed from a 1996 report from the Nutritional Prevention of Cancer Trial, which found that 200 micrograms (mcg) of selenium a day reduced the risk of prostate cancer by a startling 63 percent. A series of observational studies followed; although the results were mixed, many suggested that selenium might help.
When results are mixed or surprising, the next step is a careful randomized clinical trial. Beginning in 2001, the National Cancer Institute recruited over 35,000 men age 50 and above to test the effects of selenium and vitamin E, which had also shown mixed results against prostate cancer. The men were randomly assigned to take 200 mcg of selenium, 400 international units (IU) of vitamin E, both selenium and vitamin E, or a placebo every day.
The Selenium and Vitamin E Cancer Prevention Trial (SELECT) was conducted at over 400 research centers in the U.S., Puerto Rico, and Canada at a cost of over $114 million. Results were not expected until 2011, but in late 2008 an independent monitoring group halted the trial because neither supplement was beneficial. In fact, there was a hint that selenium might be responsible for a slight increase in diabetes and that vitamin E might be linked to a slight increase in prostate cancer.
SELECT was expensive, and it produced disappointingly negative results. Still, the trial was very important and very productive. It tells us definitively that neither selenium nor vitamin E has a role in preventing prostate cancer. Scientists will continue to monitor the volunteers for at least three years to conduct additional studies on prostate cancer and other diseases of male aging.
Like the SELECT volunteers, you should stop your supplements. Unfortunately, selenium and vitamin E have joined the ever-lengthening list of supplements that have failed careful, objective scientific testing. -- Harvey B. Simon, M.D. Editor, Harvard Men's Health Watch