Harvard Health Letter

What's up…and what's not

What's up... and what's not

Selenium for prostate cancer prevention

Selenium, a mineral with antioxidant properties, became a cancer prevention candidate in the mid-1990s after a secondary analysis of results from a skin cancer prevention study found that the men assigned to take selenium pills had 63% fewer cases of prostate cancer than men in the placebo group.

Optimism about antioxidants was running high. Other studies also indicated that the mineral might protect the prostate. And at a cost of about a nickel a day, or less than $20 a year, selenium would be an incredible bargain even if the risk reduction was modest. There had also been some results suggesting that vitamin E might protect against prostate cancer. So in 2001, the National Cancer Institute (NCI) launched the Selenium and Vitamin E Cancer Prevention Trial, or SELECT. A total of 35,000 men were enrolled and randomly assigned to take selenium (200 micrograms daily), vitamin E (400 international units), both, or placebo pills. We'd have to wait awhile — final results weren't expected till 2013 — but SELECT was supposed to be the last word on selenium.

As it turned out, the wait wasn't nearly so long, and the word, not very encouraging. In October 2008, NCI officials told the SELECT participants to stop taking their pills. An interim analysis had shown that neither selenium nor vitamin E alone, nor the pills together, lowered prostate cancer risk. Moreover, the data hinted at possible harm, with the vitamin E pills increasing prostate cancer risk slightly, and the selenium pills possibly leading to diabetes. However, the numbers were small, so these results could have been by chance.

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