Selenium is an essential trace mineral—that is, it’s needed in small amounts to maintain good health and must be consumed, since the body does not manufacture it. The Recommended Dietary Allowance (RDA) is only 55 micrograms a day (a microgram is one-thousandth of a milligram). It is plentiful in foods and in baker’s yeast. Among its purported benefits: It prevents cancers—notably prostate cancer. But does that claim hold up?
What the studies show
Early studies observed that in areas where foods are richer in selenium, there are fewer cancer-related deaths; researchers also found an association between low blood selenium levels and an increased risk of several cancers. Some studies have found associations between higher selenium levels in toenails (a measure of long-term selenium intake) and a lower risk of certain cancers, including bladder cancer.
In the Nutritional Prevention of Cancer Trial in 1996, selenium (200 micrograms a day from baker’s yeast) dramatically reduced the risk of prostate cancer, mostly in men with low initial selenium levels. And in the Physicians’ Health Study, men with the highest blood selenium levels were only half as likely to develop advanced prostate cancer as men with the lowest levels.
But in 2008, the important Selenium and Vitamin E Cancer Prevention Trial (SELECT) found no benefit against prostate cancer from either supplement over five years. In 2011, after three more years of follow-up, researchers found that the men who had taken vitamin E (400 IU a day of the synthetic form) had a 17 percent increased risk of prostate cancer, compared to those taking a placebo. This suggests that effects of supplements can show up years after people stop taking them. Then in a further analysis of SELECT data in 2014, researchers found that in men with low baseline levels of selenium, vitamin E supplements increased the risk of prostate cancer by 63 percent (and more than doubled the risk of high-grade cancer). This is a good example of how high-dose supplements can have complex and unpredictable effects and interactions.
A 2011 review by the Cochrane Collaboration of 55 studies on selenium found that in most observational studies, men who consumed the most selenium in their diet tended to have a lower risk of several cancers; there was minimal effect in women. But the more reliable clinical trials, using supplements, failed to confirm the benefits. “There is no convincing evidence that selenium supplements can prevent cancer,” the researchers concluded.
The FDA has approved this highly qualified health claim regarding selenium and prostate cancer: “Selenium may reduce the risk of prostate cancer. Scientific evidence concerning this claim is inconclusive. Based on its review, FDA does not agree that selenium may reduce the risk of prostate cancer.”
Bottom line advice
Eat foods rich in selenium, such as whole grains and nuts, but skip the supplements, because of the inconsistent research results. The effects of the supplements depend largely on your initial selenium status (as well as genetic factors and when the mineral is taken in terms of the stage of cancer development), according to several recent papers. That is, if your diet supplies adequate amounts, additional selenium is unlikely to do any good and could be risky. Most Americans already get more than the RDA.