Many people take selenium supplements, encouraged by some promising early research suggesting that this antioxidant mineral may help prevent cancer (notably prostate) and heart disease.
In recent years, however, well-designed studies have not found benefits, and some have suggested potential harm. Here’s the latest, mostly disappointing, news:
A new review by the Cochrane Collaboration of 55 studies on selenium intake and cancer 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 Cochrane researchers questioned the quality of these studies. And 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, though they called for more research. Two years ago, after reviewing the research, the Food and Drug Administration (FDA) reached similar conclusions specifically regarding selenium and prostate cancer.
A recent Canadian study in the Journal of Clinical Oncology tested selenium supplements (200 micrograms a day), combined with vitamin E and soy (both also with proposed anti-cancer potential), in men with a precancerous condition that put them at high risk for invasive prostate cancer. After three years, the men taking the supplements were just as likely to develop invasive cancer as those taking a placebo.
Research on selenium’s effect on blood cholesterol (as well as heart disease) has produced conflicting results. A recent study in the Annals of Internal Medicine tested various doses of selenium supplements in 500 older people in Britain, where selenium intake tends to be low.
After six months, levels of total and LDL (“bad”) cholesterol decreased, but the small drops are unlikely to be clinically significant. Moreover, the researchers cautioned that the findings do not justify selenium supplements, particularly in countries where selenium intake is adequate, such as the U.S., and because the safety of selenium pills is uncertain.
Bottom line: Eat foods rich in selenium, such as whole grains and nuts. Brazil nuts are the richest source; one medium nut has 95 micrograms, more than the recommended dietary allowance (RDA)—limit yourself to a couple a day.
But skip the supplements, because of the inconsistent research results. Some research has linked higher selenium intake with increased cancer risk and blood cholesterol levels. And several studies have suggested that high doses may increase the risk of diabetes. The effects of the supplements may depend largely on your initial selenium status. That is, if your diet supplies adequate amounts, additional selenium is unlikely to do any good and may be risky. Most Americans already get more than enough.