What selenium is: 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 such as whole grains and nuts and in baker’s yeast. When first discovered in 1817, selenium was considered a poison because it is toxic in large amounts.
Claims, purported benefits: Prevents cancers—notably prostate cancer—and wards off many chronic diseases and conditions, including heart disease, diabetes, rheumatoid arthritis, Alzheimer’s disease and pregnancy-related problems.
What the studies show
Prostate cancer. 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 the important Selenium and Vitamin E Cancer Prevention Trial found no benefit against prostate cancer from either supplement; those taking selenium had a slightly increased risk of diabetes. For this reason, the study was halted in late 2008. A study in the Journal of Clinical Oncology in 2009 found that higher selenium levels in the blood may actually worsen prostate cancer in some men who already have the disease. A 2011 study found that men at high risk for prostate cancer who took selenium supplements, in tandem with vitamin E and soy, were as liable to develop invasive cancer as men who took a dummy pill, or placebo.
When it comes to dietary selenium, a 2011 review of 55 studies by the Cochrane Collaboration discovered that in the majority of observational studies, men who got the most dietary selenium were at reduced risk for several cancers. But the quality of the studies was questionable.
The Food and Drug Administration (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.”
Heart disease. A research review published in the American Journal of Clinical Nutrition found no reason to recommend selenium supplements to prevent heart disease. A 2011 Annals of Internal Medicine study found only minor declines in cholesterol, which don't warrant the use of selenium supplements, particularly in countries where residents consume enough selenium (like the U.S.). A 2013 review by the Cochrane Collaboration concluded that selenium supplements do not reduce the risk of cardiovascular disease and may slightly increase the risk of type 2 diabetes.
Diabetes. A 2012 Harvard study published in Diabetes Care linked higher toenail selenium with a reduced risk of type 2 diabetes, but only four percent of the subjects took selenium supplements. However, according to a 2012 review article in the Lancet, some studies suggest that higher selenium intake increase diabetes risk.
Side effects: The safe upper limit for adults is 400 micrograms a day. Dosages of 1,000 micrograms or more a day can cause nausea, vomiting, hair loss and other problems. As mentioned, a few studies have found that selenium (200 micrograms daily) may raise the risk of diabetes. And two recent studies linked high blood levels of selenium with modest increases in cholesterol levels.
Bottom line: Opt for selenium-rich foods and leave the supplements on the shelves. Most Americans already get more than the RDA. If you do take a selenium supplement, don’t take more than 200 micrograms a day.