Q: I had been taking a selenium pill every day to try to reduce my risk of prostate cancer. But now I’ve read that selenium doesn’t decrease prostate risk and it can cause diabetes. My blood sugar has always been normal, but I’m concerned.

A: Supplements have been taking a big hit lately, and with good reason. As randomized clinical trials have been completed, one supplement after another has been a flop. Selenium supplements are one of the more recent flops.

Vitamins have been the greatest disappointment. First, antioxidant supplements proved worthless (or worse). Next, B vitamins that lower blood homocysteine levels failed to protect the heart. And now, men have reason to rethink that old standby, a daily multivitamin.
Selenium is a mineral, not a vitamin. It’s present in soil, but amounts vary from region to region. From soil, it finds its way into plants and then works its way up the food chain. Good sources include whole grains, tomatoes, onions, garlic, seafood, nuts, meat, and poultry. The Recommended Dietary Allowance for men is only 55 micrograms, but many men get less.
The hope that selenium might protect against prostate cancer stems from earlier studies. But the most recent research has proven otherwise. Selenium supplements should not be taken to help protect against prostate cancer. The National Cancer Institute study of selenium and vitamin E in 32,400 men showed that the men taking selenium supplements actually had a slightly higher risk of prostate cancer. For men taking vitamin E, the prostate cancer risk was 17% higher.

The concern about an association of selenium supplements and diabetes stems from a 2007 report from the same NPC trial that put selenium on the map. The scientists found that selenium supplements were linked to an increased risk of diabetes, but the effect was largely limited to the volunteers who had the highest blood selenium levels before they started taking selenium. And another 2007 study also linked high blood selenium levels to an increased risk of diabetes.

Bottom line: There is no scientific basis for taking selenium supplements. Since your blood sugar is normal, it’s very unlikely that your prior use of selenium will increase your future risk of diabetes. Of course, this assumes you have stopped taking it.

— Howard LeWine, M.D., Harvard Medical School

Last Updated: 04/08