Selenium's impact on prostate cancer and diabetes

Q. I've been taking a selenium pill every day to try to reduce my risk of prostate cancer. But now I've read that selenium can cause diabetes. My blood sugar has always been normal, but I'm concerned. Should I continue taking selenium?

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. 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 one of the few supplements still in play; the others are vitamin D and fish oil.

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 1996, when a randomized clinical trial, the Nutritional Prevention of Cancer (NPC) trial, reported that a daily selenium dose of 200 micrograms reduced the risk of dying from prostate cancer by 63%. Two subsequent reports extended the follow-up to 7.6 years and still found a 49% lower risk of prostate cancer among the men who took selenium.

Other studies followed. Several, including two large Harvard studies, were optimistic, but others were not. Still, a meta-analysis of 16 studies of selenium found that the supplement appeared to reduce a man's risk of developing prostate cancer by 26%.

That's the good, or at least hopeful, news. The concern 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.

The possible link between selenium and diabetes raises important concerns, and it got a lot of publicity. But two other 2007 reports that put selenium in a favorable light got less attention. Between 1991 and 1993, French scientists evaluated 1,389 people age 60 to 71. The volunteers underwent six additional evaluations over the next nine years. At the end of the study, the researchers found that the subjects whose blood selenium levels fell the most also experienced the greatest decline in mental function. And in the other report, Italian scientists linked low blood selenium levels to poor muscle strength in 891 people age 65 and over.

None of these studies proves that selenium is either helpful or harmful. But they do show how hard it is to pass judgment on the basis of just a few reports. Fortunately, the National Cancer Institute is sponsoring a study of selenium and vitamin E in 32,400 men. When the results are available, we may be able to give selenium a thumbs up or thumbs down — but that may take another four years. Until then, each man will have to decide for himself. Since you know your blood sugar is normal, you may decide to continue taking selenium, but a man at risk for diabetes might reach a different decision.

Marc Garnick, M.D., is an internationally renowned expert in medical oncology and urologic cancer, with a special emphasis on prostate cancer. He is a Clinical Professor of Medicine at Harvard Medical School and maintains an active oncology practice at Beth Israel Deaconess Medical Center. Dr. Garnick serves as Editor in Chief of Perspectives on Prostate Diseases, a quarterly report from Harvard Health Publications.

Vitamins and Minerals
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