Selenium supplements may raise the risk of squamous cell skin cancer slightly.

Between 1983 and 1993, researchers studied more than 1,300 residents of the Southeast who had been treated for a previous squamous or basal cell skin cancer. (Levels of selenium in the soil are low in that part of the country.) Each got a selenium supplement (200 mcg a day) or a placebo to see if the nutrient would cut their risk of a subsequent cancer.

After 10 years, the researchers found no difference in skin cancer rates. But after 13 years, they detected a 25 percent higher risk of squamous cell cancers in the selenium takers than in the placebo takers.

What to do: This study also found a (unexpected) lower risk of prostate, colon, and lung cancers in selenium takers. In fact, researchers have since launched a trial in 32,000 men to see if selenium (and vitamin E) can protect the prostate. In the meantime, if you've already had a squamous cell cancer, stick to the latest recommended selenium intake (55 mcg a day).

J. Nat. Cancer Inst. 95: 1477, 2003.

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