A Cochrane Systematic Review of randomised controlled clinical trials have found that taking selenium offer no protective effect against non-melanoma skin cancer or prostate cancer. In addition, there is some indication that taking selenium over a long period of time could have toxic effects.
These conclusions were reached after researchers scanned the medical literature, looking for trials that studied the effects of taking selenium supplements and observational studies on selenium intake. The researchers located 49 prospective observational studies and six randomised controlled trials.
Looking at the data from observational studies gave some indication that people may be marginally protected from cancer if they had a higher selenium intake than those with a lower intake, and that the effect was slightly greater for men than women. "These conclusions have limitations because the data came from a wide variety of trials, and so it is difficult to summarise their findings," says lead researcher Dr Gabriele Dennert of the Institute for Transdisciplinary Health Research, Berlin, Germany, who coordinated the work of the international team of experts.
When the team of researchers looked at the more carefully conducted randomised controlled trials, any sign of benefit disappeared. "In fact, the results of the Nutritional Prevention of Cancer Trial and the Selenium and Vitamin E Cancer Prevention Trial raised concerns about possible harmful effects from long-term use of selenium supplements," says Dennert.
The researchers believe that there is a need for more research looking at selenium's effect on liver cancer and think that it would be worth investigating the possible gender differences that appear to be present in the uncontrolled studies.
"However, we could find no evidence to recommend regular intake of selenium supplements for cancer prevention in people whether or not they already have enough selenium," says Dennert.