Long-term results from a clinical trial conducted in Italy suggest that an antioxidant compound may play a role in preventing colorectal cancer. In the trial, participants were randomly assigned to take a placebo or a selenium-based antioxidant compound supplement for 5 years following the removal of precancerous growths in the colon called metachronous adenomas. Participants who took the supplement had a statistically significantly reduced risk of adenoma recurrence compared with participants who took the placebo. The results were reported at the American Association for Cancer Research Frontiers in Cancer Prevention Research conference in Houston.

The reduction in risk extended out 13 years after participants had stopped taking the supplement.

There were 411 participants in the trial, and data on survival and the results of at least one total colonoscopy were available for 311 patients. The supplement used in the trial, manufactured by Pharma Nord, was a combination of selenomethionine, zinc, vitamin C, and relatively high doses of vitamins A and E.

Overall, participants who took the supplement had a 41 percent reduced risk of developing adenomas over the long term. Adenomas returned in 4.2 percent of patients in the supplement arm compared with 7.2 percent in the placebo arm. The reduction was especially pronounced in patients who had previously had advanced adenomas removed, which for this report included 111 participants in the antioxidant arm and 109 in the placebo arm. Among these participants, those who took the supplement had a nearly 90 percent reduction in the risk of developing advanced adenomas compared with those who took the placebo. Participants with prior advanced adenomas did appear to obtain the greatest benefit, and there was little to no toxicity associated with the supplement.

Other data from observational and clinical trials examining the effect of selenium on colorectal cancer have yielded mixed results, showing either a decreased risk or no effect. Selenium also appeared to have no effect in prevention trials for skin and prostate cancer.