Clinical Question: Can colorectal cancer risk be decreased with antioxidant supplements?

Setting: Outpatient (any)

Study Design: Meta-analysis (randomized controlled trials)

Synopsis: The researchers conducted this analysis using standard methodology, searching five databases for all randomized trials comparing beta-carotene, vitamin A, vitamin C, vitamin E, or selenium with no treatment or placebo on the development of colorectal adenoma, a cancer precursor. They also searched for unpublished studies. They used The Cochrane Collaboration methodology for conducting the meta-analysis but did not provide details of how they chose studies for inclusion or how they abstracted the data. They assessed the research for quality, identifying studies as high or low quality based on study design. There was no publication bias.

The eight trials used in this analysis studied a total of 17,260 participants, although most of the participants (88 percent) were in one high-quality study. This study enrolled participants without previous adenoma; the rest enrolled participants with previously removed colorectal adenomas or previous colorectal cancer.

Overall, there was no evidence that antioxidants prevented the development of colorectal adenoma. High-quality studies showed no effect or a slight increase, whereas small, low-quality studies found antioxidants to be beneficial. When analyzed separately, none of the individual antioxidants had a beneficial effect on adenoma rates. Vitamin E, used in the largest study, produced a statistically significant increase in the risk of colorectal adenoma (relative risk 1.7, 95% confidence interval, 1.1 to 2.8).

Bottom Line: Antioxidant supplementation for up to six years does not decrease the risk of colorectal adenomatous polyps and, by extension, colorectal cancer. Vitamin E may increase the risk of colorectal adenoma. (Level of evidence: 1a–)