Antioxidants do not prevent colorectal cancer
Q Can colorectal cancer risk be cut with antioxidant supplements?
The researchers conducted this analysis using standard methodology and searched five databases for all randomised trials comparing ß-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 They also searched for unpublished studies.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 report that they used the Cochrane Collaboration methodology for conducting the meta-analysis but do not give details of how they chose studies for inclusion or how they abstracted the data. They assessed the research for quality, identifying the studies as high quality or low quality according to study design. There was no publication bias.
The eight trials used in this analysis included a total of 17,260 participants, though most of the patients (88 per cent) were in a single high-quality study. This study enrolled patients without previous adenoma; the rest of the studies enrolled participants with previously removed colorectal adenomas (six studies) or previous colorectal cancer (one study).
Overall, there was no benefit of antioxidant supplementation on the development of colorectal adenoma. High-quality studies showed no effect or a slight increase of risk of adenoma with antioxidants; the small, low-quality studies found a benefit with antioxidants. When analysed 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 per cent CI 1.1-2.8).
Level of evidence
1a (see www.infopoems.com/levels.html)
Bjelakovic G et al. Meta-analysis: antioxidant supplements for primary and secondary
prevention of colorectal adenoma. Aliment Pharmacol Ther 2006;24:281-291.
Antioxidant supplementation for up to six years does not decrease the risk of colorectal adenomatous polyps and so, by extension, does not reduce the risk of colorectal cancer. Vitamin E may increase the risk of colorectal adenoma.