Can colorectal cancer risk be decreased with antioxidant supplements?

Antioxidant supplementation for up to 6 years does not decrease the risk of colorectal adenomatous polyps and thus, by extension, does not reduce the risk of colorectal cancer. Vitamin E may increase the risk of colorectal adenoma. (LOE = 1a-)

Bjelakovic G, Nagorni A, Nikolova D, Simonetti RG, Bjelakovic M, Gluud C. Meta-analysis: antioxidant supplements for primary and secondary prevention of colorectal adenoma. Aliment Pharmacol Ther 2006;24:281-291.  [PMID:16842454]

Meta-analysis (randomized controlled trials)

Self-funded or unfunded

Outpatient (any)

The researchers conducted this analysis used standard methodology and searched 5 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 They also searched for unpublished studies.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 report that they used The Cochrane Collaboration methodology for conducting the meta-analysis but they don't give details of how they choose 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 8 trials used in this analysis included a total of 17,260 participants, though most of the patients (88%) 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 (6 studies) or previous colorectal cancer (1 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 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% CI, 1.1-2.8).