Systematic review and meta-analysis: primary and secondary prevention of gastrointestinal cancers with antioxidant supplements.
Aliment Pharmacol Ther. 2008 Jun 30. Epub 2008 Jun 30. PMID: 18616690
The Cochrane Hepato-Biliary Group, Copenhagen Trial Unit, Centre for Clinical Intervention Research, Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark.
Background The evidence on whether antioxidant supplements prevent gastrointestinal cancers is contradictory. Our aim was to assess the beneficial and harmful effects of antioxidant supplements in preventing gastrointestinal cancers. Methods Using the Cochrane Collaboration methodology we reviewed the randomised trials comparing antioxidant supplements with placebo or no intervention on the occurrence of gastrointestinal cancers. We searched electronic databases and reference lists until October, 2007. Our outcome measures were gastrointestinal cancers, overall mortality, and adverse events. Outcomes were reported as relative risks (RR) with 95% confidence interval (CI) based on random-effects and fixed-effect models meta-analyses. Results We identified 20 randomised trials (211,818 participants) assessing beta-carotene, vitamin A, vitamin C, vitamin E, and selenium. The trial quality was generally high. The antioxidant supplements were without significant effect on the occurrence of gastrointestinal cancers (RR 0.94, 95% CI 0.83-1.06, I(2) = 54.0%). The heterogeneity seemed to be explained by bias risk (low-bias risk trials RR 1.04, 95% CI 0.96-1.13 compared to high-bias risk trials RR 0.59, 95% CI 0.43-0.80 test of interaction P<0.0005) and type of antioxidant supplement (beta-carotene potentially increasing and selenium potentially decreasing cancer risk). Antioxidant supplements had no significant effect on mortality in a random-effects model meta-analysis (RR 1.02, 95% CI 0.97-1.07, I(2) = 53.5%), but significantly increased mortality in a fixed-effect model meta-analysis (RR 1.04, 95% CI 1.02-1.07). Conclusion We could not find evidence that the studied antioxidant supplements prevent gastrointestinal cancers. On the contrary, they seem to increase overall mortality.