A systematic review and meta-analysis (pooled analysis) of previously published randomised trials in this week’s issue of THE LANCET provides strong evidence that antioxidant supplements (such as vitamin supplements) are not effective in protecting against gastro-intestinal cancer.
Some combinations of supplements may slightly increase gastro-intestinal cancer risk, whereas selenium may be associated with a risk reduction.
The human diet is a complex mix of oxidants and antioxidants. Excess oxidants can cause cancer by inducing gene mutations. Goran Bjelakovic (University of Niss, Serbia and Montenegro) and colleagues from The Cochrane Hepato-Biliary Group reviewed all randomised trials comparing antioxidant supplements with placebo for prevention of gastrointestinal cancers.
The investigators identified 14 randomised trials (totalling over 170,000 participants). Overall, the results did not show any protective effect of supplementation with b-carotene, vitamins A, C, E, and selenium (alone or in combination) compared with placebo on oesophageal, gastric, colorectal, pancreatic, and liver cancer incidences. In half the trials (categorised as high quality), there was a small but statistically significant increase (6% relative risk value) in mortality among people taking antioxidants compared with placebo; the results also showed that two combinations of supplements were associated with increased mortality risk: b-carotene and vitamin A (30% increase in relative risk), and the combination of b-carotene and vitamin E (10% increase in relative risk). Four of the trials suggested that selenium was associated with a reduction in gastro-intestinal cancer risk, but this may be due to bias.
Dr Bjelakovic comments: “We could not find evidence that antioxidant supplements can prevent gastrointestinal cancers; on the contrary, they seem to increase overall mortality. The potential preventive effect of selenium should be studied in adequate randomised trials”.