Bjelakovic G et al. (2004) Antioxidant supplements for prevention of gastrointestinal cancers: a systematic review and meta-analysis. Lancet 364: 1219–1228

Although diets rich in fruit and vegetables have been linked to a lower cancer incidence, the role of antioxidant supplements in cancer prevention is controversial. A recent analysis by Bjelakovic and colleagues from the Cochrane Hepato-Biliary Group indicates that common dietary supplements do not offer protection against gastrointestinal cancer and might, in fact, increase overall mortality.

The authors combined the results of 14 randomized trials comparing β-carotene, vitamins A, C and E and selenium (alone or in various combinations) with placebo, in a total of over 170,000 participants. The trials ranged from small (n = 226) to large (n = 39,876); 7 of the 14 trials were of high methodological quality. In each case, daily antioxidant supplements or placebo had been given orally for 1–12 years.

Overall, there was no significant difference between the placebo and supplements groups in the incidence of esophageal, gastric, colorectal, pancreatic or liver cancers. A possible exception was selenium, which was associated with a lower incidence of hepatocellular carcinoma, albeit in mostly low-quality trials.

In a fixed-effect model based on six of the high-quality trials, a small but statistically significant increase in mortality was shown. In particular, β-carotene, in combination with either vitamin A or vitamin E, was linked with an elevated mortality risk. Using a random-effects model, no significant increase in mortality was shown. The findings should be considered preliminary since the study covered only a proportion of all trials reporting mortality as an outcome. Furthermore, supplement doses in excess of recommended levels may be a factor in increased mortality.