Efficacy of Antioxidants in GI Cancer Prevention
Does supplementation with antioxidant vitamins prevent gastrointestinal (GI) cancer?
There is no evidence that supplementation with beta carotene or vitamins A, C, or E prevents GI cancer. Data for selenium is inconsistent and based on poor-quality studies, and supplementation with this mineral should not be recommended routinely. Most importantly, combinations of antioxidant vitamins appear slightly to increase overall mortality rates.
Until recently, vitamin E and other antioxidants had been considered safe and possibly effective in the prevention of heart disease and various malignancies. However, a recent meta-analysis1 of studies of vitamin E supplementation for prevention of heart disease found no benefit and even a possible increase in risk at dosages above 400 IU per day. In the current Cochrane review, Bjelakovic and colleagues identified 14 randomized controlled trials of more than 170,000 patients that compared beta carotene, selenium, and vitamins A, C, and E with placebo for prevention of GI cancer.
No single antioxidant or combination of antioxidants significantly reduced the incidence of esophageal, gastric, colorectal, pancreatic, or hepatic cancer. When the results of all antioxidants and antioxidant combinations for a particular cancer were integrated, there was no effect on the incidence of that cancer. There was a trend in favor of selenium for prevention of esophageal, colorectal, and hepatocellular cancer, but it was not statistically significant, and the studies were of limited quality. Most importantly, when the results for all studies were combined using one statistical approach, overall mortality was increased in patients taking antioxidants (relative risk [RR], 1.06; 95 percent confidence interval [CI], 1.02 to 1.10). However, only a trend was noted when a more conservative approach was used (RR, 1.06; 95 percent CI, 0.98 to 1.15). When the selenium trials were excluded, both analyses showed a statistically significant increase in mortality, which was particularly strong in patients taking beta carotene and vitamin A (RR, 1.29; 95 percent CI, 1.14 to 1.45) or beta carotene and vitamin E (RR, 1.10; 95 percent CI, 1.01 to 1.20).