Our body cannot synthesize all compounds that are essential for health. Therefore such compounds must be taken through diet. Oxidative stress may cause cell damage that is implicated in chronic diseases like cancer. Gastrointestinal cancers are among the most common cancers worldwide. The poor prognosis of patients diagnosed with gastrointestinal cancers made primary prevention a potentially attractive approach. The evidence on whether antioxidant supplements are effective in decreasing gastrointestinal cancers is contradictory.
In this review prevention with antioxidant supplements of oesophageal, gastric, small intestinal, colorectal, pancreatic, liver, and biliary tract cancers is assessed. The review includes 20 randomised clinical trials. In total, 211,818 participants were randomised to antioxidant supplements (beta-carotene, vitamin A, vitamin C, vitamin E, and selenium) versus placebo. Trial quality was exceptionally good.
Based on properly designed and conducted randomised clinical trials, convincing evidence that beta-carotene, vitamin A, vitamin C, and vitamin E or their combinations may prevent gastrointestinal cancers is not found. A total of 2057 of 95084 participants (2.2%) randomised to antioxidant supplements and 1548 of 78935 participants (2.0%) randomised to placebo developed gastrointestinal cancers. These antioxidant supplements even seem to increase mortality. A total of 17114 of 122,501 participants (14.0%) randomised to antioxidant supplements and 8799 of 78693 participants (11.2%) randomised to placebo died. Selenium alone may have preventive effects on gastrointestinal cancers. This finding, however, is based on trials with flaws in their design and needs confirmation in properly conducted randomised clinical trials.