A review of the studies did not find antioxidants inhibit chemotherapy, but doctors say there is enough reason to be concerned about harm and no persuasive evidence of benefit.
So researchers are warning patients not to take antioxidants during their treatments.
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Cancer patients should avoid the routine use of antioxidant supplements during radiation and chemotherapy because the supplements may reduce the anticancer benefits of therapy, researchers concluded in a commentary published online May 27 in the Journal of the National Cancer Institute.
Radiation and many chemotherapy agents work to kill cells by inducing free radicals that damage DNA and proteins. Therefore, there is a possibility that taking antioxidant supplements, such as vitamin E or ƒÒ-carotene, may interfere with the therapies and reduce their anticancer activity. On the other hand, some investigators hypothesize that antioxidant supplementation may protect healthy tissues and reduce the side effects of treatment. Despite two decades of research into this question, no clear answer has appeared.
To evaluate the potential harms or benefits of antioxidant supplementation, Brian D. Lawenda, M.D., of the Naval Medical Center San Diego and colleagues reviewed all of the randomized trials they could identify that tested the effect of antioxidant supplements on radiation therapy or chemotherapy.
In the case of radiotherapy, they identified nine studies that addressed the question, including two meta-analyses. However, only three studies were randomized controlled trials designed to look at the clinical effect of antioxidant therapy on radiation. In the largest of the randomized trials, antioxidant supplementation was associated with a reduction in overall survival. One antioxidant agent, amifostine, which is already approved by the U.S. Food and Drug Administration to increase radioresistance in healthy salivary gland tissues, may protect normal tissues without increasing tumor radioresistance. Lawenda and colleagues caution that the question needs to be studied further before a solid conclusion can be made.
The authors identified 16 randomized controlled trials that examined the impact of antioxidant supplementation on chemotherapy. Six of the trials were placebo-controlled.
Of the studies that included information on response rates, none reported a decrease in response in the antioxidant arm of the trial compared with the control arm. However, Lawenda and colleagues caution that none of the trials was large enough to reliably detect such differences.
Despite some intriguing studies that have suggested the benefit of adjunctive antioxidant treatments in cancer patients, the totality of the available evidence is equivocal at best and leaves us with serious concerns about the potential for harm, the authors write.
