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Colorectal cancer: "antioxidants" have no preventive effect

"Antioxidants" have no place in the prevention of colorectal cancers. Furthermore, they may be harmful.

In the absence of an overt deficiency, "antioxidants" have no proven beneficial effect in preventing cardiovascular disease or cancers. A British review examined their efficacy in preventing adenomatous polyps and colorectal cancer.

Five trials involving around 1300 people with a history of polyps compared antioxidants with a placebo, over a period ranging from 18 months to 5 years. The antioxidants evaluated were selenium, beta-carotene, and vitamins A, C and E. These trials revealed that the "antioxidants" were no more effective than the placebo in preventing the recurrence of polyps.

Nine other trials involving more than 140,000 people compared one or several antioxidants (selenium, beta-carotene, vitamins C and E and zinc) to no treatment. The subjects were monitored for between 5 and 12 years. Colorectal cancers were as frequent in those taking antioxidants as in those not taking them.

Adverse effects were observed with the antioxidants: itching (with vitamin A + C + E combinations), nosebleeds, hair loss (with selenium), and skin discoloration with beta-carotene.

Strokes occurred more frequently in people taking vitamin E. Another study showed a higher mortality in groups taking vitamins A and E or beta-carotene compared with those not taking antioxidants.

Antioxidants have no place in disease prevention: they have no proven efficacy, but are likely to cause harm, especially vitamin E.

©Prescrire 1 June 2013

""Antioxidants": no preventive effect against colorectal cancer" Prescrire Int 2013; 22 (139): 160. (Pdf, subscribers only).

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