A decades-old medical mystery - why antioxidants such as vitamin E and beta carotene seemed to accelerate the growth of early lung tumors in high-risk populations such as smokers, rather than protect them from cancer, as theory suggests - may have been solved, according to research published on Wednesday.

In essence, "antioxidants allow cancer cells to escape cells' own defense system" against tumors, biologist Per Lindahl of Sweden's University of Gothenburg and a co-author of the study told reporters. That lets existing tumors, even those too small to be detected, proliferate uncontrollably.

The findings imply that "taking extra antioxidants might be harmful and could speed up the growth of (any) tumors," said biologist and co-author Martin Bergo of Gothenburg, adding, "If I had a patient with lung cancer, I would not recommend they take an antioxidant."

The study, published in Science Translational Medicine, did not examine whether antioxidants can also initiate lung cancer, rather than accelerate the growth of existing tumors. Nor did it examine whole foods naturally high in antioxidants. But it adds to a growing pile of research challenging the health benefits of taking vitamin supplements except in cases of malnutrition.

Last month, a review of dozens of studies found "no clear evidence of a beneficial effect of supplements" on heart disease, cancer or mortality. An accompanying editorial in the Annals of Internal Medicine warned that "beta carotene, vitamin E, and possibly high doses of vitamin A supplements are harmful," while other antioxidants "are ineffective."

The new antioxidant study "seems quite sound," said Eliseo Guallar, professor of epidemiology at Johns Hopkins School of Public Health and a co-author of the editorial. "It's unfortunate that the public has the idea that vitamin supplements are good and antioxidants are better."

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