Antioxidant vitamins are widely assumed to be cancer fighters even though research in smokers has found high doses may actually raise their risk of tumors. Now a new study may help explain the paradox.

Swedish scientists gave antioxidants to mice that had early-stage lung cancer, and watched the tumors multiply and become aggressive enough that the animals died twice as fast as untreated mice.

The reason: The extra vitamins apparently blocked one of the body’s key cancer-fighting mechanisms, the researchers reported Wednesday.

The scientists stressed that they can’t make general health recommendations based on studies in mice, but said their work backs up existing cautions about antioxidant use.

“You can walk around with an undiagnosed lung tumor for a long time,” said study co-author Martin Bergo of the University of Gothenburg. For someone at high risk, such as a former smoker, taking extra antioxidants “could speed up the growth of that tumor.”

Antioxidants are compounds that help protect cells from certain types of damage, and antioxidant-rich fruits and vegetables certainly are healthy. The question is the health effect of extra-high doses in pill form. Studies in people have shown mixed results but haven’t proven that vitamin supplements prevent cancer, and a few have suggested the possibility of harm. One study in the 1990s found beta-carotene increased the risk of lung cancer in smokers. Nor are smokers the only concern: A 2011 study found Vitamin E supplements increased men’s risk of prostate cancer.

As for people who already have cancer, the National Cancer Institute says: “Until more is known about the effects of antioxidant supplements in cancer patients, these supplements should be used with caution.”

But biologically, scientists couldn’t explain why antioxidants might harm. Wednesday’s report in the journal Science Translational Medicine is a first step to do so.

The research doesn’t examine whether antioxidants might help prevent tumors from forming in the first place — only what happens if cancer already has begun.

The researchers gave Vitamin E, in a range of supplement doses, or an antioxidant drug named N-acetylcysteine to mice engineered to have lung cancer.

The antioxidants did prevent some cell damage. But doing so prevented a well-known tumor-suppressing gene named p53 from getting the signal to do its job, explained study co-author and Gothenburg biologist Per Lindahl.

The antioxidants “allow the cancer cells to escape their own defense system,” he said.