vitamins are widely assumed to be cancer fighters even though research in
smokers has found high doses may actually raise their risk of tumours.
Now a new study may help explain the paradox.
Swedish scientists gave antioxidants to mice that had early-stage
lung cancer, and watched the tumours multiply and become aggressive enough
that the animals died twice as fast as untreated mice.
Cautions about anti-oxidant use
The reason: The extra vitamins apparently blocked one of the body's key
cancer-fighting mechanisms, the researchers reported.
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
cancer tumours often harmless
"You can walk around with an undiagnosed lung tumour 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 tumour."
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. The report in the journal Science
Translational Medicine is a first step to do so.
The research doesn't examine whether antioxidants might help prevent tumours
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 tumour-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 defence
system," he said.