Antioxidants are supposed to be the good cops in the fight against cancer-causing free radicals. But in fact, antioxidant supplements may give benign tumours the ammunition they need to grow malignant, according to a study published Wednesday in Science Translational Medicine.
Researchers at the University of Gothenburg, Sweden, set out to investigate why previous studies have shown increases in lung cancer in smokers who took antioxidant supplements compared to smokers given placebos.
Using mice with early stages of lung cancer, the researchers analyzed differences in cell growth between untreated mice and those given antioxidants.
They found that lung tumours in the antioxidant group became more invasive and killed the mice twice as fast compared with lung tumours in mice that did not receive antioxidants.
In the same study, antioxidants also sped up cancer growth in human lung-cancer cells cultivated in vitro, said lead author Dr. Martin Bergo at the University of Gothenburg. Antioxidants appeared to accelerate cancer growth in mice and human cells by destroying free radicals – including a key protein that suppresses tumour growth, he said.
Free radicals damage tumour cells as well as healthy cells, Bergo explained. By knocking out free radicals with antioxidants, "We're helping the tumour by removing the toxic substance," he said.
Bergo acknowledged that results from mice studies do not always apply to humans. Nevertheless, lung tumours engineered in mice "are powered by the same mechanics that develop lung cancer in humans," he said.
Large human trials have found links between antioxidant supplements and lung-cancer growth in patients at high risk for lung cancer. In a study of 29,000 Finnish smokers, published in the New England Journal of Medicine in 1994, those who took daily supplements of beta carotene for five to eight years were significantly more likely to die from lung cancer or heart disease than smokers given a placebo.
The same journal published a U.S. study two years later showing a 46-per-cent increased risk of lung-cancer deaths among participants taking a combination of vitamin A and beta carotene compared with those on placebos. The study involved 18,000 smokers and asbestos workers, and was halted early due to the high rate of cancer deaths.
Antioxidants have been linked to other cancers as well. In 2011, a study of 35,000 men published in the Journal of the American Medical Association found that men who took 400 international units of vitamin E daily for seven years had more prostate cancers than men who took a placebo.
These and other studies looked at antioxidant supplements, not antioxidants from food sources such as carrots (beta carotene), red peppers (vitamin C) or almonds (vitamin E), Bergo noted.
In the mice study, Bergo and colleagues monitored the effects of vitamin E as well as acetylcysteine, a synthetic antioxidant drug prescribed in inhaled form to patients with chronic obstructive pulmonary disease (COPD), a lung disease often caused by smoking.
Acetylcysteine is water-soluble, whereas vitamin E is oil-based, but both accelerated tumour growth in mice and human cells.
"The only thing they have in common is their antioxidant properties," Bergo said, adding that the findings may be applicable to other antioxidants.
Further research is needed to determine whether antioxidants may put smokers and other patients at increased risk for cancer, Bergö said. "The really pressing issue is to look at this in COPD patients who are taking acetylcysteine."
He cautioned that it would be premature to tell people to stop taking antioxidants until researchers have more data.
"I think that a normal, healthy, varied diet is the way to go," Bergo said, "but I can't make recommendations about diet to the general population based on this study."