TUESDAY, May 20, 2003 (HealthDayNews) -- Antioxidants have long been touted for their cancer-fighting properties.
But a new study has confirmed earlier findings that one antioxidant -- beta-carotene -- may actually increase the risk of certain cancers in people who smoke or drink.
The researchers found a combination of smoking, drinking and the use of beta-carotene supplements is linked to a possible recurrence of colorectal cancer. Previous studies had found the same connection with lung cancer.
However, beta-carotene cut the risk of colorectal cancer in non-smokers and non-drinkers.
The latest study, which appears in the May 21 issue of the Journal of the National Cancer Institute, "certainly raises the issue of whether beta-carotene supplements are worthwhile," says Dr. John A. Baron, a professor of medicine at Dartmouth Medical School and leader of the team that did the research.
But Baron says the results can't be extended to all cancers. "Each cancer has its unique characteristics; all have their own causative factors," he explains.
His study did find, however, that intestinal polyps, which can become cancerous, multiplied for those who smoked, drank and took a beta-carotene supplement. But the polyps dwindled in supplement takers without those habits.
Beta-carotene, found naturally in fruits and vegetables such as carrots, mangos and oranges, is transformed into vitamin A in the body and has been widely promoted as a natural cancer preventative. But two large-scale studies, reported several years ago, found that beta-carotene supplementation actually increased the risk of lung cancer for smokers, particularly for those who also drink.
Baron's study followed 864 people who had had intestinal polyps removed. Some were randomly assigned to receive beta-carotene supplements, while others received vitamin C and E supplements or the two vitamins plus beta-carotene. Still others acted as a control group, getting a placebo. All the participants listed whether they smoked or drank and how much.
After four years, nonsmokers and nondrinkers reduced the recurrence of polyps by 44 percent, the researchers report.
But the recurrence rate for beta-carotene takers who smoked increased by 36 percent. The increase for nonsmoking drinkers was 11 percent. And the increase for those who smoked and drank was nearly doubled.
The researchers can't say for sure why coupling beta-carotene supplements with smoking and drinking seems to boost the risk for colorectal cancer. One possibility is that beta-carotene can change from an antioxidant to a different type of molecule with damaging effects in some people.
Baron says one message of the study is that cancer prevention is a tricky business. "A careful mix of animal studies, epidemiology and clinical trials is needed" to design appropriate strategies, he says.
Dr. Bernard Levin, vice president for cancer prevention at the University of Texas M.D. Anderson Cancer Center, calls the finding "unexpected."
"The common wisdom has been that antioxidants might be good for you or at least do no harm," he says. "Some that actually cause an accelerating effect is unexpected."
Supplements such as beta-carotene may have a role in cancer prevention, Levin says, but they should not be high on the list.
A better -- and more obvious -- first step is not to smoke, he says. Physical activity also seems to help prevent colorectal cancer. And for people at high risk for the cancer, aspirin therapy can help, he says.
A low-fat diet that's high in fiber and low in red meat seems to be a good idea, Levin adds. And there's some evidence that multivitamin supplements might be helpful. "The emphasis should be on a varied diet," Levin says.
More information
You can learn about colorectal cancer and measures to prevent it from the National Institutes of Health and the National Cancer Institute.
