Yet another study has struck a blow to the idea that beta carotene supplements might help prevent cancer, although researchers said this one, published yesterday, at least showed they did no harm.

I-Min Lee of Brigham and Women's Hospital in Boston and colleagues examined women taking part in the Women's Health Study. As part of the study, more than 39,000 women age 45 and older were given beta carotene, vitamin E or aspirin, which was used as a dummy pill.

Lee's team looked at the beta carotene part of the trial, which was stopped after just over two years.

"Among women randomly assigned to receive beta-carotene, there were no statistically significant differences in incidence of cancer, cardiovascular disease or total mortality after a media of 4.1 years," they wrote in a report published in today's Journal of the National Cancer Institute. "There was no benefit or harm from beta-carotene supplementation for a limited period on the incidence of cancer and of cardiovascular disease."

Decades ago, doctors noticed that people who ate the most fruits and vegetables had the lowest risk for cancer and heart disease. They thought vitamins might have something to do with this, because they can act as antioxidants, wiping up free radicals--charged particles that damage cells and cause disease.

The best-known antioxidant vitamins, A, C and E, have all been tested to see if people who took them in supplement form might lower their risk for disease.

Results for vitamins C and E are mixed. But several studies suggested that beta carotene--which the body converts to vitamin A--did no good.

And in two studies, one involving 29,000 older Finnish men who smoked, taking supplements of beta carotene was actually linked to higher rates of lung cancer.

"Clearly, beta-carotene supplements do not prevent cancer," James Marshall of the Arizona Cancer Center at the University of Arizona wrote in a commentary.

He said it was not a case of the supplements being ineffective, or of the patients not taking their supplements, because blood tests were used to check. "The dose more than tripled the blood beta-carotene levels of the experimental subjects," he wrote.

Marshall said beta carotene might be a marker for something else in fruits and vegetables that works to prevent cancer.

"There are thousands of nutrients and hundreds of carotenoids in the foods that we consume," he wrote. "Clearly, focusing as readily as we did on beta-carotene was a mistake."

Research presented earlier this year at a meeting of the American Association for Cancer Research showed that supplements, from ordinary multivitamins to specially formulated vegetable-based capsules, may slow cancer.