Antioxidant vitamins fail to prevent polyps.

Antioxidant vitamins have received a lot of attention for their putative role as anticancer agents. Last spring, however, the antioxidant story became complicated when researchers reported that such vitamins did not protect men who smoked from getting lung cancer. This week, another scientific team weighs in on the question of whether antioxidant vitamins shield against colorectal cancer.

Epidemiologist E. Robert Greenberg of Dartmouth Medical School in Hanover, N.H., and his colleagues report that certain antioxidant vitamins offer no defense against developing a wartlike growth called a polyp in the large intestine or rectum. This type of polyp is a precursor of invasive colorectal cancer.

The researchers studied men and women who had already had one such growth removed and thus faced a higher than average chance of developing another polyp and colorectal cancer. They randomly assigned the volunteers, who had very similar diets at the study's start, to one of four groups. Those in the control group received a placebo capsule each day containing an inactive substance. The three treatment groups received either beta carotene alone, vitamins C and E, or beta carotene plus vitamins C and E. Neither the researchers nor the patients in the study knew who was getting the vitamins and who was taking the placebo.

To monitor the appearance of new growths, the patients underwent a procedure called colonoscopy, in which doctors view the colon through a flexible tube. The researchers discovered that more than a third of the recruits developed polyps during a 3-year period. However, people taking vitamins fared no better than those popping placebos: The rate of occurrence of new polyps was about the same in each of the four groups. Greenberg and his colleagues describe their results in the July 21 New England Journal of Medicine (NEJM).

"The findings don't provide any support for the idea that taking vitamins will lower your risk of colorectal tumors," Greenberg told Science News.

Still, even Greenberg acknowledges that the antioxidant story is far from over. Antioxidants neutralize damaging free radicals, molecules that contain an unstable oxygen atom, and thus may prevent cancer. The group's results "don't prove conclusively that vitamins are ineffective," Greenberg points out. "It might be that the study did not last long enough to see an effect."

This study looked only at the ability of antioxidant vitamins to stave off another polyp in people who had already had a growth removed, adds Norman I. Krinsky an antioxidant researcher at Tufts University School of Medicine in Boston. Giving antioxidants at that point may already be too late, he says. The jury is still out on the question of whether antioxidant vitamins can prevent such growths -- or colorectal cancer -- in people who have never had them, Krinsky says.

An editorial written by NEJM editors Marcia Angell and Jerome P. Kassirer warns against an "all-or-nothing" interpretation of this study's findings. To prove any kind of scientific theory -- such as the antioxidant model of cancer protection -- scientists must conduct a number of studies. These trials "should be considered tentative until a body of evidence accumulates pointing in the same direction," they say.

Meanwhile, numerous epidemiological studies show that a diet rich in fresh fruits and vegetables lowers the risk of malignancy, including colon cancer, says Arthur G. Schatzkin of the National Cancer Institute (NCI) in Bethesda, Md. Such a diet contains antioxidant vitamins as well as many other cancer-fighting components, he points out.

Indeed, a study by researchers at Harvard Medical School in Boston suggested that folate, a micronutrient in fresh fruits and leafy vegetables, was the crucial ingredient in protecting people against colorectal cancer (SN: 6/5/93, p.358).

Until the vitamin controversy is sorted out, NCI recommends eating at least five fruits and vegetables per day.