Claire Bonithon-Kopp, M.D., and colleagues from the European Cancer Prevention Organization Study Group, published their paper in the Oct. 14 issue of the British medical journal The Lancet. The researchers gave fiber supplements, calcium supplements or an inactive substance (in this case, sugar) to different groups among 552 patients who earlier had precancerous growths called adenomatous polyps surgically removed from the colon.

Adenomatous polyps are small, abnormal, mushroom-shaped growths. Because some polyps are likely to turn cancerous if not removed from inside the colon, researchers have for years sought a way to prevent them from occurring or from returning after they have been removed.

After three years on the supplements, the patients were checked for any new polyps. At least one new polyp was found in 29 percent of those getting the fiber supplements, compared to 20 percent of those getting the dummy supplements, making those getting the fiber supplements about one-third more likely to develop new polyps. About 16 percent of study participants who took calcium supplements developed new polyps.

"[O]ur study, along with the two American trials, suggests that low-fat, high-fibre diet and supplementation with wheat-bran fibre or ispaghula husk may not be effective strategies for the prevention of colorectal adenoma recurrence," the researchers write. "However, our findings should not prevent recommendations for high consumption of vegetables, fruits and cereals, because this approach has potentially beneficial effects on other chronic disease, especially coronary heart disease."

Michael Thun, M.D., vice president of epidemiology and surveillance research for the ACS, agrees with the findings on fiber supplements but says a diet high in fruits and vegetables protects against other cancers.

"There is no question that new evidence over the last several years has weakened the belief that fruits and vegetables or a specific type of fiber in them is strongly protective against colorectal cancer.  This study supports that trend.

"But that trend will probably not change overall ACS nutritional guidelines because the guidelines pertain to choosing a diet that is high in plant foods, for cancer prevention generally, not a particular component of plant foods for prevention of colorectal cancer," Dr. Thun says. "There is substantial evidence that a diet high in fruits and vegetables helps reduce the risk of other cancers, such as the oral cavity, esophagus, larynx and the stomach."

Dr. Thun adds that this study is a good example of the way nutrition science advances ? with researchers asking clearly focused questions and then reevaluating recommendations after getting the results of trials.

"Because of the complexity of nutrition and diet it will be some decades before we have a clear and coherent picture about all the ways diet affects cancer risk," he says. "In the meantime, those who want to take advantage of what is known on the subject can continue to eat a plant-based diet high in fruits and vegetables. That will help protect them against more than just one kind of cancer and provide other health benefits as well."