Most people consider fiber to be part of a healthy diet; some believe it also protects against colon cancer. But fiber's protective effect has yet to be substantiated, and some researchers now think it's time to admit that fiber does not protect against colon cancer. Others say that published studies aren't rigorous enough and only clinical trials can provide satisfactory answers.
"The hypothesis that fiber intake is important in reducing colon cancer risk was interesting, but the reality is that the data have not provided much support for that," said Walter Willett, M.D., Dr.P.H., professor of epidemiology and nutrition at the Harvard School of Public Health, Boston.
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"Our study," said Willett, "was the longest follow-up study done, and we could use multiple measurements of diet. We looked at adenomas as well as colon cancer and there was no hint at all that fiber was reducing the risk."
The study's first author agreed. "I don't think it's appropriate to strongly recommend fiber as a preventive for colon cancer," said Charles Fuchs, M.D., an assistant professor of medicine at the Dana Farber Cancer Institute, Boston. "All prospective studies as they relate to colon cancer are null, and, although there are positive retrospective studies, I don't think that's enough to draw any meaningful conclusions."
British Disagree
But in Europe, British researchers disputed the U.S. study's findings. J. Gordon McVie, director general of Britain's Cancer Research Campaign in London, claimed that the study was too small and not representative of the general public. He told news conference attendees, the "message" is unaltered: a high-fiber, low-fat diet protects against bowel cancer.
John Potter, M.D., Ph.D., director of cancer prevention research at Fred Hutchinson Cancer Research Center, Seattle, cited several obstacles to getting at fiber's effects. First, fiber is found in a variety of foods, including fruits, vegetables, nuts, cereals, and legumes. Some types of fiber may help protect against colon cancer, but others may not. Second, fiber intake is a derived variable; researchers use food-frequency questionnaires and food tables to come up with a single variable that attempts to describe intake. Third, differences between the sexes and differences by age will attenuate any findings.
"All this, in turn, will mean that an association is observed in some studies but not others," Potter said in an e-mail. "We need a better understanding of what fiber is . . . and better ways to characterize the component substances."
Potter chaired the American Institute for Cancer Research's panel on food, nutrition, and the prevention of cancer. AICR's 1997 report noted three categories of "fiber": cereals (grains), starches, and non-starch polysaccharides/fiber. According to the report, research shows no effect of cereal intake and "possible" (the lowest category of certainty) effects of starches and NSP/fiber.
Peter Greenwald, M.D., director of NCI's Division of Cancer Prevention, said that the time for conclusions on the issue is still in the future. "You always have questions in studies on dietary fiber," he noted. "Dietary assessment is difficult. I think we should wait until [clinical trials] are completed before taking sides."
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Results of two clinical trials of fiber and its effects on colon cancer should be published within the next year. NCI's polyp prevention trial, led by Arthur Schatzkin, M.D., Dr.P.H., senior investigator in NCI's epidemiology branch, enrolled more than 2,000 people in eight centers across the United States. Participants followed either a low-fat, high-fiber diet or their pre-surgery diet. The second trial, based in Arizona, compared a fiber supplement with placebo for the prevention of recurrent adenomas.
David Alberts, M.D., associate dean for research at the University of Arizona College of Medicine and the Arizona Cancer Center, Tucson, is the lead investigator on the fiber supplement trial, a phase III trial in the analysis phase.
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Deoxycholic Acid
Alberts' group is also analyzing deoxycholic acid, which, said Alberts, is "two steps removed" from a known colon cancer carcinogen. In a 1996 phase II trial, Alberts and others found that that a diet high in wheat fiber reduced deoxycholic acid concentrations by nearly 50%.
"I am a little concerned [about the Harvard study making people dismiss fiber altogether]," said Alberts. "They were looking at one end point. Almost everyone who's studied this has shown that cereal fiber has other healthful aspects," he said.
Schatzkin concurred. "It does not follow from [the Harvard] study that you should not continue to eat a diet high in fruits, vegetables, beans and some grains. That diet may protect against colorectal cancer it's not known but that study does not rule out that such a diet might be good for you."
Willett also agreed for the most part. "There are well-documented reasons to consume whole-grain, high-fiber foods but not for colon cancer prevention."
What to Do?
How should people reduce the risk of colon cancer if not with fiber? Physician recommendations include receiving regular screening, reducing red meat consumption, avoiding smoking, getting regular exercise, and eating a variety of fruits and vegetables.
If fiber studies are so complex that consensus has yet to be reached, why continue?
"I think we need to link the nutritional studies with science to discover mechanisms," said Greenwald. "We need a more intensive program on nutrition related to cancer risk, and it has to be soundly based in basic science as well as epidemiology."
Said Schatzkin, "If we can reach a higher level of certainty that changing your diet would reduce the incidence of one cancer or several cancers, that would push the knowledge to a higher level."
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