Dec. 13, 2005 -- There are plenty of good reasons to eat a fiber-rich diet, but lowering colorectal cancer risk may not be among them.
A newly published pooled analysis of 13 studies involving more than 725,000 people found little evidence of a protective benefit for dietary fiber against colon cancer.
The analysis was largely conducted by researchers from the Harvard School of Public Health and Harvard Medical School. It is published in the Dec. 14 issue of the Journal of the American Medical Association.
"There are more questions to be answered but clearly this adds to the growing body of evidence finding that high fiber intake does not lower the risk of colorectal cancer," lead researcher Yikyung Park, ScD, tells WebMD.
Large Studies Mixed
For years it was believed that the opposite was true, based on the results of a handful of relatively small studies.
A 1999 Harvard study involving more than 80,000 nurses was among the first to question the link. Several large studies that followed also failed to show a protective benefit for fiber against colorectal cancer.
Adding to the confusion, two other large studies, both published in May of 2003, found that a high-fiber diet did appear to protect against the cancer. One included more than half a million Europeans and the other included nearly 34,000 Americans.
"Because of these discordant results, the debate continues on whether dietary fiber consumption decreases colorectal cancer risk," Park and colleagues write.
The pooled studies in the latest analysis included 725,628 men and women followed for between six and 20 years.
A total of 8,081 cases of colorectal cancer were identified during this period. The major source of dietary fiber varied across studies, with cereal the most common source of fiber in studies from Europe and fruits and vegetables as the main sources in the U.S. studies.
Fiber's Proven Benefits
Although eating a high-fiber diet did appear to protect against the cancer at first glance, the protective benefit disappeared when the researchers adjusted for other factors indicative of a healthy lifestyle. Compared with people who ate low-fiber diets, fiber eaters were more likely to take multivitamins and eat foods rich in the B-vitamin folate, and they ate less red meat.
Park says it is possible that studies showing a protective benefit against colon cancer may not have fully adjusted for other healthy lifestyle choices that could play a role in risk.
"People who eat high-fiber diets also tend to have healthier lifestyles and eat healthier diets, overall," she says. "All of these things together may influence risk."
She adds that even if fiber does not have a major impact on colorectal cancer, there is convincing evidence that dietary fiber helps prevent heart disease, type 2 diabetes, the colon disease diverticulitis, and other several chronic conditions.
"It is important to eat a high-fiber diet," she says.
John A. Baron, MD, agrees. But he says there are still unanswered questions about the role of different types of dietary fiber in colon cancer prevention.
Baron wrote an editorial accompanying the analysis in which he argued that the various studies have done a poor job of differentiating between different types of fiber.
"It is important to understand that different fibers do different things," he tells WebMD. "But it does appear increasingly unlikely that any type of fiber has much of an impact on colorectal cancer."