High-Fiber Diets May Not Keep Colorectal Polyps Away

From the WebMD Archives

April 19, 2000 -- Switching to a high-fiber diet does not prevent the return of colorectal polyps, a kind of growth that can lead to cancer, according to the findings of two long-awaited studies just published in TheNew England Journal of Medicine.

If diet had reduced the rate of recurrence in people who had already had polyps, this would indicate that diet could cut the risk of colorectal cancer, the second-leading cause of cancer death in the U.S.

"We were surprised and very disappointed" in the findings, Arthur Schatzkin, MD, tells WebMD. Schatzkin, chief of the nutritional epidemiology branch of the National Cancer Institute in Bethesda, Md., helped lead one of the studies, called the Polyp Prevention Trial.

These polyps, or adenomas, are growths on the inner wall of the colon or rectum. About 5% to 10% of polyps will become cancerous. In the first three years after being diagnosed with a polyp, people have as much as a 50% risk of developing more.

The Polyp Prevention Trial involved more than 2,000 men and women at least 35 years old at eight hospitals throughout the country. The other clinical trial, called the Wheat Bran Fiber Study, was headed by the Arizona Cancer Center in Tucson and carried out by the Phoenix Colon Cancer Prevention Physician Network. It had about 1,400 participants aged 40 to 80. Participants in both trials had recently had polyps removed.

Volunteers were divided into two groups for the Polyp Prevention Trial. One ate a 20%-fat diet that also included 18 grams of fiber and 3.5 servings of fruits and vegetables for each 1,000 calories the volunteers consumed daily. They also received intensive counseling on how to change their eating patterns. Men and women in the second group were only given a brochure telling them to eat healthily.

Each participant was followed for four years, and subjects in both groups had almost the same rate of polyp recurrence: 39.7% of those on the special diet had polyps come back, compared to 39.5% in the control group. Polyps were detected by colonoscopy, a screening test in which the physician examines the rectum and colon by looking through a telescope-like instrument inserted into the rectum.

Volunteers for the Wheat Bran Fiber Study were also divided into two groups. For three years, one group consumed a high-fiber cereal (containing 13.5 grams of fiber) every day; the other group ate a low-fiber (2 grams) cereal. After two years, all the volunteers were allowed to get up to 25% of their daily fiber supplement from a fiber bar. Otherwise, they ate their usual diets, which were not low in fat.

Colonoscopy screenings found that 51.2% of those in the low-fiber group and 47% of those in the high-fiber group had new polyps.

"The bottom line, no matter how we looked at it, was there were no significant positive effects of wheat bran fiber on adenoma recurrence," says Maria Elena Martinez, PhD, an investigator in the Wheat Bran Fiber Study.

Still, researchers are not suggesting that people toss their bran muffins in favor of cheese omelets. "Don't give up on diet," Schatzkin says. "There's lots of evidence ... that adopting a diet that's low in animal fats and saturated fats, and high in whole grains and rich in fruits and vegetables, can improve overall health and reduce the risk of chronic disease. The problem is, we just have not yet been able to pin this down for colon cancer."

For colon cancer prevention, "screening is still going to be the No. 1 thing," Martinez tells WebMD. "I think the overall picture is that fiber does appear to prevent other important disease -- [heart] disease and diabetes. Giving up on fiber is not the right thing."

"These are beautifully done studies. Unfortunately, they don't answer all the questions," says Bernard Levin, MD, who reviewed the work for WebMD. "Is it possible that people who start this [diet] in their 20s and 30s would derive benefit? Secondly, [the studies] mostly address the issue of small polyps." Levin is vice president for cancer prevention at the University of Texas MD Anderson Cancer Center in Houston.

These studies did have limits. For example, neither could examine whether eating a healthier diet or extra fiber would affect either the progression of polyp growth or the tendency of polyps to become cancerous. Also, both studies followed the subjects for only a few years. The two research groups intend to follow the volunteers for several more years.

While Levin is disappointed in the studies' results, he tells WebMD: "We should not go overboard and say everyone can go out and eat hamburgers and grilled cheese sandwiches. I think one should be moderate and say these diets have benefits for other health purposes."

Vital Information:

  • Polyps are growths on the inner wall of the colon or rectum that can eventually develop into colon cancers.
  • In two recent studies, patients who had had polyps removed and ate diets high in fiber had the same rate of polyp recurrence as patients who did not eat high-fiber diets.
  • Although fiber does not protect against polyp recurrence, it is still part of a healthy diet and can help prevent heart disease and diabetes.