High-Fiber Diets May Not Keep Colorectal Polyps Away
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
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
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.