A low-fat, high-fiber diet is often recommended as a way of preventing polyps in the lining of the colon -- growths associated with an increased risk of colon cancer. But new research casts doubt on this oft-repeated advice.
Adopting a low-fat diet rich in fruits, vegetables and other sources of
fiber does not prevent the development of intestinal polyps in people who have
already had polyps removed, US researchers report.
But before you put away the bran and reach for the pancake mix -- experts
say there are still plenty of reasons to eat a healthy diet.
Colorectal polyps are growths on the lining of the colon or rectum. Many
people aged 50 and older develop polyps, but only some of them become cancerous.
Diet is thought to play a role in the development of colon cancer, since the
disease is much less common in countries where people eat lots of fruits and
vegetables and fewer meats than in western countries.
In one of two studies on diet and intestinal polyp risk published in the
April 20th issue of The New England Journal of Medicine, researchers studied
more than 2,000 men and women aged 35 and older who had already had at least one
colorectal polyp removed. Participants were randomly assigned to continue eating
their regular diets or to follow a diet containing 20% of calories from fat, 18
grams of dietary fiber and 3.5 servings of fruits and vegetables per 1,000
calories.
About 4 years into the study, researchers did not detect any significant
difference in the development of colorectal polyps. Despite the different diets,
the percentage of individuals who had at least one polyp during the study was
similar in both groups: 39.7% in the low-fat, high-fiber group, and 39.5% in the
normal-diet group.
"We (found) no difference between the groups in our study," the study's lead
author, Dr. Arthur Schatzkin, of the National Cancer Institute in Bethesda,
Maryland, said in an interview with Reuters Health. But he said that the
findings do not rule out the possibility that diet may have an effect on the
development of colorectal polyps and cancer.
"It's possible that dietary changes affect the process before the
development of polyps," he said. Or a low-fat, high-fiber diet may affect the
progression of polyps into cancer, according to Schatzkin. "Our study can't rule
that out," he said.
He also noted that the study may not have been long enough to detect any
effects of dietary changes. Schatzkin said that the findings do not change the
fact that a low-fat, high-fiber diet has been shown to be good for health in
general.
In the second study, Dr. David S. Alberts, of the University of Arizona at
Tucson, and colleagues compared the effects of adding wheat-bran to the diet.
More than 1,400 men and women with a history of colorectal polyps were randomly
assigned to a low- or high-dose of wheat-bran each day. After about 3 years,
wheat-bran did not appear to make much of a difference. About half of each group
developed at least one polyp in their large intestine.
"There may be many reasons to eat a diet that is low in fat and high in
fiber, fruits and vegetables or to supplement the diet with a food high in
cereal fiber, but preventing colorectal (polyps), at least for the first 3 to 4
years, is not one of them," Dr. Tim Byers, of the University of Colorado School
of Medicine in Denver, writes in an editorial that accompanies the study.
But according to Melanie R. Polk, a registered dietitian who is the director
of nutrition education at the American Institute for Cancer Research in
Washington, DC, there are plenty of reasons to eat a low-fat diet that contains
lots of fruits, vegetables and fiber. Many studies have shown that this sort of
diet is linked to a decreased risk of many types of cancers, she said in an
interview with Reuters Health.
Although Polk did not dispute the findings of the current studies, she said,
"We really should not react with regards to our daily eating patterns based on a
couple of studies."
Schatzkin, the author of one of the studies, agreed on the need to look at
the big picture. "It's important to recognize that cancer is a long process," he
said. "We looked at only a part of that process."
