Two large studies cast doubt on the widely held belief that eating low-fat, high-fiber food will lower the risk of colon cancer.
Such a diet is recommended by health groups for many reasons, but evidence of the anti-cancer benefit has been unclear.
To help resolve doubts, researchers conducted two large experiments, putting people on different diets and counting potentially cancerous growths in their colons and rectums for up to four years. The researchers were disappointed to find no apparent effect from the low-fat, high-fiber diet or high-fiber supplements.
The two new studies were published Thursday in The New England Journal of Medicine.
Similar results were reported in a study published in the journal in January 1999. That study tracked the colon and rectal health of 88,757 women who participated in the Harvard-based Nurses Health Study over 16 years. It found that the risk of cancer of the same regardless of how much fiber the participants ate, and researchers said they believed those findings apply to men as well.
However, the issue is still not settled. Animal experiments and some studies of large populations suggest that fruits, vegetables and fiber do indeed help ward off such cancers. And even if they do not, experts maintain that this kind of food clearly carries other benefits, including preventing obesity, heart disease and possibly some other kinds of cancer.
"It's not a case of choosing the disease you don't want to get," said Melanie Polk, a dietitian at the American Institute of Cancer Research in Washington. "If we eat a diet that is high in vegetables, fruits, grains and beans we ... will protect our overall health."
About 130,000 cases of colon and rectal cancers were diagnosed last year in the United States. About 56,600 people died from the disease, which is second only to lung cancer in causing cancer deaths.
One of the new studies, conducted at the National Cancer Institute and eight other health centers, watched 1,905 patients for recurring adenomas, or polyps, which are precancerous growths in the colon, or large intestine. One group of patients limited fat to 20 percent of total calories and ate five to eight servings of fruit or vegetables daily; a virtually identical group kept up its usual eating habits.
Polyps were removed from both groups at the start. Over four years, they came back in 39 percent of patients in both groups. The average number per patient and size were about the same.
"It was very disappointing," said Dr. Arthur Schatzkin, a lead researcher at the National Cancer Institute. "A positive result would have been a very strong statement."
In the second study, of 1,303 patients, University of Arizona researchers also found no lower risk of colorectal cancer from a diet heavy on a high-fiber, wheat-bran cereal.
Researchers have theorized that a low-fat, high-fiber diet chemically neutralizes cancer agents, makes protective changes to cells, or curtails bile acids that irritate intestinal lining and promote polyps.
The two new studies did not consider actual cancers partly because doctors can readily remove polyps and do so under the accepted standard of good medical care. If undetected, only 5 to 10 percent of polyps turn cancerous within 10 years.
The researchers said low-fat, high-fiber diets could conceivably act on colon cancer in the later stages of development. Such diets may work only when eaten for more than just four years.
However, in an accompanying editorial, Dr. Tim Byers, at the University of Colorado School of Medicine, was not as hopeful.
"There may be many reasons to eat a diet that is low in fat and high in fiber, fruits, and vegetables ... but preventing colorectal adenomas, at least for the first three or four years, is not one of them," Byers said.
On the Net:
National Colon Cancer Research Alliance: http://www.nccra.org
Colon Cancer Alliance: http://www.ccalliance.org
American College of Gastroenterology: http://www.acg.gi.org