April 20, 2000

Two Studies With Fiber Find No Protection for the Colon

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Fiber Does Not Help Prevent Colon Cancer, Study Finds (Jan. 21, 1999)

Issue in Depth: Nutrition

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By GINA KOLATA

wo rigorous studies involving thousands of people have failed to confirm one of the most widely held beliefs about diet and health: that eating low-fat, high-fiber foods can reduce the risk of colon and rectal cancer.

The studies found that neither eating a low-fat diet with abundant fiber, much of it from fruits and vegetables, nor eating extra fiber in the form of wheat bran made any difference in colon cancer risk. And while some researchers say there is still a chance that fiber helps prevent colon cancer, others say it is time to stop telling people that it can.

Experts on colon cancer said the research, led by scientists at the National Cancer Institute and the University of Arizona, left them stunned. Colon and rectal cancer is the second-leading cause of cancer-related death in the United States, after lung cancer, and is diagnosed in 130,000 Americans a year. Many people hope that a high-fiber diet will protect them.

"It was totally comfortable to believe that low-fat, high-fiber diets would be beneficial," said one public health expert, Dr. Gilbert S. Omenn, executive vice president for medical affairs at the University of Michigan, who was not involved in the research. "Well, here we are. There's not a shred of evidence from these trials."

Dr. Barnett Kramer, deputy director of the division of cancer prevention at the National Cancer Institute, said the surprising results showed "the need to rigorously put belief systems to the test," especially "when you are making recommendations to literally hundreds of millions of people."

Public health experts said fiber might nonetheless play another role long attributed to it: reducing the risk of heart disease and diabetes. It may also help prevent diverticulitis and diverticulosis, ailments that entail the formation of sacs in the wall of the intestine. But some researchers say the latest studies show that it is time to abandon the idea that fiber can help prevent colon cancer.

In both studies, financed by the cancer institute, the subjects were men and women who had had at least one polyp, a tiny noncancerous growth, removed from the colon. Such people face a heightened risk of developing new polyps. And since many colon cancers start out as polyps, people who have had polyps also face an increased risk of developing colon cancer. The question was, Can a special diet protect these people from developing new polyps? If so, cancer experts reasoned, the diet might protect them from colon cancer.

One study, lasting four years, involved 2,079 people randomly assigned to eat low-fat, high-fiber diets with lots of fruits and vegetables or to follow their usual diets. In the other study, lasting three years, 1,429 participants were randomly assigned to eat high-fiber wheat bran cereals or wheat bran fiber bars, or to eat cereals or bars that looked and tasted the same but were low in fiber.

The results, being published today in The New England Journal of Medicine, showed that those who ate the high-fiber diets were no less likely to develop new polyps over the next few years than those who did not.

"These are pretty stunning articles," Dr. Omenn said.

Dr. Kramer said it remained possible that a high-fiber diet could alter colon cancer risk: the effect might occur either before polyps develop or very late in the development of colon cancer, outside the period covered by the studies. But that is only a hypothesis, he said.

And Dr. David Hunter, director of the Harvard Center for Cancer Prevention, said that although "there is no reason to think there is any harm associated with cereal fiber," it appears that "it's not going to prevent colon polyps or cancer, at least in the short term or over a decade."

The hypothesis linking fiber to a reduced risk of colon cancer dates from 1971, with a paper by Dr. Denis P. Burkitt, a British missionary surgeon who noticed that poor rural Africans had much less colon cancer than affluent Westerners. The reason, Dr. Burkitt proposed, was diet: the Africans ate much more fiber. Over the years, evidence supporting the hypothesis steadily mounted.

There was a biological rationale for why it might be so: Fiber makes the stool more bulky. A bulkier stool will dilute the cancer-causing substances that are in foods and that are excreted in the stool, sparing the colon an assault by these chemicals. Fiber also makes the stool move faster, an effect that should reduce the time that the colon is in contact with cancer-causing chemicals in the stool.

Biochemical data added support to the hypothesis: Fiber binds bile acids, which are chemicals produced by the liver and secreted into the bowel. Bile acids can damage cells and possibly cause cancer. The more fat people eat, the more bile acids they secrete. So a low-fat diet might work with a high-fiber one to protect against colon cancer, researchers thought.

In addition, fiber is metabolized in the intestines to produce a class of short-chain fatty acids that seem to protect against carcinogens.

Animal studies bolstered the notion. Rodents, fed high concentrations of carcinogens in laboratory studies, were protected against colon cancer if they also ate high-fiber diets.

Studies of humans, however, were inconsistent. Some showed that people who developed colon cancer or polyps ate more fat and less fiber than those who did not.

But several recent studies, one involving 88,000 nurses, failed to detect any effect of diet on colorectal cancers or polyps. The problem with these studies was that they were observational: the subjects chose their diets and were simply observed. It may be that those who chose high-fiber diets happened to be different in some other way from those who did not, and that this difference might have negated diet's effects on their colon cancer risk.

In planning the current studies, said Dr. Arthur Schatzkin, chief of the nutritional epidemiology branch at the cancer institute, "we had high expectations and good rationale." But, he said, "we got absolutely null results."

Dr. Schatzkin, who directed the study in which subjects ate either diets high in fiber, including fruits and vegetables, or their ordinary diets, said the fiber hypothesis might still turn out to be true. "You could say diet works earlier -- before they develop the first polyp," he said. "Or maybe diet works later in the process."

But the leader of the second study, Dr. David S. Alberts, director of cancer prevention at the University of Arizona's Cancer Center, said, "I think we've definitely disproved the fiber hypothesis for colon cancer."