Study doubts fiber's ability to fight cancer

Healthy lifestyle, not diet alone, may help cut colorectal illness


People who eat high-fiber diets run a lower risk of colorectal cancer than those who don't -- but it's probably not the fiber that's protecting them, scientists reported today in one of the largest studies of the subject.

Researchers seeking to resolve one of the most debated issues in cancer prevention found that colorectal cancers were 16 percent less common among people consuming the most fiber than among those getting the least.

But the advantage probably comes from other healthy lifestyle choices made by fiber eaters, who also tend to smoke less, exercise more, avoid red meats and consume more fruits, vegetables, whole grains, calcium and folic acid.

In fact, when researchers ran controls for these factors, evidence of fiber as a cancer fighter disappeared.

"People who eat high fiber tend to have a healthier lifestyle and a more healthy diet," said Yikyung Park, an epidemiologist now with the National Cancer Institute. "Those things together may reduce the risk of colon cancer."

Colorectal cancer, a term that encompasses tumors of the colon and rectum, is the nation's second-leading cancer killer, claiming 56,000 lives last year. Only lung cancer kills more.

The researchers, mainly from the Harvard School of Public Health and affiliated institutions, pulled together 13 previous studies that tracked a total of 725,000 men and women for periods ranging from six to 20 years.

Their report, appearing in today's Journal of the American Medical Association, is the third major study in the past six years to cast doubt on decades of claims made by health advocacy groups, cereal companies and nutritionists that fiber is a key to preventing colon cancer.

But the debate is unlikely to end here. A large European study of a half-million people followed for more than a decade recently found a statistical link between high-fiber diets and a lower risk of colorectal cancer.

"There is still a controversy," Park said. "We need to look into more specifics, like the types of fibers people eat, the sources of fiber and whether people with certain characteristics may have some benefit."

The fiber theory gained a foothold in 1969, when a British researcher noted that societies in rural Africa had much lower rates of colorectal cancer than those in industrialized nations. He credited the high-fiber content of the African diet --heavy in vegetable products and short on meat.

Though subsequent studies fell on both sides of the question, the notion had a logical appeal: Fiber, the indigestible ingredients of plant products, speeds the passage of waste products through the colon and dilutes whatever carcinogens are present.

Another theory holds that fiber triggers production of compounds that may be healthy for the colon.

Now, many doctors say the case for fiber as cancer fighter was probably overstated -- lumped together with fiber's clearer benefits against heart disease and diabetes.

"I think this is one of those areas where something that was perhaps beneficial in other respects was oversold," said Dr. Jean-Pierre Raufman, director of gastroenterology at the University of Maryland Medical Center.

"From the beginning, it was a little too wishful and a little too simplistic to think you only needed to eat more fiber" to prevent colon cancer, Raufman said. "It's a more complex disease than that."

The American Cancer Society is unlikely to alter its recommendations that people eat foods rich in fiber, said Marjorie L. McCullough, an epidemiologist with the group and one of the study's authors. Aside from cardiovascular benefits, high-fiber foods prevent obesity -- a suspected cause of cancer -- by giving people the sensation of being full.

For years, specialists have emphasized the importance of early detection in preventing colorectal cancer deaths. Colonoscopies and other screening tools give doctors a chance to remove small tumors and precancerous polyps before they turn deadly.

"Screening will probably do more to impact on the colon cancer risk than any dietary or lifestyle issue," said Dr. Michael Choti, director of the Johns Hopkins Colon Cancer Center.

Even so, doctors want to identify dietary choices that may prevent the cancer in the first place. The task has been maddeningly difficult, in part because it's hard to separate the effects of one food from another.

"It's so hard to tease out the confounding factors," Choti said. For example, further studies may show that fiber prevents cancer in people with a particular genetic makeup, but not in others, Choti said. If that's true, the effect may not show up in large population studies such as the one reported today.

In the 1980s, the National Cancer Institute drew criticism when it endorsed a claim by the Kellogg Company that its high-fiber cereals prevented certain types of cancer.

Although NCI later backed off the endorsement, the cereal company still claims on its Web site that a high-fiber diet helps prevent colon cancer and heart disease.

Meanwhile, Internet health sites are rife with similar claims. A site maintained by the Johns Hopkins Bayview Medical Center advises that insoluble fiber -- from sources such as fruits, vegetables and wheat bran -- speeds the elimination of wastes and "reduces the risk of colon cancer."

In an editorial accompanying today's article, Dr. John A. Baron of Dartmouth Medical School said the confusion may partly result from the many sources of dietary fiber.

Insoluble fiber from wheat and other sources has inhibited the growth of bowel cancers in lab animals, he noted. Human studies showed differing results -- with some finding a benefit in soluble fibers from fruits and vegetables, and others finding protection in cereal fibers.

"Studies like that of Park et al. provide valuable help, but unfortunately there is much to do," he wrote.

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