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LONDON - Evidence is mounting that fiber might not prevent colon cancer after all, with a new study suggesting that one type of supplement might even be bad for the colon.
The theory that a high-fiber diet wards off the second-leading cancer killer has been around since the 1970s, but the evidence was never strong. The concept began to crumble last year when the first of three major U.S. studies found it had no effect.
In the latest study, published this week in The Lancet medical journal, European researchers found that precancerous growths, or polyps, were slightly more likely to recur in those taking a certain fiber supplement.
The findings demonstrate the difficulty of trying to figure out the relationship between nutrition and disease, said Dr. Michael Thun, who heads epidemiological research for the American Cancer Society.
Fiber is particularly complicated, he said, because there are various types and they all could act differently.
"The concept of a healthy diet continues to be the recommendation for overall health," Thun said. "But the painful process of clarifying which ingredients in food do what will take us decades to sort out."
Thun said the American Cancer Society will revisit its recommendations on fiber and colon cancer in light of the growing body of evidence eroding support for the theory that it wards off the disease.
Experts recommend a low fat, high-fiber diet rich in fruits and vegetables and whole grains because it has been shown to reduce the risk of heart disease, high blood pressure, diabetes and some other cancers.
"There is definitely something dietary going on with bowel cancer, but we haven't really been able to fix on what it is," said Dr. Tim Key, a cancer researcher at Oxford University who was not connected with the study. "The cause of colorectal cancer is very far from understood."
The latest study, conducted by scientists at the University of Bourgogne, France, does not address the effect of a high-fiber diet, but of supplements of one type of fiber ispaghula husk, a compound similar to psyllium that is not part of the average diet.
Psyllium, a grain grown in India, is found in some over-the-counter laxatives and fiber supplements.
The study, involving 552 Europeans who previously had precancerous growths in the bowel, found that 29 percent of those receiving the supplement got at least one new tumor within three years. That compares with 20 percent of those given fake granules.
The findings may or may not be related to the role fiber in general plays in bowel cancer but, considered together with other recent studies, the plausibility of a protective role looks less likely.
"This does produce more evidence for the negative side," said Dr. Lesley Walker, a scientist at the London-based Imperial Cancer Research Fund, which was not connected to the research.
"But we still haven't got the totality of the evidence we want," Walker said. "There are still some important ongoing studies under way on the fiber question that should give us some solid answers."
Sorting out the influence of genes, food, pollutants, living habits and other factors requires drawing together information from many different scientific approaches. Those include lab experiments, rat studies, observations of large groups of people and human experiments.
Information from all of these kinds of science went into the rise and fall of the idea that fiber prevents colon cancer.
It started when scientists noticed that Westerners get more colon cancer than poor people in rural Africa. While the differences between these two populations are too numerous to count, an obvious one was the Africans' higher consumption of fiber.
Over time, many lines of evidence seemed to support the theory.
People see their risk of colon cancer rise when they move from places with low rates to areas where it's more common and they adopt the local eating habits.
Furthermore, the idea made sense. Fiber makes the stool bulkier and perhaps more likely to dilute cancer-causing substances. It also causes them to flow more quickly through the digestive system.
The data seemed convincing enough for health agencies to recommend high-fiber foods as one way of preventing colon cancer.
Then last year, the first major study putting the theory to the test, in which researchers based at Harvard School of Public Health studied 88,757 nurses for 16 years, concluded fiber doesn't help.