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Fiber May Prevent Colon Cancer After All

2 Studies Contradict Earlier Research Showing No Benefit of Fiber on Colon Cancer Risk
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WebMD Health News

May 1, 2003 -- There's no question that a high-fiber diet is good for you. The question is, how good is it at preventing colon cancer?

While there's convincing evidence that dietary fiber helps prevent heart disease, diabetes, diverticulosis, and other diseases, studies have produced mixed results in its role in reducing risk of the second most common cancer in the U.S. Some find that a high-fiber diet lowers cancer risk, while others indicate it offers no protection. A few even suggest that dietary fiber increases cancer risk.

What's more consistent is the two newest findings, both published in the May 3 issue of The Lancet. Although done by different research teams, both produced remarkably similar results, offering renewed hope for fiber's role against colon cancer.

In one, researchers at the National Cancer Institute looked at the diet of 34,000 people participating in a study of early detection for various cancers. All were free of precancerous colorectal polyps when the trial began. The researchers found that those eating a high-fiber diet -- upwards of 36 grams of fiber each day -- were 25% less likely to develop polyps than those eating fewer than 12 grams.

The people who ate a higher-fiber diet were also more likely to exercise, smoke less, drink less alcohol, were more likely to have used aspirin more frequently, and ate less red meat.

The other study, conducted on 520,000 people in 10 European countries and called the largest study ever, also initially found a 25% reduced rate in colorectal cancer in those eating high-fiber diets of about 35 grams daily compared with those eating less than 15. But upon further analysis, researchers said that low-fiber eaters who doubled their intake could reduce risk by 40% -- and the protective effect was greatest on the left side of the colon, where most cancers originate.

"And in our study, the people eating the most fiber were consuming as much dietary fat as those eating the lowest amounts," lead researcher of the British study, Sheila A. Bingham, PhD, tells WebMD. "And they still got the protective effects."

Why is that important? Because it's just one of the many issues that have contributed to the fiber-for-colon cancer debate and mixed study results, which began in the 1970s when a missionary doctor compared colon cancer rates between America and Africa. He attributed the high cancer incidence in affluent western countries to diets high in animal fats, and the low cancer rates in poorer countries such as Africa to the diets high in plant-based fiber.

"The way I see it, we have observed that diets high in fiber are associated with a lower colon cancer risk," says Marji McCullough, ScD, nutritional epidemiologist at the American Cancer Society and a fiber-for-cancer investigator. "But pinpointing the exact reason has been challenging. It's hard to exactly determine what is in red meat -- whether the fat or something else -- that seems to increase colon cancer risk."

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