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Colorectal Cancer Health Center

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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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."

Fiber is believed to help reduce risk in two ways: It keeps you regular to remove toxins from the intestines, and bacteria living in the gut feeds on it -- producing beneficial byproducts to keep the colon healthy.

So why so many conflicting results over the years?

Most of the trials that show no benefit to a high-fiber diet -- at least four well-publicized studies in 1999 and 2000 -- looked at either cereal fibers or fiber supplements such as Metamucil. "Bran fibers may be not be in the same proportions or even be the same type of fiber that might occur in a natural diet," McCullough tells WebMD. "And it may be that supplemental fiber is not helpful at all." In fact, one study published in The Lancet in October 2000 found that these fiber supplements actually increased the risk of colon polyps. Meanwhile, most studies that show a protective benefit investigated fiber that comes from fruits and vegetables, as well as whole grains.

And while the "bottom line" findings may differ, there are similarities between these new Lancet results and even some studies to indicate no benefit from fiber: Measurable protection in the 30 grams-plus range.

"Actually, we found that there was a 35% reduced lower risk in people who ate something like 33 grams a day vs. those who had 14," says Edward Giovannucci, MD, of Harvard Medical School, a researcher on the ongoing Health Professionals and Nurses Health studies, whose findings suggested no benefit from a high-fiber diet. "But we statistically controlled for other factors, including smoking, aspirin use, red meat, alcohol. And when those factors were taken into account, the benefit from fiber went entirely away. That's why our studies suggested no benefit."

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