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For years, Americans have been advised to eat a high-fiber diet to lower the risk of colon cancer. Early theories suggested that dietary fiber was protective against colon cancer and possibly breast cancer. But inconsistent findings from numerous studies have led to disagreement on the role of dietary fiber in preventing cancer.  

What does this mean for you? Read on to learn more about current research and expert recommendations for including fiber in your diet.

The Findings on Fiber and Colon Cancer

Since the early 70s, scientists proposed that a diet high in fiber could help prevent colon cancer. Colon cancer rates are significantly lower in cultures where people eat a large amount of high-fiber foods, so most of the research on fiber was directly related to colon cancer. 

In theory, fiber protects against colon cancers like this:

  • Insoluble fiber from foods such as wheat bran adds bulk to stools and moves them through the GI tract swiftly, reducing the contact time with potential toxins in the colon.
  • A bulky stool high in water can also dilute potential carcinogens.
  • Fiber can also discourage growth of harmful bacteria and encourage healthy bacteria in the colon.

Studies on fiber have been inconclusive, however. Some show that higher fiber intakes are linked to a reduced colon and colorectal cancer, while others have not. The first setback for this theory came in 1999 from the Nurses’ Health study. Nurses who ate more dietary fiber did not have a lower incidence of colon cancer.  

The following year, a study indicated that eating a large amount of fiber each day (the recommended 25 to 38 grams) could reduce the risk of colon cancer by 40%. But in 2005, another observational study concluded that high dietary fiber intake was not associated with a reduced risk of colorectal cancer.

Since then, more studies continue to show that colorectal cancer risk can be lowered with higher fiber intakes from fruit and vegetables.

Fiber Research: Breast and Prostate Cancers

Research has not turned up a strong link between fiber and prostate and breast cancers. The majority of studies have not found that eating dietary fiber significantly reduces breast cancer risk.

Few studies have examined the association between fiber and prostate cancer risk. But those that have showed that dietary fiber does not reduce prostate cancer risk.

Role of Dietary Fiber in a Cancer-Preventing Diet

Alice Bender, MS, RD, a nutritionist for the American Institute for Cancer Research, says the evidence on fiber and colon cancer, while mixed, is strong enough to make recommendations. “Our 2007 expert report looked at all the studies and concluded that foods high in fiber, not necessarily the fiber itself, can lower risk for colorectal cancer,” she tells WebMD.

“We know that a plant-based diet rich in fruits, nonstarchy vegetables, legumes, and whole grains is associated with a lower risk of a number of the most common cancers – colorectal, stomach, mouth, pharynx, larynx, and esophageal” Bender says.

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