The idea that fiber protects against colon cancer was first proposed many years ago when researchers found that Africans who ate high fiber diets had a low incidence of colon cancer. Since then dozens of studies have supported the protective role of fiber. There are plenty of reasons fiber might lower colon cancer risk: Fiber increases fecal bulk and may dilute potential cancer-causing substances; fiber reduces the time the stool spends in the intestine, which might limit the colon’s exposure to carcinogens. Fiber may also alter the intestinal environment in other beneficial ways. Yet, several recent studies have cast doubt on the theory that fiber protects against colon cancer.
- The well-known Nurses Health Study tracked more than 88,500 female nurses over a 16-year period, dividing the women into five groups based on their estimated intake of fiber. By the end of the study, 787 new cases of colon cancer occurred. Surprisingly, the women who ate the most fiber—nearly 25 grams per day—were just as likely to develop colon cancer and pre-cancerous tumors as those who ate the least (about 10 grams per day).
- In a study published in 2000, researchers randomly assigned 1,429 men and women with a history of colon polyps (a pre-cancerous condition) to eat either a high-fiber or low-fiber wheat-bran supplement in addition to their ordinary diet. After three years, researchers found at least one polyp in about 50 percent of the participants in each group.
- In a well-designed study published in The New England Journal of Medicine, 2,079 people with a history of polyps were randomly assigned to eat either a low-fat, high-fiber diet (consisting largely of fruits and vegetables) or a low-fiber diet. After four years, a similar number of people in each group—about 39 percent—developed at least one recurrent polyp.
Indeed, such results are confusing, but they are far from definitive. First, the amount of fiber may have been insufficient to exert an effect. Subjects in the randomized studies may not have been followed long enough to detect whether the polyps found in the high-fiber subjects have the same potential for malignancy as the polyps from the low-fiber subjects. It’s also possible that introducing dietary changes, such as increased fiber intake, later in adult life may be ineffective at reducing the risk of colon cancer.
Bottom Line: While the relationship between fiber and colon cancer is unclear, a fiber-rich diet has other definite benefits. A diet high in soluble fiber can reduce elevated cholesterol levels, and fiber-rich foods can smooth out blood sugar spikes in people with diabetes. A high-fiber diet can also aid in preventing constipation, which reduces the risk of diverticulitis. Finally, foods rich in fiber -- fruits, vegetables, legumes, and whole grains -- are important sources of vitamins, minerals, and phytochemicals.
For more Alerts and Special Reports, please visit the Colon Cancer Topic page.
