Does Fiber Prevent Colon Cancer?

Eating cereal can increase your fiber intake.

A few years ago, a large Boston-based study found that dietary fiber did not prevent colon cancer. Suddenly, headlines proclaimed, “Fiber Has No Benefit!” People started wondering if it was worth the effort to add more fruits, vegetables, beans, cereals, and whole grains--all good sources of fiber--to their diet. But preventing cancer isn’t the only reason to consider adding fiber. And despite what the Boston study said, doing so may still help lower your cancer risk.

How Fiber May Help

Best known for its ability to help people stay regular, fiber has not been a high-priority nutrient for most Americans. The average American gets only about 13 grams of fiber every day. But there has long been a clear link between fiber and certain health benefits. In addition to preventing constipation, those benefits include lowering cholesterol and improving blood sugar levels for people with diabetes. So for years, the U.S. Department of Agriculture has recommended that people get between 25 and 35 grams of fiber daily. Now, some experts are pointing to studies other than the Boston one that suggest that fiber can offer some protection against developing cancer.

Fiber is often referred to as either soluble or insoluble. Soluble fiber absorbs water to form a gel. Soluble fibers are found in these foods.

• Oats

• Certain fruits

• Beans

• Psyllium, which comes from the seed of a type of plantain

Insoluble fibers do not absorb water and are found mainly in whole grains and cereals. Although experts think it may be for different reasons, both soluble and insoluble fiber may offer protection against cancer.

Barry Goldin, PhD, is a researcher who focuses on diet and cancer. “We know that people who eat more fiber produce more stool,” Goldin says. “The belief is that fiber may prevent colon cancer if the increased stool is absorbing or diluting cancer-causing material.” This material is called carcinogenic. Insoluble fiber, Goldin says, may help because it moves food through the body quicker. That may shorten the time it takes for these carcinogens to leave the body.

There is also another theory about how fiber may help. It focuses on what happens when bacteria found in the intestines break fiber down. “The fiber ferments,” Goldin says, “into products that may be protective to the colon.” The fermentation of soluble fibers is thought to make the colon more acidic. The acidic nature helps it resist carcinogens.

How Much Do We Really Know?

One reason for the confusion over how much protection fiber offers is the challenge of accurately measuring fiber in people’s diets. There is no standard method to measure the various fibers in food. There is also no easy way to know how much fiber reaches the colon, where it can have anticancer effects. In studies that focus on fiber, people are often given food questionnaires to record how much fiber they eat. The questionnaires differ. That can produce different findings.

Katherine Tucker, PhD, is director of the Dietary Assessment Research Program at the USDA Human Nutrition Research Center on Aging in Boston. Tucker explains, “A survey may ask how much white bread and dark bread one eats. But people could translate ‘dark’ bread as pumpernickel or rye--which, because of the color, may look high in fiber but is actually made from mostly refined white flour.”

Tucker says that the amount of fiber in assorted breads can vary so much that a less-detailed survey could produce flawed results--especially if many other fiber-rich foods are also not listed on the survey.

In addition, some studies ask people to remember how much fiber they ate in the past--sometimes as many as 10 to 20 years ago. These are details that are not easy to remember and not always guaranteed to be correct.

Tucker says that using surveys with more detailed questions about fiber-rich foods can improve accuracy. She adds that researchers should also include a larger number of people with high-fiber intakes to better gauge an effect on colon cancer, if there is one.

Conflicting Reports

Another way to research the effects of fiber is to compare people in different countries. Such comparisons seem to indicate that those who eat more fruits, vegetables, and grains have the lowest rates of colon cancer. In the 1970s, observational studies by researcher David Burkitt found that colon cancer was rare in African nations. He also saw that the African diet was rich in fiber and low in refined carbohydrates, such as sugar.

More controlled studies, though, show conflicting results. A landmark study, published in the January 1999 issue of The New England Journal of Medicine, followed more than 88,000 women for 16 years. The study looked at total fiber, as well as the individual effects of fruit, vegetable, and cereal fibers. The researchers found no link between lower colon cancer rates and dietary fiber. Two other studies published in the April 20, 2000, issue of The New England Journal of Medicine also found no link.

In contrast, Patricia Bebo, MS, RD, and colleagues conducted a review of 16 studies on fiber and colon cancer. When they did, they noted that 73% of those studies found total dietary fiber protective against cancer and colon polyps. When they broke down the types of fiber studied, they found that fruit fiber was the most protective. Vegetable fiber was next. Cereal fiber was the least protective. A handful of studies that classified fiber as soluble and insoluble found both types protective against colon cancer.

Bebo says it’s important to look at all the studies that are being done, not just a few. Regardless of how well done a particular study is, she points out, it gives only a partial picture.

Putting More Fiber in Your Diet

Arthur Schatzkin, MD, PhD, who was lead author of one of the studies that showed no benefit from fiber, concludes that despite the negative findings, the researchers are not suggesting that dietary change is ineffective in lowering the risk for colorectal cancer. While recent studies raise doubts about the importance of fiber, he says, the earlier studies that found a benefit are still important. “I wouldn’t say it’s the end of the story. I’d say we’re less sure now and have to go back and do better studies.” And besides, fiber has other undisputed benefits.

The American Gastroenterological Association (AGA) has noted that studies showing a protective effect of fiber involve fiber intake 3 times what the average American gets. Therefore, the AGA recommends eating 30 to 35 grams of total fiber daily from various sources. Those should include 5 to 7 servings of fruits and vegetables and “generous portions of whole-grain cereals.”

Here is a sampling of fiber-rich foods you can add to your diet to increase your daily intake of fiber.

10 to 15 grams of fiber

• 1 cup cooked chickpeas (garbanzo beans), kidney beans, lentils, or lima beans

• ½ cup Kellogg’s All-Bran cereal

6 to 7 grams of fiber

• ½ cup cooked prunes

• ¾ cup bran flakes

3 to 4 grams of fiber

• 1 medium apple with skin

• 1 orange

• 1 slice 100% whole wheat bread

• 1 cup cooked brown rice

• 1 packet instant oatmeal

• ½ cup cooked broccoli, carrots, or green peas

• 1 small baked potato with skin

Including a variety of high-fiber foods in your diet and exercising regularly are preventive measures to help lower your risk for colorectal cancer. Early detection is another key factor. The earlier colorectal cancer is found, the better the chance it can be cured. If the cancer spreads outside the colon, it is much harder to treat.