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, and maybe even protecting against cancer. In 2008 the U.S. Department of Agriculture recommended that men ages 14 to 50 get 38 grams of fiber a day and after age 51 get 30 grams per day. Women ages 19 to 50 should get 25 grams of fiber a day, and 21 grams per day after age 51.

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.

One way fiber may affect cancer risk is based on the fact that people who eat more fiber produce more stools. Fiber may prevent colon cancer if the increased stool is absorbing or diluting cancer-causing material. This material is called carcinogenic. Insoluble fiber 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 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.

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