Although the food you eat affects every system in your body, your digestive tract bears the initial brunt of your choices. To keep it running smoothly and disease-free, aim for a diet high in fiber. Unfortunately, most Americans eat only about half the 20 to 35 grams they need each day, even though fiber is readily available in raw and cooked fruits and vegetables, as well as in grain products such as breads, cereals, pasta, and rice.
Fiber comes in two forms, soluble and insoluble. Soluble fiber, found in fruits, vegetables, brown rice, oats, and barley, lowers blood cholesterol levels and slows the entry of glucose into the bloodstream, an important factor in preventing or controlling diabetes. Insoluble fiber, found mainly in whole grains, fruit and vegetable peels, high fiber cereals, and wheat and corn bran, keeps your digestive tract in order. Insoluble fiber soaks up water, adding the bulk that pushes possible cancer-causing substances (carcinogens) out of the intestine.
Fiber’s presumed power of protecting against colorectal cancer has received a lot of publicity. This type of cancer is probably the most significant diet-related disorder of the digestive tract and the third leading cause of cancer deaths among both men and women. For years, health experts have speculated that a high-fat, low-fiber diet encourages the production and concentration of carcinogens in the colon, but that fiber-rich food clears them out. Yet the research remains controversial. “A diet high in fiber is almost always one that is, at the same time, low in fat,” says Bennett E. Roth, M.D., a gastroenterologist at the University of California, Los Angeles School of Medicine. “So it may be that it’s the reduction in fat that’s beneficial rather than the fiber itself. But that issue hasn’t been resolved yet.”
The National Cancer Institute’s National Cancer Polyp Prevention Trial, for example, failed to find a lowered incidence of colorectal polyps among more than 2,000 participants on a high-fiber diet. Critics have pointed out, however, that the study lasted only three to four years, while colon cancer takes decades to develop. Thus, the effect of a high-fiber diet wasn’t yet apparent. In addition, they note, while polyps are a risk factor for colorectal cancer, most of them do not become malignant.
“Most research supports the protective effect of a diet high in fruits, vegetables, beans, and grains,” says Polk, citing the American Institute for Cancer Research’s own 1997 report, “Food, Nutrition and the Prevention of Cancer: A Global Perspective,” which reviewed 4,500 international studies before coming to this conclusion.