Not until the 1960s, however, did scientists seriously begin to investigate the role of fiber in health. At that time, studies showed rural Africans, with diets higher in fiber than the typical American or European diet, had a lower incidence of colon cancer, diverticulosis, hemorrhoids, gallstones, appendicitis, diabetes and some forms of heart disease than Americans or Europeans. Studies since generally have indicated a positive relationship between a high-fiber diet and good health, although it has been difficult to separate the effects of fiber from other dietary and lifestyle factors that may play a role in health. A high-fiber diet appears to reduce disease risk by increasing fecal bulk, decreasing the transit time of food through the gastrointestinal tract, reducing blood cholesterol levels and helping to control blood sugar levels. With their distinct physical characteristics, insoluble and soluble fibers work differently to produce these results.
Insoluble fibers seem to have their greatest impact on the health of the colon or large intestine.
Large amounts of insoluble fibers increase fecal bulk and draw water into the large intestine. The result is a larger, softer stool that exerts less pressure on the colon walls and is eliminated more quickly. In addition, large amounts of insoluble fibers dilute the concentration of potential carcinogens that may be present in the stool, and the decreased transit time reduces the exposure of the intestinal wall to those substances. Furthermore, insoluble fibers alter the pH of the large intestine, interfering with microbial activity that produces carcinogens. The combined effect may be a reduced risk of colon cancer.