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. Complementing the action of insoluble fibers, some soluble fibers also add to fecal bulk and increase its water content. But soluble fiber's potential for reducing blood cholesterol levels recently has grabbed the spotlight. Margo Denke, M.D., a nutrition research scientist at the University of Texas Southwestern Medical Center, estimates that a diet low in fat and high in soluble fiber may reduce an individual's blood cholesterol by 3 percent to 6 percent.
Soluble fibers appear to reduce blood cholesterol in two ways.
First, they prevent the reabsorption of vital bile acids from the small intestine. To replace the lost bile acids, cholesterol is drawn from the body, thereby reducing its cholesterol supply. Second, the fermentation of soluble fibers in the intestine produces short-chain fatty acids which block the synthesis of cholesterol. Studies suggest soluble fibers also may help control the rise in blood sugar following a meal and reduce insulin requirements in some patients with diabetes mellitus. By increasing the viscosity of gastrointestinal contents, soluble fibers retard gastric emptying, slowing the absorption of glucose in the process.