What Is It?
Insoluble fiber is a subclass of dietary fiber. Like its soluble cousin, insoluble fiber differs from starch because the chemical bonds that join individual sugar units can not be digested by enzymes in the human gastrointestinal tract. Insoluble fiber is considered a “noncarbohydrate carbohydrate” since the components that make up insoluble fiber are lignins, cellulose, and hemicelluloses. All of these compounds form the structural parts of plants and do not readily dissolve in water and are not metabolized by intestinal bacteria. Bran fiber is rich in hemicelluloses, while a cotton ball is pure cellulose.
Insoluble fiber is important because it provides mass to the stool, helping to ease elimination. The fiber absorbs water and holds onto it in the intestine. When enough fiber is consumed, the water-retaining property helps to enlarge and soften the stool. As a result, less pressure is required to expel the stool.
As a result of its ability to increase fecal bulk and decrease intestinal transit time, insoluble fiber decreases the risk for diverticulosis, a condition in which small pouches form outside of the intestinal wall and may become infected. Insoluble fibers, especially certain types of hemicelluloses, are the best fibers for increasing stool size. Bran, which is the fibrous covering of grain kernels, is rich in hemicelluloses. Bran layers form the outer covering of all grains, so whole grains are good sources of insoluble fiber. Wheat products are especially beneficial in increasing fecal bulk, while brown rice is useful in decreasing intestinal transit time. A high fluid intake is also important with a high fiber intake to help move the bulk efficiently through the colon.
In addition to gastrointestinal benefits, evidence suggests that cellulose, hemicellulose, and lignins serve a major protective function in colon cancer. Researchers hypothesize that carcinogens are diluted by fluid, attracted and bound to the fibers, and then quickly excreted as the fibers pass through the gastrointestinal tract for elimination. Dietary fiber from fruits and vegetables has demonstrated the most protective effect in human studies.
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