Fiber is either in a soluble or insoluble form.
Soluble fibers include pectins, gums, mucilages, and some hemicelluloses. These compounds are found inside and around plant cells and exist as gum arabic, guar gum, locust bean gum, and pectins. Soluble fiber is found in cereals and a variety of foods such as salad dressings, jams, and jellies.
Soluble fibers eaten in high amounts can decrease blood cholesterol.
Soluble fiber ingested in large amounts can decrease blood cholesterol. The mechanism is due, in part, to the ability of soluble fiber to inhibit bile recycling in the intestinal tract. Bile, which is formed from cholesterol, is pulled into the feces for elimination, rather than eventually accumulating in the blood.
Soluble fiber is also beneficial in moderating levels of blood glucose. When consumed in large amounts, soluble fiber slows glucose absorption from the small intestine. The fiber, such as oatmeal, is processed slowly and produces a slow increase in blood glucose after eating. This effect may be helpful in the management of diabetes because it is part of a diet that helps to regulate blood glucose.
Insoluble fiber cannot be digested by enzymes in the human gastrointestinal tract. Lignins, cellulose, and hemicelluloses are insoluble fibers.
Insoluble fiber is important because it provides mass to the stool, helping to ease elimination. 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.