The other category of fiber, insoluble fiber, is not appetizing to the colonic bacteria and leaves the body in much the same form as it entered. Insoluble fiber has a local effect on the digestive tract, increasing size and weight of the feces through water absorption. This increases the frequency of bowel movements, stimulates peristaltic movement, and reduces the time it takes for food to travel through the digestive system. A diet rich in insoluble fiber is associated with a decreased risk of colon and rectal cancer, decreased constipation, and a reduction in blood pressure.
Insoluble fibers are cellulose, hemicellulose, and lignin. Cellulose is a form of plant carbohydrate found in fruit and vegetable pulp, skin, stems, and leaves and the outer covering of nuts, seeds, and grains. Hemicellulose is a polysaccharide made from hexoses, pentoses, and acid forms of hexoses and pentoses, as well as glucose units. It is found along with cellulose. Lignin is a fiber that is not a carbohydrate, although it is found in the cell walls of plants. Sources include wheat bran and woody portions of fruits and vegetables. Lignin is associated with a lower incidence of breast and ovarian cancers.