Fiber is the indigestible residue in plants that you eat and is composed of the carbohydrates? (car-buh-HY-druts) cellulose (CELL-you-loas), pectin,hemicellulose (heh-mi-CELL-you-loas), and the noncarbohydrate lignin (LIG-nun). You need enough fiber to promote regular bowel movements and improve the biochemical environment of the large bowel. If you don't eat enough fiber, your digestive system won't function as well as it should, which could lead to disease. 'Insoluble' (in-SAHL-you-bull) fiber, which doesn't dissolve in water, is found in such foods as wheat bran and is a good bowel regulator. 'Soluble' (SAHL-you-bull) fiber dissolves in water and lowers cholesterol levels, which is good for the heart. Some cholesterol in the body is converted into digestive juices called 'bile acids,' which are soaked up in the digestive tract by fiber. If you haven't eaten enough fiber, these acids turn back into cholesterol, which is reabsorbed into the blood. The main sources of dietary fiber are fruits, vegetables, beans, and whole grains. If you eat a proper diet with adequate fiber, there's no need to take commercial laxatives, which could have undesirable side effects. People who eat a high-fiber diet have a lower incidence of diseases, especially diseases of the digestive system.