Large carbohydrate molecules form the structure of plants, and to a lesser extent, animals. A carbohydrate polymer, or polysaccharide, is a string of sugar molecules linked together. The cell walls of plants are constructed of elaborate polysaccharides made from 12 basic sugars. Cellulose is the main structural carbohydrate; it is a polymer of glucose units linked together to form a tough fiber. Vegetarian ruminants utilize special stomachs that host bacterial populations that break down cellulose.
There are several types of carbohydrate polymers in fruit and vegetables that are not digested. This material passes through the GIT as bulk fiber, undergoing modification and digestion by colon microorganisms. Several fibers have benevolent roles. The benefit seems to be the absorption or neutralization of the irritation or toxicity of other foods. Carbohydrate fiber contributes to the well-hydrated bulk of soft, easily passed stools. Increased dietary fiber over a lifetime is associated with decreased incidence of bowel cancer and cardiovascular disease.
The colon's dense population of microorganisms is important in health and disease. The bacteria feed mostly on undigested carbohydrate, and 99% of them survive best in the absence of oxygen (anaerobic bacteria). It is estimated that 10-15% of starch from cereal grains, potatoes, and up to 50% of milk sugar in most adults enters the colon undigested where it is fermented by colon bacteria. Many vegetables contain indigestible carbohydrate, welcomed by the colon flora. The gas associated with beans is produced by the fermentation of these carbohydrates. Colon fermentation produces hydrogen gas, which may distend the GIT and produce pain. Methane and carbon dioxide are other odorless gases produced by fermentation. The foul smells of colon gas are mostly volatile substances produced by the putrefaction of undigested protein, and indicate a maldigestive state. Many chemical substances are produced by colon bacteria and may be absorbed into the body. Some products are desirable, like the vitamins K, and Biotin. Other products are nutrients such as fatty acids that supply a small percentage of the calories extracted from food. Yet other substances produced in the colon may be undesirable and these include alcohols, lactic acid, and formate. The unpleasant smelling colonic gases are also absorbed and excreted by the lungs, giving the exhaled breath an unpleasant smell (halitosis).
A generous intake of dietary fiber, as indigestible carbohydrate, aids colon bacteria in incorporating ammonia in their own structure and metabolism and results in a lower body ammonia burden. Diets, deficient in fiber, and high in protein increase the body burden of ammonia. Patients suffering liver disease with elevated blood ammonia are improved by the oral intake of lactulose, an indigestible carbohydrate.
The book, Managing Diabetes 2 and the Alpha Nutrition Program talk about the behavioral issues of diet change and food control.