Colon bacteria carbohydrate sources

The large intestine extends from the ileocecal valve to the anus. It is wider than the small intestine except for the descending colon, which when empty may have the same diameter as the small intestine. Major functions of the colon are absorption of water, Na+, and other electrolytes, as well as temporary storage of excreta followed by their elimination. The colon harbors large numbers of mostly anaerobic bacteria that can cause disease if they invade tissues. These bacteria metabolize carbohydrates to lactate, short-chain fatty acids (acetate, propionate, and butyrate), and gases (CO2, CH4, and H2). Ammonia, a toxic waste product, is produced from urea and other nitrogenous compounds. Other toxic substances are also produced in the colon. Ammonia and amines (aromatic or aliphatic) are absorbed and transported to the liver via the portal blood, where the former is converted to urea (Chapter 17) and the latter is detoxified. The liver thus protects the rest of the body from toxic substances produced in the colon. Colonic bacteria can also be a source of certain vitamins (e.g., vitamin K, Chapter 36). 

The focus on bacterial fermentation of components of dietary fiber from conventional sources such as bran should not prevent us from considering other types of carbohydrate which may also be fermented by colon bacteria. For example, the host itself produces a variety of polysaccharides which reach the colon. These include mucins from the secretions which lubricate the intestinal tract and mucopolysaccharides from slonghed epithelial cells. It is difficult to obtain precise information concerning how much of this material is produced daily, but the amount could easily be comparable to the amount of polysaccharide which is ingested in the diet. Some of these host-produced polysaccharides, particularly the mucopolysaccharides, are readily utilizable by some of the same bacterial species which ferment plant polysaccharides ( ). 

Another potential source of carbohydrate for colon bacteria consists of polysaccharides which are added to foods as emulsifiers (e.g. guar gum, alginate) or which are used as stool softeners (e.g. psyllium). Plant polysaccharides such as guar gum are also being evaluated for use in the treatment of diabetes because they retard glucose absorption (, ). The amount of polysaccharide from these sources will vary from person to person but could be appreciable in some cases. 

Carbohydrate malabsorption plays a major role in diarrhea associated with SBS. Unabsorbed carbohydrates are broken down by intestinal bacteria to short-chain fatty acids (SCFAs), producing an osmotic load in the distal small intestine and colon that can lead to protracted diarrhea. However, the colon is able to use these SCFAs as a source of energy, thus complex carbohydrates may provide a significant caloric source for patients with a massive resection and a preserved colon. ... 

Because of the variety of polysaccharides which can be fermented by some Bacteroides species, it is difficult to predict with certainty which polysaccharides in the complex mixture of dietary and host-produced carbohydrates that enter the colon will be degraded most rapidly and most extensively. Further information about how these organisms make choices between different polysaccharides i vitro may help to clarify this issue. However, nutritionists who are interested in catabolism of dietary fiber components iji vivo should be aware that the bacteria may prefer other sources of carbohydrate, such as mucopolysaccharides from host secretions or even Maillard products, to the dietary polysaccharide under study, and that this preference may influence catabolism of a particular polysaccharide in ways which we cannot at present predict. Effects of this sort may be responsible for some of the individual-to-individual variation which is encountered in nutritional studies of dietary fiber utilization. 

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