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Chapter: Introduction to Human Nutrition: Digestion and Metabolism of Carbohydrates

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Physiological classification of carbohydrates entering the colon

Carbohydrates enter the colon because (1) monosac-charide transporters do not exist in the intestinal mucosa or do not function at a high enough rate; (2) the enzymes needed to digest the carbohydrates are not present in the small intestine; (3) the enzymes are present but cannot gain access to the carbohydrates; or (4) the enzymes do not digest the carbohydrates rapidly enough for them to be completely absorbed.

Physiological classification of carbohydrates entering the colon

 

Carbohydrates enter the colon because (1) monosac-charide transporters do not exist in the intestinal mucosa or do not function at a high enough rate; (2) the enzymes needed to digest the carbohydrates are not present in the small intestine; (3) the enzymes are present but cannot gain access to the carbohydrates; or (4) the enzymes do not digest the carbohydrates rapidly enough for them to be completely absorbed. In addition, a small amount of carbohydrate entering the colon consists of carbohydrate residues occurring on mucopolysaccharides (mucus) secreted by the small and large intestinal mucosal cells.

 

Some carbohydrates are always nonglycemic because the human species lacks the enzymes neces-sary for their digestion. However, a significant pro-portion (perhaps up to half ) of all carbohydrates that escape digestion in the small intestine have a chemical structure which is such that they could potentially be digested or absorbed in the small intestine, but they are variably absorbed for various reasons, examples of which are given below.

 

First, some monosaccharides and sugar alcohols are only partially absorbed because of low affinity for intestinal transporters. Xylose is taken up by the glucose transporter, but is only partly absorbed because of a low affinity. Fructose is poorly absorbed on its own, but readily absorbed in the presence of glucose. The surface area of the small intestine avail-able for absorption is reduced by diseases that cause atrophy of the intestinal mucosa, such as tropical sprue or celiac disease, or surgical resection of a portion of the intestine (e.g., for Crohn’s disease). An increased rate of intestinal transit (e.g., high osmotic load in the small intestinal lumen from undigested sugars) reduces the time available for absorption to occur.

 Second, some individuals have a low or absent intestinal lactase activity; thus, lactose is partly or completely nonabsorbed in these individuals. The availability of pancreatic amylase may be reduced in cystic fibrosis or in individuals whose pancreatic mass has been destroyed by, for example, recurrent pancreatitis.

 

Third, although starch (amylopectin or amylose) is potentially digested in the small intestine, if it is trapped inside intact cell walls or other plant cell structures, intestinal enzymes may not be able to gain access to it, and therefore it remains undigested. The digestibility of the carbohydrates in banana depends on the degree of ripeness. The starch in green banana is very indigestible, but, as the banana ripens, the starch is converted to digestible sugars.

Finally, there are many reasons why carbohydrates may not be digested rapidly enough to be completely absorbed. Some forms of retrograded or resistant starch, or foods with a large particle size, are digested so slowly that the time spent in the small intestine is not long enough for their complete digestion. Diges-tion of these carbohydrates can be altered by factors that affect transit time. The presence of osmotically active and unabsorbed molecules (such as unabsorbed sugars) will draw water into the intestine and speed the rate of transit. Substances that increase bulk, such as wheat bran, will have similar effects. Transit rate is slowed in old age and in the presence of viscous fibers. Drugs may increase or decrease the rate of transit. Certain disorders can also affect transit time, such as gastroparesis, a complication of type I diabetes.


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