The digestion and absorption of dietary carbohydrates takes place in the small intestine. These are extremely efficient processes, in that essentially all of the carbohydrates consumed are absorbed. Carbohydrates are an extremely important component of food intake, since they constitute about 45 to 50% of the typical Western diet and provide the greatest and least expensive source of energy. Carbohydrates must be digested to monosaccha-rides before absorption.
The Diet Contains Both Digestible and Nondigestible Carbohydrates
Humans can digest most carbohydrates; those we cannot digest constitute the dietary fiber that forms roughage. Carbohydrate is present in food as monosaccharides, disaccha-rides, oligosaccharides, and polysaccharides. The monosaccharides are mainly hexoses (six-carbon sugars), and glucose is by far the most abundant of these. Glucose is obtained directly from the diet or from the digestion of dis-accharides, oligosaccharides, or polysaccharides. The next most common monosaccharides are galactose, fructose, and sorbitol. Galactose is present in the diet only as milk lactose, a disaccharide composed of galactose and glucose. Fructose is present in abundance in fruit and honey and is usually present as disaccharides or polysaccharides. Sorbitol is derived from glucose and is almost as sweet as glucose, but sor-bitol is absorbed much more slowly and, thus, maintains a high blood sugar level for a longer period when similar amounts are ingested. It has been used as a weight-reduction aid to delay the onset of hunger sensations.
The major disaccharides in the diet are sucrose, lactose, and maltose. Sucrose, present in sugar cane and honey, is composed of glucose and fructose. Lactose, the main sugar in milk, is composed of galactose and glucose. Maltose is composed of two glucose units.
The digestible polysaccharides are starch, dextrins, and glycogen. Starch, by far the most abundant carbohydrate in the human diet, is made of amylose and amylopectin. Amy-lose is composed of a straight chain of glucose units; amylopectin is composed of branched glucose units. Dextrins, formed from heating (e.g., toasting bread) or the action of the enzyme amylase, are intermediate products of starch digestion. Glycogen is a highly branched polysaccharide that stores carbohydrates in the body. The structure of glycogen is illustrated in Figure 27.19. Normally, about 300 to 400 g of glycogen is stored in the liver and muscle, with more stored in muscle than in the liver. Muscle glycogen is used exclusively by muscle, and liver glycogen is used to provide blood glucose during fasting.
Dietary fiber is made of polysaccharides that are usually poorly digested by the enzymes in the small intestine. They have an extremely important physiological function in that they provide the "bulk" that facilitates intestinal motility and function. Many vegetables and fruits are rich in fibers, and their frequent ingestion greatly decreases intestinal transit time.
Carbohydrates Are Digested in Different Parts of the GI Tract
The digestion of carbohydrates starts when food is mixed with saliva during chewing. The enzyme salivary amylase acts on the a-1,4-glycosidic linkage of amylose and amy-lopectin of polysaccharides to release the disaccharide maltose and oligosaccharides maltotriose and a-limit dextrins (Fig. 27.20). Because salivary amylase works best at neutral pH, its digestive action terminates rapidly after the bolus mixes with acid in the stomach. However, if the food is thoroughly mixed with amylase during chewing, a substantial amount of complex carbohydrates is digested be-
Maltotriose a-Amylase a-limit dextrin
The digestion products of starch after exposure to salivary or pancreatic a-amylase Sugar units are indicated by hexagons.
fore this point. Pancreatic amylase continues the digestion of the remaining carbohydrates. However, the chyme must first be neutralized by pancreatic secretions, since pancreatic amylase works best at neutral pH. The products of pancreatic amylase digestion of polysaccharides are also maltose, maltotriose, and a-limit dextrins.
The digestion products of starch and glycogen, together with disaccharides (sucrose and lactose), are further digested by enzymes located at the brush border membrane. Table 27.6 lists the enzymes involved in the digestion of disaccharides and oligosaccharides and the products of their action. The final products are glucose, fructose, and galactose.
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