Carbohydrates

Carbohydrates constitute approximately 50% of the "typical" American diet. The average daily intake is 300-500 g, which amounts to an average of 1600 calories per day. The major dietary form of carbohydrate is starch, accounting for 50% of the total intake. Most dietary starch is in the form of the polysaccharide amylopectin or in the form of plant starch and consists of chains of glucose molecules linked at the «-1,6 carbons (Fig. 3). Animal starch or amylose is also present in the diet. This polysaccharide is a straight chain polymer of glucose, linked «-1,4. The disaccharides sucrose (glucose-fructose), lactose (glucose-galactose), and maltose (glucose-glucose) make up approximately 30%, 6%, and 2% of the intake, respectively. The remainder is accounted for by small amounts of glucose, fructose, and trehalose. The polysaccharide cellulose is also ingested, but its ^-1,4 linkages are not attacked by mammalian enzymes. Approximately 80% of the cellulose, however, is digested by the microbial flora of the lower bowel. Cellulose is a primary constituent of "fiber," and it is unclear how much of that digested by the flora is absorbed and how much is used by the flora.

Digestion

To be absorbed, dietary carbohydrates must be broken down into monosaccharides. Luminal digestion of starch begins in the mouth with the action of salivary «-amylase and ends in the small intestine following the action of pancreatic «-amylase. These identical enzymes have a pH optimum near 7.0, are activated by Cl", and produce identical digestion products. Most luminal starch digestion is carried out by the pancreatic enzyme, and no problems result if salivary amylase is absent. In fact, most salivary amylase is inactivated by gastric acid. However, if a starchy meal is eaten and remains unmixed in the orad stomach for a time, salivary amylase may digest more than 50% of the starch present. Amylase hydrolyzes only the interior 1,4-glycosidic bonds of amylase. Therefore, exhaustive digestion of starch by amylase yields maltose, maltotriose, and so-called «-limit

Luminal digestion Membrane digestion Product

Glucoamylase Isomaltase Sucrase Lactase Trehalase ( Starch ) Amylas% a-Dextrins a1-6. 5 95

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