30.8 ). Starch is a particularly important source of calories, especially in developing countries, and is found predominantly in cereal products. Disaccharides are a second class of carbohydrate nutrients that includes sucrose (consisting of glucose and fructose) and lactose (consisting of glucose and galactose), the latter being an important caloric source in infants. It is, however, a key principle that the intestine can absorb only monosaccharides and not larger carbohydrates. Finally, many food items of vegetable origin contain dietary fiber, which consists of carbohydrate polymers that cannot be digested by human enzymes. These polymers are instead digested by bacteria present largely in the colonic lumen (see ), thereby allowing salvage of their caloric value.
Dietary disaccharides are hydrolyzed to their component monomers directly on the surface of small intestinal epithelial cells by brush border digestion, mediated by a family of membrane-bound, heavily glycosylated hydrolytic enzymes synthesized by small intestinal epithelial cells. Brush border hydrolases critical to the digestion of dietary carbohydrates include sucrase, isomaltase, glucoamylase, and lactase (
Table 30.1). Glycosylation of these hydrolases is believed to protect them to some extent from degradation by luminal pancreatic proteases. However, between meals the hydrolases are degraded and must therefore be resynthesized by the enterocyte to participate in digesting the next carbohydrate meal. Sucrase/isomaltase and glucoamylase are synthesized in quantities that are in excess of requirements, and assimilation of their products into the body is limited by the availability of specific membrane transporters for these monosaccharides (see
Uptake of Carbohydrates). Lactase, in contrast, shows a developmental decline in expression after weaning. The relative paucity of lactase means that digestion of lactose, rather than uptake of the resulting products, is rate limiting for assimilation. If lactase levels fall below a certain threshold, the disease of lactose intolerance results.
•Fig. 30.8 Structureofamylopectinandtheactionofamylase.Thebluecirclesrepresentglucosemonomerslinkedbyα-1,4bonds.Theblackcirclesrepresentglucoseunitslinkedbyα-1,6bondsatthebranchpoints.
Lactose intolerance isrelativelycommoninadultsfromspecificethnicgroups,suchasAsians,AfricanAmericans,andHispanics.Thedisorderreflectsanormaldevelopmentaldeclineintheexpressionoflactasebyenterocytes,particularlywhenlactoseisnotaconsistentcomponentofthediet.Insuchindividuals,consumptionoffoodscontaininglargequantitiesoflactose(e.g.,milk,icecream)canresultinabdominalcramping,gas,anddiarrhea.Thesesymptomsreflectarelativeinabilitytodigestlactose;thusitremainsinthelumen,andwaterisretained.Somelactose-intolerantpatientsbenefitfromoraladministrationofabacteriallyderivedlactaseenzymebeforeingestingdairyproducts.
Digestion of starch occurs in two phases. The first takes place in the lumen and is actually initiated in the oral cavity via the activity of salivary amylase, as discussed in . Salivary amylase, however, is not essential for starch digestion, although it may assume greater importance in neonates or patients in whom the output of pancreatic enzymes is impaired by disease. Quantitatively the most significant contributor to the luminal digestion of starch is pancreatic amylase. Both enzymes hydrolyze internal α-1,4 bonds in both amylose and amylopectin, but not external bonds nor the α-1,6 bonds that form the branch points in the amylopectin molecule (see