The Lack of Some Digestive Enzymes Impairs Carbohydrate Absorption

Impaired carbohydrate absorption caused by the absence of salivary or pancreatic amylase almost never occurs because these enzymes are usually present in great excess. However, impaired absorption due to a deficiency in membrane disaccharidases is rather common. Such deficiencies can be either genetic or acquired. Among congenital deficiencies, lactase deficiency is, by far, the most common. Affected individuals suffer from lactose intolerance, a condition in which the ingestion of milk products results in severe osmotic (watery) diarrhea. The mechanism responsible is depicted in Figure 27.22. Undigested lactose in the intestinal lumen increases the osmolality of the luminal contents. Osmolality is further increased by lactic acid produced from the action of intestinal bacteria on the lactose. Increased luminal osmolality results in net water secretion into the lumen. The accumulation of fluid distends the small intestine and accelerates peristalsis, eventually resulting in watery diarrhea.

Congenital sucrase deficiency results in symptoms similar to those of lactase deficiency. Sucrase deficiency can be inherited or acquired through disorders of the small intestine, such as tropical sprue or Crohn's disease.

Lactic acid production by bacteria

Lactase deficiency

Accumulation of lactose in intestinal lumen

Increased luminal osmolality

Fluid accumulation in lumen

Luminal distension

Enhanced peristalsis

Watery diarrhea

The mechanism for osmotic diarrhea resulting from lactase deficiency.

Dietary Fiber Plays an Important Role in GI Motility

Dietary fiber includes indigestible carbohydrates and carbohydrate-like components mainly found in fruits and vegetables. The most common are cellulose, hemicellulose, pectins, and gums. Cellulose and hemicellulose are insoluble in water and are poorly digested by humans, thus, providing the bulkiness of stool.

Dietary fiber imparts bulk to the bolus and, therefore, greatly shortens transit time. It has been proposed that dietary fiber reduces the incidence of colon cancer by shortening GI transit time, which, in turn, reduces the formation of carcinogenic bile acids (e.g., lithocholic acid). Because dietary fiber also binds bile acids, which are formed from cholesterol, fiber consumption can result in a lowering of blood cholesterol by promoting excretion.

The Luminal Lipid Consists of Both Exogenous and Endogenous Lipids

Lipids are comprised of several classes of compounds that are insoluble in water but soluble in organic solvents. By far the most abundant dietary lipids are triacylglycerols, or triglycerides. They consist of a glycerol backbone esteri-fied in the three positions with fatty acids (Fig. 27.23A). More than 90% of the daily dietary lipid intake is in the form of triglycerides.

The other lipids in the human diet are cholesterol and phospholipids. Cholesterol is a sterol derived exclusively from animal fat. Humans also ingest a small amount of plant sterols, notably ^-sitosterol and campesterol. The phospholipid molecule is similar to a triglyceride with fatty acids occupying the first and second positions of the glycerol backbone (Fig. 27.23B). However, the third position of the glycerol backbone is occupied by a phosphate group coupled to a nitrogenous base (e.g., choline or ethanolamine), for which each type of phospholipid molecule is named.

Bile serves as an endogenous source of cholesterol and phospholipids. Bile contributes about 12 g/day of phos-pholipid to the intestinal lumen, most in the form of phos-phatidylcholine, whereas dietary sources contribute 2 to 3 g/day. Another important endogenous source of lipid is desquamated intestinal villus epithelial cells.

Different Lipases Carry Out Lipid Hydrolysis

Lipid digestion mainly occurs in the lumen of the small intestine. Humans secrete an overabundance of pancreatic lipase. Depending on the substrate being digested, pancreatic lipase has an optimal pH of 7 to 8.0, allowing it to work well in the intestinal lumen after the acidic contents from the stomach have been neutralized by pancreatic HCO3" secretion. Pancreatic lipase hydrolyzes the triglyceride molecule to a 2-monoglyceride and two fatty acids (Fig. 27.24). It works on the triglyceride molecule at the oil-water interface; thus, the rate of lipolysis depends on the surface area of the interface. The products from the partial di-

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