Chapter 21: Digestion, Absorption, and Transport of Carbohydrates

The digestive process begins in the mouth where salivary amylase initiates the breakdown of starch into smaller fragments called alpha-dextrins, though gastric acid subsequently inactivates this enzyme. In the small intestine, pancreatic amylase continues fragmenting starch into maltose, maltotriose, and limit dextrins. The final stage of carbohydrate digestion occurs at the brush border of the intestinal epithelium, where specialized disaccharidases complete the hydrolysis: glucoamylase cleaves alpha-1,4 glycosidic bonds, the sucrase-isomaltase complex breaks down multiple disaccharides, lactase hydrolyzes lactose through beta-1,4 bonds, and trehalase processes trehalose. Monosaccharides produced from digestion are absorbed through specific transport mechanisms: glucose and galactose utilize sodium-dependent cotransporters, while fructose relies on GLUT5 facilitated diffusion. Once absorbed, these sugars circulate via distinct glucose transporter proteins in different tissues—GLUT1 in red blood cells and the blood-brain barrier, GLUT2 in hepatic and pancreatic tissues, GLUT3 in neurons, and insulin-responsive GLUT4 in muscle and adipose cells. Clinical abnormalities reveal the importance of these systems: lactose intolerance results from lactase deficiency either congenitally, in adulthood, or secondarily from intestinal injury; fructose malabsorption stems from GLUT5 defects; and sucrose intolerance involves disaccharidase insufficiency. The chapter also addresses dietary carbohydrate management, including the distinction between soluble and insoluble fiber, the cholesterol-lowering properties of beta-glucans and pectins, and the glycemic index concept for diabetes control. Undigested carbohydrates undergo fermentation by colonic bacteria, producing short-chain fatty acids and gases that provide colonocyte fuel. Additional clinical relevance includes cholera toxin effects on intestinal ion transport and the life-saving application of oral rehydration therapy combining glucose and sodium.