Carbohydrate digestion breaks down dietary polysaccharides and disaccharides into monosaccharides that can be absorbed across the intestinal epithelium. The process begins in the mouth and is completed in the small intestine.
Digestion in the Mouth
Salivary alpha-amylase initiates starch digestion in the mouth. This endoglycosidase hydrolyzes internal alpha-1,4-glycosidic bonds in amylose and amylopectin, producing maltose, maltotriose, and limit dextrins. Salivary amylase is inactivated by stomach acid, so its action is limited to the brief period food is in the mouth and upper stomach.
Digestion in the Small Intestine
Pancreatic alpha-amylase is secreted into the duodenum and continues starch breakdown, producing the same mixture of maltose, maltotriose, and limit dextrins. The final stage of carbohydrate digestion occurs at the brush border membrane of enterocytes, where specific disaccharidases hydrolyze disaccharides into monosaccharides.
Maltase-glucoamylase hydrolyzes maltose and maltotriose to glucose. Sucrase-isomaltase is a bifunctional enzyme that hydrolyzes sucrose to glucose and fructose, and also cleaves alpha-1,4 and alpha-1,6 bonds in limit dextrins. Lactase hydrolyzes lactose to glucose and galactose. Trehalase hydrolyzes trehalose, a disaccharide found in mushrooms and insects.
Absorption of Monosaccharides
Glucose and galactose are transported across the apical membrane of enterocytes by SGLT1, a sodium-dependent glucose transporter that couples sugar import to the sodium gradient maintained by the Na+/K+-ATPase. This secondary active transport allows glucose absorption even when luminal concentrations are low.
Fructose is absorbed by GLUT5, a facilitative transporter that does not require sodium or energy. Once inside the enterocyte, all monosaccharides exit across the basolateral membrane into the portal circulation primarily via GLUT2.
Regulation of Carbohydrate Digestion
The rate of carbohydrate digestion and absorption is influenced by the glycemic index of foods, which reflects how quickly carbohydrates raise blood glucose levels. Simple sugars and refined starches have a high glycemic index, while fiber-rich foods slow digestion and produce a lower glycemic response. Soluble fiber forms viscous gels that delay gastric emptying and reduce glucose absorption. Once absorbed, glucose enters metabolic pathways for energy production or storage.
Lactose Intolerance
Lactose intolerance results from lactase deficiency, leading to malabsorption of lactose. The unabsorbed lactose reaches the colon, where it is fermented by bacteria, producing gas, bloating, and diarrhea. Primary lactase deficiency is the most common type and develops after weaning in most of the world’s population, with prevalence varying by ethnicity. Congenital lactase deficiency is rare and presents in newborns. Secondary lactase deficiency can follow intestinal infections or inflammatory conditions.
