Carbohydrate Absorption

Carbohydrate digestion begins in the mouth with chewing and the action of salivary amylase, which partially breaks down dietary starch into oligosaccharides. This mechanical and enzymatic activity prepares carbohydrates for further processing. The partially digested carbohydrates are then incorporated into chyme and passed into the duodenum for continued breakdown.

When chyme enters the stomach, the acidic environment inactivates salivary amylase, halting carbohydrate digestion temporarily. This pause in enzymatic activity occurs until the partially digested carbohydrates move into the duodenum, where pancreatic amylase resumes the breakdown process in the alkaline environment.

Brush border enzymes—maltase, sucrase, and lactase—are located on the microvilli of enterocytes and catalyze the final breakdown of disaccharides into monosaccharides at the site of absorption. Maltase splits maltose into glucose, sucrase hydrolyzes sucrose into glucose and fructose, and lactase breaks down lactose into glucose and galactose. By the time carbohydrates reach the jejunum, they are almost entirely reduced to monosaccharides.

Glucose and galactose are absorbed via secondary active transport using sodium-glucose symporters at the apical surface of enterocytes, requiring two sodium ions and one glucose or galactose molecule to bind for cellular entry. Fructose, however, enters through facilitated diffusion mediated by GLUT5 transporters, which does not require energy and relies on a concentration gradient between the intestinal lumen and enterocyte cytoplasm.

The sodium-potassium pump at the basolateral membrane maintains the sodium gradient essential for secondary active transport of glucose and galactose. It expels sodium ions from the cell in exchange for potassium ions, creating the concentration difference that drives sodium-glucose symporters to function. This indirect support is critical for efficient glucose and galactose uptake at the apical membrane.

Once inside enterocytes, all monosaccharides are transported across the basolateral membrane into capillaries of the villi via GLUT2 transporters using facilitated diffusion. This process does not require additional energy and ensures rapid transfer into the bloodstream. The capillaries then carry monosaccharides via the hepatic portal vein to the liver for metabolism, storage, or distribution to peripheral tissues.

The small intestine maximizes carbohydrate absorption through villi and microvilli that significantly increase surface area for nutrient uptake. A dense capillary network within villi enables swift transport of absorbed nutrients into the bloodstream. Brush border enzymes are strategically positioned on microvilli to immediately hydrolyze disaccharides into monosaccharides at the site of absorption, minimizing nutrient loss and supporting metabolic states of the body the absorptive state.