Carbohydrates are essential macronutrients that serve as the body's primary energy source. Their digestion begins in the mouth, where salivary amylase partially breaks down complex carbohydrates such as starch into smaller oligosaccharides. This mechanical and enzymatic activity prepares carbohydrates for further processing in the gastrointestinal tract.

After being swallowed, the partially digested carbohydrates mix with gastric secretions in the stomach. However, the acidic environment inactivates salivary amylase, halting carbohydrate digestion temporarily. Once the chyme enters the duodenum, the pancreas secretes pancreatic amylase, which resumes the enzymatic breakdown of starch into smaller oligosaccharides and disaccharides. Brush border enzymes—maltase, sucrase, and lactase—located on the microvilli of enterocytes catalyze the final breakdown of disaccharides into monosaccharides (glucose, fructose, and galactose) at the site of absorption.

Digestion of Carbohydrates: From Polysaccharides to Monosaccharides

Pancreatic amylase acts rapidly in the alkaline environment of the duodenum, hydrolyzing starch into maltose, maltotriose, and dextrins. These intermediate products are subsequently acted upon by specific brush border enzymes. For example, maltase splits maltose into glucose molecules, 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, fructose, and galactose.

Efficient enzymatic activity ensures that virtually all dietary carbohydrates are converted into monosaccharides, making them ready for absorption by the time they enter the jejunum.

Absorption of Monosaccharides in the Small Intestine

Monosaccharides are absorbed primarily in the jejunum and ileum by specialized mechanisms involving membrane transport proteins. At the apical surface of enterocytes, glucose and galactose are absorbed via secondary active transport using sodium-glucose symporters (SGLT1). This process depends on a sodium gradient maintained by the sodium-potassium pump at the basolateral membrane, which expels sodium ions from the cell in exchange for potassium ions. The binding of two sodium ions and one glucose (or galactose) molecule to the symporter triggers their entry into the cell.

Fructose, however, is absorbed differently. It enters enterocytes through facilitated diffusion mediated by GLUT5 transporters. This process does not require energy, as it relies on a concentration gradient between the intestinal lumen and the cytoplasm of enterocytes.

Once inside the enterocytes, all monosaccharides are transported across the basolateral membrane into the capillaries of the villi via GLUT2 transporters. Facilitated diffusion ensures the rapid transfer of these monosaccharides into the bloodstream without additional energy expenditure. From there, capillaries in the villi transport the absorbed monosaccharides via the hepatic portal vein to the liver, where they are metabolized, stored, or distributed to peripheral tissues based on metabolic demands.

Adaptations of the Small Intestine for Efficient Absorption

The small intestine is structurally adapted to maximize carbohydrate digestion and absorption. The presence of villi and microvilli significantly increases the surface area, enhancing nutrient uptake. Additionally, the dense capillary network within the villi allows for the swift transport of absorbed nutrients into the bloodstream.

The brush border enzymes on the microvilli are strategically positioned to immediately hydrolyze disaccharides into monosaccharides at the site of absorption. This close proximity to the absorptive transporters minimizes the loss of monosaccharides into the intestinal lumen. Furthermore, the sodium-potassium pump indirectly supports glucose and galactose absorption by maintaining the sodium gradient essential for secondary active transport.