Carbohydrate Digestion: Enzymes and Anatomical Sites from Mouth to Small Intestine

Stepwise Digestion

1. Mouth: Salivary amylase (ptyalin) initiates starch hydrolysis → maltose, maltotriose, α-limit dextrins. Inactivated by gastric acid.
2. Small Intestine – Pancreatic Enzymes: Pancreatic amylase continues starch breakdown. Maltase, sucrase, isomaltase, lactase (brush border) hydrolyze disaccharides/oligosaccharides to monosaccharides.
3. Brush Border Enzymes (Microvilli):
   • Maltase → glucose + glucose
   • Sucrase → glucose + fructose
   • Lactase → glucose + galactose
   • Isomaltase → α-1,6 bonds in limit dextrins
4. Absorption: SGLT1 (glucose/galactose + Na⁺), GLUT5 (fructose), GLUT2 (basolateral exit). Villi and microvilli amplify surface area ~600-fold.

Clinical Correlates

• Lactase Non-persistence: Primary (genetic) or secondary (villous atrophy) → lactose intolerance.
• Congenital Sucrase-Isomaltase Deficiency (CSID): Rare autosomal recessive.
• Celiac Disease: Villous atrophy → generalized malabsorption including carbohydrates.