Carbohydrate digestion and absorption

Dietary Carbohydrates - The Starting Lineup

• Polysaccharides: Starch (plants), glycogen (meats), and fiber (indigestible).
  • Starch is composed of amylose (α-1,4 glycosidic bonds) and amylopectin (α-1,4 and α-1,6 bonds).
• Disaccharides: Sucrose (glucose-fructose), lactose (glucose-galactose), and maltose (glucose-glucose).
• Monosaccharides: Glucose, fructose, and galactose are the final products of digestion and the only forms absorbed by enterocytes.

⭐ Lactase non-persistence (adult-type hypolactasia) is common in many populations, leading to lactose intolerance. It is NOT an allergy.

Luminal Digestion - The Starch Smashers

• Initiators: Digestion starts with salivary α-amylase (mouth) and is completed by pancreatic α-amylase (small intestine).
• Mechanism: These enzymes hydrolyze internal α-1,4 glycosidic bonds in starch.
• Products: Yields maltose, maltotriose, and α-limit dextrins.
• Limitations: Amylase cannot cleave terminal α-1,4 bonds, α-1,6 branch points, or disaccharide bonds like sucrose and lactose.

⭐ Salivary amylase is inactivated by low gastric pH ( < 4.0). Therefore, the bulk of starch digestion relies on pancreatic amylase in the duodenum.

Brush Border Hydrolysis - The Final Snip

Final digestion occurs at the enterocyte surface via brush border enzymes (disaccharidases). Oligosaccharides from luminal digestion are broken into absorbable monosaccharides before transport.

• Lactase: Breaks down lactose → glucose + galactose.
• Sucrase-Isomaltase Complex:
  • Sucrase: Cleaves sucrose → glucose + fructose.
  • Isomaltase: Cleaves isomaltose (α-1,6 bonds) → 2 glucose.
• Maltase-Glucoamylase: Cleaves maltose & maltotriose → glucose.

Monosaccharide Absorption:

• Glucose & Galactose: Apical uptake via SGLT1 (Na⁺-dependent).
• Fructose: Apical uptake via GLUT5.

⭐ Lactose intolerance, due to lactase deficiency, causes osmotic diarrhea as undigested lactose pulls water into the lumen.

Monosaccharide Absorption - The Glucose Gateway

• Apical Membrane (Lumen → Enterocyte):
  • Glucose & Galactose: Enter via SGLT1 (secondary active transport).
    • Co-transported with $Na^+$; gradient is maintained by the basolateral $Na^+/K^+$ pump.
  • Fructose: Enters via GLUT5 (facilitated diffusion).
• Basolateral Membrane (Enterocyte → Portal Blood):
  • All three monosaccharides exit via GLUT2 (facilitated diffusion).

⭐ High-Yield Fact: Oral rehydration solutions contain glucose and sodium. The SGLT1 co-transporter enhances both solute and water absorption, effectively treating dehydration from diarrhea.

Clinical Correlation - The Lactose Blues

• Lactase Deficiency: Insufficient lactase enzyme at the intestinal brush border, preventing the breakdown of lactose into glucose and galactose.
• Pathophysiology: Unabsorbed lactose is osmotically active, pulling water into the intestinal lumen → osmotic diarrhea.
• Bacterial Fermentation: Colonic bacteria ferment lactose, producing gas (H₂, CO₂, CH₄) → bloating, flatulence, and cramps.
• Diagnosis: Positive hydrogen breath test after a lactose challenge.

⭐ Most cases are due to primary lactase non-persistence, an age-dependent decline in enzyme levels, not a congenital defect.

High‑Yield Points - ⚡ Biggest Takeaways

• Salivary & pancreatic α-amylase break down starch into smaller oligosaccharides.
• Brush border enzymes, like lactase and sucrase-isomaltase, complete digestion to monosaccharides.
• Glucose and galactose are absorbed via the Na+-dependent SGLT1 transporter.
• Fructose is absorbed by GLUT5 via facilitated diffusion.
• All monosaccharides exit the enterocyte into portal circulation via GLUT2.
• Lactase deficiency causes osmotic diarrhea and bloating upon milk ingestion.