Intestinal Absorption

Intestinal Absorption

Overview

Absorption is the net passage of nutrients from the intestinal lumen, across the epithelium, into the interstitial fluid (and then blood or lymph). Surface area is maximised by: small intestinal length (~6 m), circular folds (plicae circulares), villi (finger-like projections, ×10 area), and microvilli (brush border, ×20 area). Total ×600 over flat tube. Motility affects transit time: too slow → constipation; too fast → diarrhoea and malabsorption.

Carbohydrate Absorption

Salivary and pancreatic amylases hydrolyse starch to oligosaccharides and disaccharides. Brush border enzymes (maltase, sucrase, lactase) complete hydrolysis to monosaccharides (glucose, galactose, fructose). Na⁺-glucose co-transporter (SGLT1) transports glucose and galactose into enterocytes against their concentration gradient, driven by the Na⁺ gradient (maintained by basolateral Na⁺/K⁺-ATPase). Fructose enters by GLUT5 (facilitated diffusion). All monosaccharides exit basolaterally via GLUT2. Lactase deficiency → lactose malabsorption → osmotic diarrhoea.

Protein Absorption

Pepsin (stomach) → pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidases — activated by enterokinase on duodenal brush border) → brush border peptidases → free amino acids and di/tripeptides. Amino acids and di/tripeptides are absorbed by Na⁺-coupled transporters (PepT1 for peptides). Water follows osmotic gradient by paracellular route.

Fat Absorption

Pancreatic lipase (with colipase) hydrolyses triglycerides → monoglycerides + fatty acids. Bile salts emulsify fat → form mixed micelles (bile salts + fatty acids + monoglycerides + fat-soluble vitamins ADEK + cholesterol). Micelles diffuse to brush border → lipids diffuse passively into enterocytes. Inside enterocytes: triglycerides re-synthesised → packaged into chylomicrons (with apolipoproteins) → secreted into lacteals (lymph) → thoracic duct → circulation. Fat-soluble vitamins (ADEK) require micelles for absorption. Water-soluble vitamins (B, C) absorbed by specific transporters; B₁₂ requires intrinsic factor.

Iron and B₁₂ Absorption

Iron: absorbed in proximal duodenum; dietary Fe³⁺ reduced to Fe²⁺ by ferric reductase (DCytB); Fe²⁺ enters enterocytes via DMT1; exits basolaterally via ferroportin (regulated by hepcidin — high hepcidin ↓ ferroportin = less Fe export). B₁₂: requires intrinsic factor (from parietal cells); IF-B₁₂ complex absorbed by cubilin receptors in terminal ileum; IF deficiency → pernicious anaemia.

Colon

Colon absorbs Na⁺ (electrogenic Na⁺ channels, stimulated by aldosterone) and water (~1.5 L/day). It also secretes K⁺, H⁺, and HCO₃⁻. Important for electrolyte balance and drug absorption (rectal route). Short-chain fatty acids (from bacterial fermentation of fibre) are the primary energy source for colonocytes.