Digestion and absorption (Dr JM Delalande) 2025

Digestion and Absorption

Overview

Title: Digestion and AbsorptionAuthor: Jean Marie Delalande PhDInstitution: Queen Mary University of London

Lecture Overview and Learning Objectives

• Anatomical features of the midgut

• Three phases of digestion and absorption

• Absorption of different nutrients

• Causes of general malabsorption

• Causes of specific malabsorption

Gastrointestinal Anatomy

Surface Area Comparison

• Skin: 25 m²

• Mucosae: 32 m² (approximately half the size of a badminton court)

Normal Absorptive Processes

Intestinal Epithelial Barrier (IEB)

Functions:

• Absorb nutrients (permeable)

• Control passage of pathogens/toxins (impermeable)

Regulated by:

• Outer microenvironment (microflora, chyme)

• Inner microenvironment (immune cells, fibroblasts, enteric nervous system, extrinsic nerve fibers)

Absorption Efficiency

• 95% of nutrient absorption occurs in the small intestine

Rates of Fluid Movement

• Ingested water: 2 liters

• Liver and pancreatic secretions: 2 liters

• Salivary gland secretions: 1.5 liters

• Secretions by stomach and small intestines: 3.5 liters

• Absorption in small intestine: ~8.5 liters

• Colon: 400ml absorbed; feces: 100ml

Key Structures in Absorption

• Microscopic structures of the intestinal wall:

  • Villi: Enhance surface area for absorption

  • Microvilli (brush border) on epithelial cells

  • Lacteals: Lymph vessels in villi

Nutrient Absorption Processes

Bile Salts and Digestive Enzymes

• Mouth: Salivary amylases begin digestion

• Stomach: Pepsin initiates protein digestion

• Duodenum: Bile salts emulsify triglycerides; pancreatic enzymes digest fats

Nutrient absorption types:

• Amino acids, monosaccharides, free fatty acids absorbed in the jejunum

• Limited absorption of B12, water, and sodium

Water and B12 Absorption

• Ileum:

  • Absorbs water, sodium, and B12 with the help of intrinsic factor

  • Bile acids are reabsorbed for recycling to the liver

Phases of Digestion

1. Luminal Phase

   • Food is metabolized by enzymes and gastric acids;

   • Enzymes split carbohydrates and proteins into smaller units (e.g., disaccharides, peptides)

2. Mucosal Phase

   • Pre-digested nutrients absorbed through brush border membrane to enterocytes

   • Brush border enzymes complete digestion:

     • Sucrase, lactase, maltase, limit dextrinase, glucoamylase

   • Carbohydrates enter epithelial cells via sodium-linked secondary transport

   • Fructose absorption via facilitated diffusion

3. Post-absorptive Phase

   • Sugars and nutrients transported via lymphatics and portal circulation to the bloodstream

Lipid Digestion and Absorption

Lipid Phase Processes

• Luminal Phase:

  • Lipid digestion initiated in the mouth and continued in the stomach and small intestine

  • Bile salts emulsify fats, enabling pancreatic lipase to digest them into fatty acids and monoglycerides.

• Mucosal Phase:

  • Fatty acids and monoglycerides absorbed via simple diffusion into enterocytes

  • Triglycerides reassembled and packaged into chylomicrons

• Post-absorptive Phase:

  • Chylomicrons secreted from enterocytes into lymphatics, eventually entering the bloodstream

Protein Digestion

Phases of Protein Absorption

• Luminal Phase:

  • Begins with pepsin activation in the stomach; pancreatic enzymes finalize digestion in the small intestine.

• Mucosal Phase:

  • Trypsin activates further enzymes; amino acids absorbed via sodium-linked transport across the enterocyte membrane.

• Post-absorptive Phase:

  • Amino acids transported across the basolateral membrane into circulation via facilitated diffusion

Vitally Relevant Micronutrients

Vitamins

• Fat-soluble Vitamins: (A, D, E, K) absorbed with lipids

• Water-soluble Vitamins: (B, C) often follow water absorption pathways; B12 requires intrinsic factor for absorption.

  • B12: Absorbed in the terminal ileum, binds to haptocorrin and intrinsic factor before transportation to the portal circulation.

Iron Absorption Mechanisms

• Absorption primarily in the duodenum; affected by the pH level of intestinal content and the presence of heme non-heme iron pools.

Causes of Malabsorption

Generalized Malabsorption

• Often results from small intestine diseases (e.g. Coeliac disease, Crohn's disease)

Specific Malabsorptive States

• Failure to absorb specific nutrients (e.g. lactase deficiency)

• Genetic mutations affecting absorption pathways, leading to symptoms such as diarrhea and discomfort due to undigested sugars reaching the colon.