Absorption of Monosaccharides

Overview

Step 1: Digestion to Monosaccharides (Prerequisites)

Step 2: Entry into the Enterocyte (Apical/Brush Border Membrane)

Glucose and Galactose - Secondary Active Transport via SGLT1

• Transported by SGLT1 (Sodium-Glucose Linked Transporter 1, also SLC5A1) on the apical/brush-border membrane (BBM).
• SGLT1 is a Na+-dependent cotransporter - each transport cycle moves 1 monosaccharide + 2 Na+ ions into the cell simultaneously.
• This is secondary active transport: the energy does not come directly from ATP but from the inwardly directed Na+ electrochemical gradient across the BBM. That gradient is generated and maintained by the Na+/K+-ATPase on the basolateral membrane (BLM).
• Because 2 positive charges enter the cell per cycle (both Na+ ions), the process is electrogenic - it depolarizes the membrane. Both the Na+ concentration gradient and the inside-negative membrane potential drive this transport.
• SGLT1 accepts either glucose or galactose as substrate but not both simultaneously.
• This mechanism allows glucose and galactose to be absorbed against their own concentration gradients (true uphill transport).

Fructose - Facilitated Diffusion via GLUT5

• Transported by GLUT5 (SLC2A5) on the apical membrane.
• This is passive, energy-independent facilitated diffusion with no involvement of Na+.
• Fructose cannot be absorbed against its concentration gradient - it moves only down its concentration gradient.
• SGLT1 plays no role in fructose transport.

Step 3: Exit from Enterocyte to Portal Blood (Basolateral Membrane)

• GLUT2 is a low-affinity facilitated sugar transporter.
• It accepts glucose, galactose, and fructose as substrates.
• Net release of sugars from the cell occurs only when intracellular sugar concentrations exceed those in portal blood (down-gradient release).
• The low affinity is physiologically appropriate - it ensures sugars accumulate inside the cell first before being released into the circulation.

GLUT2 Recruitment to the Apical Membrane Under High Sugar Load

• Under normal/low luminal sugar loads, GLUT2 is expressed only at the BLM.
• When the intestinal lumen faces a high sugar load, SGLT1-mediated glucose entry acts as a signal for GLUT2 to traffic to the BBM as well.
• Since SGLT1 is high-affinity but low-capacity, it becomes inefficient at high concentrations. GLUT2 at the BBM (low-affinity, high-capacity) supplements absorption.
• This also enhances fructose absorption, since GLUT5 has limited capacity but GLUT2 at the BBM can absorb fructose when luminal concentrations are high.

Summary Diagram

Summary Table

Clinical Relevance

• Glucose-Galactose Malabsorption: A rare autosomal recessive disorder caused by mutations in SGLT1. Results in severe osmotic diarrhea in neonates. The unabsorbed sugars remain in the lumen, causing osmotic water secretion.
• Lactase Deficiency (Lactose Intolerance): Failure to hydrolyze lactose to glucose + galactose means the disaccharide cannot be absorbed (SGLT1/GLUT5 only accept monosaccharides). Undigested lactose is fermented by colonic bacteria causing gas, bloating, and diarrhea.
• Alpha-glucosidase inhibitors (acarbose, miglitol): Delay intestinal monosaccharide absorption by preventing complex carbohydrate breakdown. Used in Type 2 diabetes management.
• Fructose malabsorption: Since fructose relies solely on facilitated diffusion (GLUT5), high fructose intake (e.g., HFCS in foods) can overwhelm the capacity of GLUT5, leading to osmotic diarrhea and bloating.

Why Salt is Added to ORS - and the Role of Water in Monosaccharide Absorption

The Core Principle: Na+-Glucose Cotransport (SGLT1)

SGLT1 on the enterocyte brush-border membrane cotransports 1 glucose (or galactose) molecule together with 2 Na+ ions simultaneously into the cell.

Why Salt (NaCl) is Added to ORS

The Driving Force Relationship

Na+/K+-ATPase (basolateral)
        ↓ pumps Na+ OUT of cell
        ↓ creates low intracellular Na+
        ↓ creates Na+ electrochemical gradient across apical membrane
        ↓
SGLT1 uses this gradient to pull Na+ INTO the cell
        ↓ glucose is dragged along with it (cotransport)

• The gut is secreting massive amounts of Na+, Cl-, and water into the lumen.
• The normal Na+-dependent absorption mechanisms are disrupted.
• However - SGLT1 remains functional even during secretory diarrhea. This is the crucial discovery that made ORS possible.

1. Supply Na+ to drive SGLT1.
2. SGLT1 cotransports both Na+ and glucose into the enterocyte.
3. Na+ is then pumped out basolaterally by Na+/K+-ATPase into the bloodstream.
4. Chloride follows Na+ passively (electrochemical attraction) - so chloride is also absorbed.
5. Water follows osmotically - this is the rescue mechanism.

The Role of Water in Monosaccharide Absorption

1. Water as a "Passenger" - Cotransported with SGLT1

Each monosaccharide molecule transported by SGLT1 brings along approximately 264 molecules of water.

• Yamada's Textbook of Gastroenterology states: "SGLT1 also has a major role in the absorption of luminal water. Each monosaccharide molecule is transported with ~264 molecules of water, resulting in the net absorption of 5-6 L of water per day, and thus the rationale for adding glucose to oral rehydration solutions."

2. Water as a "Follower" - Osmotic/Solvent Drag

• The blood becomes slightly hypertonic relative to the lumen.
• Water moves osmotically from lumen → enterocyte → blood to re-establish equilibrium.
• This is called osmotic coupling or solvent drag.

Why the Glucose:Na+ Ratio Matters (ORS Composition)

• Glucose:Na+ ratio is approximately 1:1 (both at 75 mmol/L) - this optimally loads SGLT1.
• Reduced osmolarity (245 vs old 311 mOsm/L) - a hypotonic solution is better absorbed; hyperosmolar fluids draw water INTO the lumen, worsening diarrhea.
• Na+ concentration at least 75-90 mmol/L is required to meaningfully drive cotransport.
• Sports drinks like Gatorade (~20 mmol/L Na+) are NOT adequate replacements - they have far too little sodium.

Summary: The "Glucose-Na+ Pump" Concept

Lumen                  Enterocyte              Blood
  |                        |                     |
  | Glucose + 2Na+  →  SGLT1  → glucose+Na+ in   |
  |  + ~264 H2O  ─────────────────────────────►  |
  |                        |    Na+/K+-ATPase     |
  |                        |    pumps Na+ out  ──►|
  |                        |    GLUT2: glucose ──►|
  |                        |    H2O follows  ────►|