Glucose absorption

Carbohydrate absorption tends to occur at the small intestine brush border:

• fructose:
  • absorbed passively down a concentration gradient
  • binds to a specific carrier protein in the apical cell membrane
  • either:
    • diffuses passively out of cells and into capillaries
    • forms lactate which then diffuses into portal blood
      
      
• glucose:
  • absorbed mainly in jejunum by active process
  • enters via a co-transporter protein on the apical side of the enterocyte
  • co-transporter requires presence of sodium ions
  • sodium ions pass down electrochemical gradient into cell to replace sodium ions which are actively being transported out of cell on basolateral membrane by Na+/K+ ATPase pump
  • glucose diffuses out of cell into intercellular space and from there to local capillaries
  • chloride ions and water accompany the movement of sodium and glucose; they may travel through the cell or through the intercellular space
    
    
• galactose: absorbed by a similar sodium-dependent co-transporter as glucose

The dependence of water and salt absorption on the absorption of glucose is the reason why oral rehydration solutions contain all three components.