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Carbohydrate absorption

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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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.

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