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.