carbohydrate absorption

Last reviewed 01/2018

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.