What is the mechanism by which bile acids are reabsorbed in the ileum?

• A. Passive diffusion
• B. Endocytosis
• C. Apical sodium-dependent bile acid transporter (ASBT)
• D. Portal circulation

Answer: (C)

The uptake of bile acids in the ileum is driven by an active, sodium-dependent transporter on the luminal (apical) surface of enterocytes, known as ASBT. This transporter uses the sodium gradient established by Na+/K+-ATPase to bring bile acids from the intestinal lumen into the cell. Once inside, they cross the basolateral membrane and enter the portal circulation via OSTα/OSTβ, delivering them back to the liver as part of enterohepatic circulation. Passive diffusion is unlikely for the ileal bile acids because they are largely conjugated and ionized at intestinal pH, and endocytosis isn’t the mechanism here. After absorption, bile acids travel in the portal blood to the liver for reuse.