bile acid
Summary
Bile acids are amphipathic steroid molecules synthesized from cholesterol in the liver, essential for emulsification and absorption of dietary fats and fat-soluble vitamins (A, D, E, K). Primary bile acids (cholic acid, chenodeoxycholic acid) are conjugated with glycine or taurine, secreted into bile, and undergo enterohepatic circulation with ~95% reabsorption in the terminal ileum.
Detail
Bile acid synthesis begins with cholesterol 7α-hydroxylase (CYP7A1), the rate-limiting enzyme, converting cholesterol into primary bile acids (cholic acid and chenodeoxycholic acid) in the liver. These are conjugated with glycine or taurine to form bile salts, which are more water-soluble and effective detergents. Bile salts are secreted into bile, stored in the gallbladder, and released into the duodenum in response to CCK following a fatty meal. In the intestine, gut bacteria deconjugate and dehydroxylate primary bile acids to form secondary bile acids (deoxycholic acid, lithocholic acid). Bile acids form micelles with phospholipids and cholesterol, solubilizing dietary lipids and fat-soluble vitamins for absorption by enterocytes. About 95% of bile acids are reabsorbed via active transport in the terminal ileum (enterohepatic circulation), recycled back to the liver via the portal vein, minimizing the need for new synthesis. The remaining 5% is excreted in feces, representing the primary route of cholesterol elimination from the body.
Clinical significance: Disruption of enterohepatic circulation (e.g., ileal resection, Crohn's disease affecting terminal ileum) causes bile acid malabsorption, leading to diarrhea (bile acid-induced colonic secretion) and steatorrhea with fat-soluble vitamin deficiencies. Cholestyramine and other bile acid-binding resins interrupt enterohepatic circulation, lowering LDL cholesterol by upregulating hepatic LDL receptors as the liver increases bile acid synthesis from cholesterol. Gallstones (cholesterol stones) can form when bile becomes supersaturated with cholesterol relative to bile acids and phospholipids. Primary biliary cholangitis and primary sclerosing cholangitis involve bile acid accumulation causing pruritus and hepatocyte injury. Bile acid diarrhea is also seen post-cholecystectomy. Ursodeoxycholic acid is used therapeutically to dissolve small cholesterol gallstones and treat primary biliary cholangitis by altering bile composition and reducing cholestatic injury.
Sources
- First Aid for the USMLE Step 1
- Guyton and Hall Textbook of Medical Physiology
- Robbins and Cotran Pathologic Basis of Disease
- Harrison's Principles of Internal Medicine
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.