Gallensalzen

Gallensalzen, or bile salts, are amphipathic derivatives of cholesterol produced by the liver and secreted into bile. They act as detergents that emulsify fats in the small intestine, enabling the digestion and absorption of lipids and fat-soluble vitamins. In bile, they are predominantly present in conjugated form, linked to the amino acids glycine or taurine to increase their water solubility and efficiency.

Chemically, the two major primary bile acids in humans are cholic acid and chenodeoxycholic acid. When conjugated, they form salts such as glycocholate and taurocholate. The intestinal microbiota can modify bile acids, producing secondary bile acids like deoxycholic and lithocholic acid; ursodeoxycholic acid is also present in small amounts and is used therapeutically in some liver diseases.

Biosynthesis and circulation: Bile acids are synthesized in the liver from cholesterol via the enzyme cholesterol 7α-hydroxylase (CYP7A1), a rate-limiting step. Conjugation with glycine or taurine yields bile salts, which are secreted into bile via transporters such as BSEP, stored in the gallbladder, and released into the duodenum after meals. Most bile salts (about 95%) are reabsorbed in the terminal ileum and returned to the liver through the enterohepatic circulation, where they are reused.

Physiology and regulation: In the intestine, bile salts form micelles with phospholipids and cholesterol, aiding lipid digestion and absorption. They also participate in regulatory pathways, notably via the FXR receptor and FGF19, which provide feedback to suppress bile acid synthesis. Cholecystokinin triggers gallbladder contraction to release bile salts into the intestine.

Clinical relevance: Alterations in bile salt pools can contribute to cholestasis, steatorrhea, and gallstone disease. Ursodeoxycholic acid is used therapeutically to treat certain cholestatic disorders and to dissolve cholesterol stones. Diseases or surgeries that reduce enterohepatic circulation, such as ileal resection, can diminish the bile salt pool and fat digestion.