Figure 1.

Schematic representation of synthesis and circulation of bile acids. Primary bile acids (BAs), cholic acid (CA), and chenodeoxycholic acid (CDCA) are synthesized in the liver from cholesterol and stored in the gallbladder. Following food intake, bile acids are released into the small intestine. Secondary bile acids are produced by the gut microbiota from modifications of primary bile acids. Deoxycholic acid (DCA) is formed from CA. Lithocholic acid (LCA) and ursodeoxycholic acid (UDCA) are formed by CDCA. Taurine conjugation of UDCA forms tauroursodeoxycholic acid (TUDCA). About 95% of the bile acids are reabsorbed in the ileum, and 5% are lost in feces. The bile acids absorbed by the enterocytes are released into the portal vein and redirected to the liver for recycling (enterohepatic circulation). Only a small portion (10%) escapes the enterohepatic circulation and reaches the systemic circulation.