Bile acid metabolism Bile acids		Bile acids, similar in structure to steroids, are classified as primary or secondary. Four different bile acids make up >95% of the total bile acid pool. Cholic acid and chenodeoxycholic acid are the 2 primary bile acids. They are synthesized in the liver from cholesterol, then conjugated with glycine or taurine (75% with glycine, 25% taurine) and excreted into the bile. There they combine with cholesterol and phospholipids to form mixed micelles. In the gut, the micelles exchange phospholipids for dietary lipids, allowing for digestion and absorption of the latter. Also in the gut, bacteria convert them to the secondary bile acids, deoxycholic acid and lithocholic acid. Bile acids are reabsorbed in the terminal ileum, returning to the liver by the portal vein. They are taken up by the hepatocyte and re-excreted into the bile canaliculi, a process called the enterohepatic circulation of bile acids, similar to that of bilirubin. In the adult, this recirculation occurs 8-10 times per day and is normally quite efficient, with a loss of only 5% of the circulating bile acids per day. Not only does the total amount of bile acids affect bile flow, but particular bile acids can significantly influence bile flow. This is termed their choleretic potential. Certain bile acids, like ursodiol, have especially high choleretic potential. The secondary bile acids formed in the gut are more hydrophobic. This increases their ability to solubilize dietary lipids and aids in their absorption. However it also leads to hepatotoxic effects when they come into contact with hepatocyte membranes. As cholestatic liver disease progresses, the proportion of hydrophobic bile acids increases, leading to even more hepatotoxicity. Return to top of page