Figure 2.

The enterohepatic circulation of bile acids (BA). Primary BA are synthesised from cholesterol in the liver and subsequently conjugated to taurine or glycine, which increases their solubility. The conjugated primary BA are then stored in the gallbladder until they are released into the small intestine by the hormone cholecystokinin (CCK) after a meal. BAs will travel through the small intestine aiding in the digestion and absorption of lipids and fat-soluble vitamins. They will be actively reabsorbed in the ileum where they will return to the liver via the portal vein. A small proportion of BAs will escape intestinal reabsorption and enter the colon. In the colon, the resident gut microbiota will promote the deconjugation and biotransformation of the primary BAs into the secondary and more hydrophilic BAs, deoxycholic acid (DCA) and lithocholic acid (LCA). The majority of secondary BA will re-enter the systemic circulation via passive reabsorption, and a small amount (5%) will be excreted in the faeces.