Bile Acid Synthesis and Enterohepatic Circulation

Bile acids are amphipathic molecules synthesised from cholesterol in the liver that emulsify dietary fat and act as signalling molecules. After their secretion into bile and release into the intestine, most are reabsorbed and returned to the liver — the enterohepatic circulation — so that a relatively small pool cycles many times each day with little net loss.

Definition

Bile acid synthesis is the hepatic conversion of cholesterol into bile acids through enzymatic pathways, and the enterohepatic circulation is the recycling loop by which secreted bile acids are reabsorbed from the distal small intestine and transported back to the liver for reuse.

Scope

The topic covers the conversion of cholesterol to primary bile acids, conjugation and secretion, intestinal modification to secondary bile acids, ileal and hepatic reabsorption, and the feedback regulation that keeps the bile acid pool stable. It treats bile acid physiology as a basic-science foundation for biliary disease, not as clinical management.

Core questions

• How is cholesterol converted into primary bile acids, and what enzymes control the rate?
• How are bile acids conjugated, secreted, and modified by gut bacteria into secondary bile acids?
• How are bile acids reabsorbed and returned to the liver, and how is the pool size regulated?
• How does feedback through nuclear receptors keep bile acid synthesis in balance?

Key concepts

• Classic (neutral) and alternative (acidic) synthesis pathways
• Cholesterol 7-alpha-hydroxylase (CYP7A1) as rate-limiting enzyme
• Primary versus secondary bile acids
• Conjugation with glycine and taurine
• Ileal apical sodium-dependent bile acid transporter (ASBT)
• Farnesoid X receptor (FXR) and FGF19 feedback
• Bile acid pool and recycling frequency

Mechanisms

In the classic pathway, cholesterol 7-alpha-hydroxylase (CYP7A1) initiates the rate-limiting step of converting cholesterol to the primary bile acids cholic and chenodeoxycholic acid; an alternative (acidic) pathway also contributes. Bile acids are conjugated to glycine or taurine, secreted across the canalicular membrane into bile, and delivered to the intestine, where bacteria deconjugate and dehydroxylate them to secondary bile acids such as deoxycholic and lithocholic acid. The apical sodium-dependent bile acid transporter in the terminal ileum reclaims most of the pool, which returns to the liver in portal blood. Bile acids act as ligands for the nuclear receptor FXR, which — together with intestinal FGF19 signalling — represses CYP7A1 and so feeds back to limit further synthesis, maintaining a stable pool that recycles several times per meal.

Clinical relevance

Bile acid physiology underlies the cholestatic syndromes, bile acid diarrhoea after ileal disease or resection, and gallstone formation, and it is the target of bile acid-based and FXR-directed therapies. This entry explains the normal biology that those conditions perturb; it is reference material and does not direct individual diagnosis or treatment.

History

The structures of the major bile acids were elucidated in the early twentieth century, work for which Heinrich Wieland received recognition, and the concept of an enterohepatic circulation with hepatic reuptake of intestinally absorbed bile acids was developed through mid-century physiology. The later identification of CYP7A1 as the rate-limiting enzyme and of FXR as a bile acid receptor connected the pathway to its feedback regulation.

Seminal works

• russell-2003
• chiang-2009
• dawson-2009

Frequently asked questions

What is the difference between primary and secondary bile acids?

Primary bile acids (cholic and chenodeoxycholic acid) are synthesised directly from cholesterol in the liver, whereas secondary bile acids (such as deoxycholic and lithocholic acid) are produced when intestinal bacteria chemically modify the primary acids.

How does the body keep the bile acid pool from being depleted?

About ninety-five percent of secreted bile acids are reabsorbed in the terminal ileum and returned to the liver through the enterohepatic circulation, so the liver only needs to synthesise enough to replace the small fraction lost in stool.