Bile acids are synthesised from cholesterol in the liver and are then conjugated with glycine or taurine. Feline bile acids are conjugated exclusively with taurine, partly explaining their high absolute requirement for this aminosulphonic acid. In contrast, rabbits conjugate their bile acids only with glycine. The conjugated bile acids are secreted into bile which then passes into the small intestine.
Bile acids participate in lipid digestion and absorption in the small intestine and the majority are then reabsorbed in the ileum. These are extracted from the portal blood in the liver and re-secreted into bile, completing the entero-hepatic circulation of bile acids.
The competence of enterohepatic circulation is usually assessed dynamically by performing a bile acid stimulation test; the patient is fasted for 12 hours and then fed a test meal and the serum bile acid concentration are measured before and two hours after the meal. The post-prandial measurement may be elevated in the following conditions:
- Hepatobiliary disease; this is a non specific test for hepatobiliary disease.
- Cholestasis, including the mild cholestasis that occurs due to hepatic hydropic change in hyperadrenocorticism in dogs and extra-hepatic biliary obstruction.
- Portosystemic shunts or microvascular dysplasia, in which the portal blood is directed into the systemic circulation, disrupting the enterohepatic circulation.
- Small intestinal bacterial overgrowth due to deconjugation of bile acids by intestinal bacteria.
- Increased post-prandial bile acid concentration may be a normal finding in Maltese terriers.
Deficiency of bile acids is extremely rare, even with severe hepatic insufficiency.