Bile acids / Bile salts & Enterohepatic circulation

In the process known as enterohepatic circulation, 95% of bile acids are reabsorbed across the intestinal epithelium and return to the liver, where unconjugated primary and secondary bile acids can be reconjugated to taurine or glycine.
Peiper et al. 2024 Cell Host & Microbe 32: 1488-1501

Hepatic conversion of cholesterol results in Primary bile acids (PBAs), that are excreted postprandially by the gallbladder. Active reuptake takes place by enterocytes in the terminal ileum and the Primary bile acids (PBAs) return to the liver via portal circulation. In the colon, Primary bile acids (PBAs) are converted to secondary bile acids by gut microbiota, and passively reabsorbed. Farnesoid X receptor (FXR / NR1H4) and Takeda G-protein coupled receptor 5 (TGR5) have a preference for secondary bile acids
Verhaar et al. 2020 Nutrients 12: 2982
Cai & Boyer 2021 Ann Transl Med 9(8): 737

The intestinal and hepatic uptake of conjugated bile acids is mediated by specific membrane transporters that retain the majority of the bile acid within the enterohepatic system, with only ~5% daily loss in the stool
Cai & Boyer 2021 Ann Transl Med 9(8): 737

Bile salts are actively reclaimed in the distal part of the small intestine. From there, they are released into the portal system and subsequently extracted by the liver. This enterohepatic cycle is critically dependent on dedicated bile salt transporters
Schubert et al. 2017 Immunological Reviews 279: 23-35

In contrast, unconjugated bile acids can diffuse across cell membranes in their protonated form
Boyer 2013 Compr Physiol 3: 1035-78

see also:
Bile acids / Bile salts pharmacokinetics
Primary bile acids (PBAs)
Secondary bile salts / Secondary bile acids (SBAs)