From primary to secondary bile acids
The liver makes primary bile acids, chiefly cholic and chenodeoxycholic acid, usually conjugated with glycine or taurine, and stores them in bile. After release into the intestine to help absorb dietary fat, most are reclaimed in the lower small intestine and returned to the liver in a cycle called enterohepatic circulation. Gut bacteria act on the fraction that escapes, first removing the conjugated amino acid and then converting primary into secondary bile acids such as deoxycholic and lithocholic acid.
This makes the bile acid pool a joint product of host liver chemistry and the microbiome, with composition shifting as the bacterial community changes.
Bile acids as signaling molecules
Beyond aiding fat digestion, bile acids act as signals. They bind the nuclear receptor FXR, which helps regulate their own synthesis and influences lipid and glucose handling, and the cell-surface receptor TGR5, linked to energy expenditure and gut hormone release. Primary and secondary bile acids differ in how strongly they engage these receptors.
This signaling role is an active research area connecting diet, microbiome, and metabolism, and many findings are still preliminary. This is general educational information, not medical advice.