Figure 1. Structure and hydrophobic/hydrophilic profile of the most common bile acids.

Cholic acid (CA) and chenodeoxycholic acid (CDCA) are primary BAs. Deoxycholic acid (DCA) and lithocholic acid (LCA) are secondary BAs. Ursodeoxycholic acid (UDCA) is a primary BA predominantly produced in bears. Hydroxyl groups that are in α-orientation are located below the steroid nucleus and are axial to the plane of the steroid nucleus. Hydroxyl groups that are in β-orientation are located above the steroid nucleus and are equatorial to the plane of the steroid nucleus. The hydrophobicity increases as follows: UDCA, CA, CDCA, DCA, LCA.