The two primary bile acids, Cholic acid and Chenodeoxycholic acid, are oxidised from cholesterol in the liver. In the intestine, the primary bile acids are then dehydroxylated to the secondary bile acids; Cholic acid to Deoxycholic acid and Chenodeoxycholic acid to Lithocholic acid and Ursodeoxycholic acid. The two primary and secondary bile acids can also be converted to their glycine and taurine conjugates to increase water solubility. By choosing deuterium labelled Chenoxycholic acid, Deoxycholic acid and Ursodeoxycholic acid it is possible to differentiate the three bile acids even though the unlabelled forms all share the same molecular weight. This allows for clear determination and identification of each bile acid, as well as the glycine and taurine conjugates.