Fig. 2.

Biosynthesis of bile acids and microbial modification of bile acid metabolism. Cholesterol is converted into two primary bile acids, cholic acid (CA) and chenodeoxycholic acid (CDCA), in the liver and further conjugated to glycine or taurine. The bile salts that escape active transport in the distal ileum become substrates for biotransformation reactions by intestinal bacteria. In the intestine, especially in the colon, these conjugated species are first deconjugated to CA or CDCA, and then dehydrogenated and dehydroxylated by gut bacteria into secondary bile acids such as DCA and LCA