ABSTRACT
This chapter examines the role that intestinal bacteria play in the metabolism of cholesterol, phytosterols, and steroid hormones. The complex populations of predominantly anaerobic bacteria that colonize the digestive tract, especially the terminal ileum and large bowel, exhibit a wide range of metabolic activities. Compared to the excretion of bile acids, cholesterol, and phytosterols relatively small amounts of steroid hormones escape enterohepatic circulation and enter the colon and are subsequently excreted. Direct evidence for the role of intestinal bacteria in the metabolism of steroid hormones has been derived from the comparison of steroidexcretion patterns in germfree and conventional rats. Steroids of the pregnane, androstane, and estrogen series enter the bowel, generally as sulfate or glucuronic acid conugates and are degraded by a combination of these metabolic pathways. The development of phytosterol metabolism in the human gut is therefore very similar to that for cholesterol and is strongly associated with the development of the gut flora.
