Physiology, Bile Acids

Bile acids are steroidal acids found in bile. Primary bile acids are steroids produced via the liver, specifically in peroxisomes. There, the acids conjugate/connect to hydrophilic amino acids, namely glycine/taurine (i.e., conjugated bile acids called glycocholic and taurocholic acids, respectively); alongside sodium/potassium, they are termed bile salts. Secondary bile acids, however, are made via colonic bacteria. Of the organic compounds found in bile, bile acids constitute the large majority. Right after being synthesized, bile acids are secreted into bile and concentrated for storage in the gallbladder. Eating then stimulates cholecystokinin release, which causes gallbladder contraction--releasing its bile acids into the duodenum through the sphincter of Oddi.[9] The more bile acid that gets secreted, the faster that bile flows. The primary purpose of bile acids is to facilitate the digestion of fat via its surfactant properties, which emulsify fats into micelles. Hormonally, bile acids are also ligands for the farnesoid X receptor (FXR) and GPBAR1 (TGR5). In sum, the three main functions of bile acids are to (1) emulsify fat, (2) excrete cholesterol, and (3) have an antimicrobial effect.