Bile acid

Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver.^[1] Bile acids are conjugated with taurine or glycine residues to give anions called bile salts.^[2][3][4]

Primary bile acids are those synthesized by the liver. Secondary bile acids result from bacterial actions in the colon. In humans, taurocholic acid and glycocholic acid (derivatives of cholic acid) and taurochenodeoxycholic acid and glycochenodeoxycholic acid (derivatives of chenodeoxycholic acid) are the major bile salts. They are roughly equal in concentration.^[5] The salts of their 7-alpha-dehydroxylated derivatives, deoxycholic acid and lithocholic acid, are also found, with derivatives of cholic, chenodeoxycholic and deoxycholic acids accounting for over 90% of human biliary bile acids.^[5]

Bile acids comprise about 80% of the organic compounds in bile (others are phospholipids and cholesterol).^[5] An increased secretion of bile acids produces an increase in bile flow. Bile acids facilitate digestion of dietary fats and oils. They serve as micelle-forming surfactants, which encapsulate nutrients, facilitating their absorption.^[6] These micelles are suspended in the chyme before further processing. Bile acids also have hormonal actions throughout the body, particularly through the farnesoid X receptor and GPBAR1 (also known as TGR5).^[7]

It is to be noted that bile acid synthesis is the only manner in which humans or other mammals may excrete excess cholesterol, as the parent compound of all bile acids is cholesterol.