Bile acids are endogenous steroids from the liver that have an emulsifying effect on lipids in fat digestion. Bile acids are largely reabsorbed into the liver in the intestine. If this reabsorption is disturbed, for example, by inflammation, bile acidosis syndrome sets in.
What are bile acids?
Bile acids are endogenous steroids that are irreplaceable as end products of cholesterol metabolism and form an important component of bile. As derivatives of cholesterol, they play a role primarily in fat digestion and the absorption of lipids. The liver produces bile acids in its hepatocytes from the starting material cholesterol. Hydroxylation reactions and oxidative shortening take place for this purpose. Chenodeoxycholic acid and cholic acid are the only primary bile acids in the human body. Conjugated bile acids are also called bile salts or secondary bile acids. Approximately 200 to 500 milligrams of bile acids are synthesized in the liver day after day in a healthy person and released into the intestine as needed. Bile acids participate in the enterohepatic circulation and are thus reused several times. They circulate between the liver and intestine, where they are reabsorbed into the liver. Their reabsorption occurs in the ileum.
Anatomy and structure
Bile acids are an important component of bile, much of which is composed of water. Cholic acid is a primary bile acid. In bile, these acids do not exist freely. They are first conjugated with glycine or taurine in the liver to form amides. The conjugation yields tauroic and glycocholic acids, also called tauro- and glycocholate. These substances are the anions of cholic acid and are also called bile salts. They are temporarily stored in the gallbladder. Via the Vater’s pupil and the bile ducts, the bile salts enter the duodenum in pulsating movements. Bacterial breakdown of the stored glycine and taurine occurs. During this cleavage, the hydroxyl group on the side chain is removed, resulting in deoxycholic acids. These deoxycholic acids are also called secondary bile acids. Primary and secondary bile acids are reabsorbed about six to ten times in the terminal ileum.
Function and tasks
Bile acids are soluble in both water and fats. After eating, they are released from the bile into the small intestine as needed. There, they stabilize emulsions, which are mixtures of immiscible substances. This means that they have an emulsifying effect specifically on dietary fats because they form micelles with them. They reduce the surface tension of water and emulsify water-insoluble components in the intestine, such as lipids. In this way, they increase the attackability of fats for enzymes and thus create ideal conditions for absorption. In particular, bile acids enable the decomposition of fats by the water-soluble enzyme lipase. Thanks to bile acids, the human body can also excrete excess cholesterol. The group of primary bile acids consists of cholic acid and chenodeoxycholic acid, about 95 percent of which are reabsorbed after fulfilling their tasks. Secondary bile acids are all products of the primary bile acids generated by processes external to the liver. Bile acids are reabsorbed by ionic and nonionic diffusion. Transport back into the portal blood occurs across the basolateral membrane by anion exchangers and cytosolic transport proteins. Approximately 0.6 grams of bile acid is lost daily in the stool. This loss is replenished by cholesterol biosynthesis in the liver. The secondary bile acid deoxycholic acid is structurally related to steroid hormones. Therefore, speculations exist for secondary bile acids about their involvement in hormone balance. In particular, there is speculation about an antagonistic interaction with glucocorticoids.
Diseases
When the ratio of bile acid to cholesterol in the gallbladder is less than 13:1, cholesterol may precipitate. This phenomenon results in the formation of gallstones, also known as cholesterol stones. In many cases, gallstones do not cause any symptoms and thus go unnoticed for a long time. When the stones become lodged, they usually cause colic or inflammation and therefore need to be removed. Bile can build up in the bile ducts as a result of gallstones.There is then an increased concentration of bile acid in the blood. Increased formation of bile acid, on the other hand, occurs in colon cancer. Another phenomenon occurs when parts of the small intestine are removed or are regularly affected by chronic inflammation. Thus, bile salts are no longer sufficiently reabsorbed, since 98 percent of reabsorption takes place in the small intestine. After the removal of parts of the intestine or in the case of a chronic inflammatory bowel disease such as Crohn’s disease, patients therefore suffer from impaired fat digestion. The majority of bile salts are no longer reabsorbed but excreted in the stool. This phenomenon manifests itself in fatty stools with a large volume, also known as chologenic diarrhea. Bile acid enters the colon, which it should not actually reach due to the processes of reabsorption. This bile acid leakage syndrome can irritate the colon and increase the risk of colon cancer. Usually, bile acid loss syndrome is mainly a result of damage to Bauhin’s valve. In addition, if blood bile levels are decreased, liver disease may be present. For example, in liver damage caused by alcoholism, liver cells synthesize significantly fewer bile acids.
