Lipid digestion bile acids

Bile acids The major metabolites of cholesterol, which are synthesized in the liver and stored in the gallbladder for use as emulsifiers in the digestion of lipids. Primary bile acids are those synthesized directly from cholesterol in the liver Secondary bile acids are metabolites of primary bile acids produced by the action of intestinal bacteria. 

Various mechanisms have been proposed to explain the hypocholesterolemic effect of GA (Annison et al., 1995 Tiss et al., 2001). Some studies have suggested that the viscosity of fermentable dietary fiber contributes substantially to the reduction of lipids in animals and humans (Gallaher et al., 1993 Moundras et al., 1994). However, other studies suggested that this property is not related to plasma lipids (Evans et al., 1992). The mechanism involved is clearly linked to increased bile acid excretion and fecal neutral sterol or a modification of digestion and absorption of lipids (Moundras et al., 1994). 

Bile acids aid in the digestion of dietary lipids. They are made in the liver and secreted into the small intestine in the bile where they emulsify lipids. 

Bile helps in the digestion and absorption of fats. Its constituent bile acids (BAs) have detergent properties, and some can be carcinogenic. BAs can act as signalling molecules, entering the nuclei and reacting with the nuclear receptors and this could enhance or reduce BA synthesis. In this way, they control their own levels as well as those of their precursor, cholesterol. This controls cholesterol homeostasis and BA and lipid synthesis. 

A and B are in cis position relative to each other (see p. 54). One to three hydroxyl groups (in a position) are found in the steroid core at positions 3, 7, and 12. Bile acids keep bile cholesterol in a soluble state as micelles and promote the digestion of lipids in the intestine (see p.270). Cholic add and cheno-deoxychoMc acid are primary bile acids that are formed by the liver. Their dehydroxylation at C-7 by microorganisms from the intestinal flora gives rise to the secondary bile acids lithocholic acid and deoxycholic acid. 

Much of the cholesterol synthesis in vertebrates takes place in the liver. A small fraction of the cholesterol made there is incorporated into the membranes of he-patocytes, but most of it is exported in one of three forms biliary cholesterol, bile acids, or cholesteryl esters. Bile acids and their salts are relatively hydrophilic cholesterol derivatives that are synthesized in the liver and aid in lipid digestion (see Fig. 17-1). Cholesteryl esters are formed in the liver through the action of acyl-CoA-cholesterol acyl transferase (ACAT). This enzyme catalyzes the transfer of a fatty acid from coenzyme A to the hydroxyl group of cholesterol (Fig. 21-38), converting the cholesterol to a more hydrophobic form. Cholesteryl esters are transported in secreted lipoprotein particles to other tissues that use cholesterol, or they are stored in the liver. 

The products of lipid digestion—free fatty acids, 2-monoacylglycerol, and cholesterol—plus bile salts, form mixed micelles that are able to cross the unstirred water layer on the surface of the brush border membrane. Individual lipids enter the intestinal mucosal cell cytosol. 

Bile salts (bile acids) are the major excretory form of cholesterol. These polar compounds are formed in the liver by converting cholesterol into the activated intermediate cholyl CoA and then combining this compound with either glycine, to form glycocholate, or taurine, to form taurocholate. The detergentlike bile salts are secreted into the intestine where they aid the digestion and uptake of dietary lipids. 

Cholesterol also serves as a precursor to other important molecules. Bile acids aid in lipid absorption during digestion. Steroid hormones all derive from cholesterol, including the adrenal hormones that maintain fluid balance Vitamin D, which is an important regulator of calcium status and the male and female sex hormones. Although humans wouldn t survive in one sense or another without cholesterol metabolites, cholesterol brings with it some well-known side effects. Doctors find cholesterol derivatives, being essentially insoluble in water, in the deposits (plaque) that characterize diseased arteries. 

Bile is produced by hepatocytes from several essential components, including water, bile acids, cholesterol, phospholipids and bilirubin. Most of these substances are absorbed in the distal ileum and delivered to the hepatocyte via the portal vein. The liver excretes approximately 500-600 mL of bile each day, most of which is stored in the gallbladder. Bile acids have an important function in emulsifying lipids in the digestive tract, which improves digestion by pancreatic lipases. 

Emulsification/absorption of dietary lipid in the intestine. Bile acids are stored in the gallbladder and released into the duodenum when cholecys-tokinin is released. In the small intestine, bile acids help to solubilise monoglycerides and fatty acids which are formed as the result of the digestion of dietary triglyceride, thereby enhancing the absorption of lipids and the fat-soluble vitamins A, D, E and K. 

The mechanism for the inhibition of cholesterol absorption is thought to involve competitive transfer to the micellar phase during absorption from the intestinal lumen. Phytosterols in the micellar phase may also act as emulsifying agents that selectively inhibit the transfer of cholesterol and other lipids (e.g., carotenoids and vitamins) and, thereby, limit their absorption. The exact kinetics governing the sterol competition for transfer are not known, but dietary sterols are absorbed differently in the digestive tract 40-50% for cholesterol, 12-16% for campesterol, 4-5% sitosterol, and <0.5% for phytostanols (37). Before absorption, esterified sterols are hydrolysed effectively in the upper intestine (191). Absorbed phytosterols are excreted by the liver into the bile but are hardly converted to bile acids (192). Numerous studies in animals and humans approved the safety of phytosterols and phytostanols (37). 

Bile Salts Enable the Digestion of Lipids Cholesterol is the precursor of both steroids and bile salts and is an integral component of cell membranes. It is eliminated from the body via conversion to bile salts and direct secretion into the bile. In fact, the word cholesterol (from the Greek chole (bile) and stereos (solid)) was used originally to describe the material of which gallstones are made. In the process of degradation, it is converted to the primary bile acids cholic acid and chenodeoxycholic acid in approximately equal amounts. The salts of these acids are excreted in bile. They perform two important functions in the digestive tract ... 

Taurine, which has an amino group in the 6-carbon and a sulfonic acid group instead of COOH, is present in the CNS and as a component of certain bile acids participates in digestion and absorption of lipids in the gastrointestinal tract. 

---