Triacylglycerol emulsification

Pasquier B, Armand M, Guillon F, Castelain C, Borel P, Barry JL, Pieroni G and Lairon D. 1996. Viscous soluble dietary fibers alter emulsification and lipolysis of triacylglycerols in duodenal medium in vitro. J Nutr Biochem 7 293—302. 

The digestion of triacylglycerols in adult nonruminant mammals has been described as initiated in the mouth by hngual lipase released in the sahva at the base of the tongue (52). Up to 6% of the fatty acids are hydrolyzed and initiate emulsion formation in the stomach. The digesta (called chyme at this location) is released from the stomach slowly into the duodenum to ensure complete mixing with the bile salts and emulsification. Lipolysis occurs by association of pancreatic lipase and co-lipase at the surface of the bile salt-stabihzed emulsion. Amphipathic molecules (fatty acids, sn-2 monoacylglycerols, and lysolecithins) are produced and associate with the bile salts to form water-soluble micelles from which absorption occurs. 

Bile salt molecules secreted by the gallbladder are essential for the emulsification and absorption of fats. They are the salt forms of bile acids, which are the major product of cholesterol catabolism in the liver. Bile salts form micelles as their hydrophobic face contacts the fat (triacylglycerol), and their polar face maintains contact with the aqueous enviromnent. This micelle formation allows water-soluble digestive enzymes to digest the entrapped triacylglycerol molecnle, releasing fatty acids that are readily absorbed by the digestive system. 

Pasquier, B., Armand, M., CasteUan, C., GuiUon, F., Borel, P., Lafont, H., and Lairon, D. (1996). Emulsification and lipolysis of triacylglycerols are altered by viscous soluble dietary fibres in acidic gastric medium in vitro. Biochem. J. 314, 269-275. 

The major fats in the diet are triacylglycerols and, to a lesser extent, phospholipids. These are hydrophobic molecules and have to be emulsified to very small droplets (micelles section 4.3.2.2) before they can be absorbed. This emulsification is achieved by hydrolysis to monoacyl- and diacylglycerols and free fatty acids, and also by the action of the bile salts (section 4.3.2.1). 

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