Triacylglycerols digestion, absorption

The products of triacylglycerol digestion, mainly monoacylglycerol and long-chain fatty acids, interact with bile salts to form micelles, which comprise bile salts/ monoacylglycerols/phospholipids and fatty acids. The micelle aids the absorption of monoacylglycerol and fatty... 

In developed conntries, fat provides about 40% of the total energy consnmed but in some individuals this percentage may be mnch higher. Almost all of this is in the form of triacylglycerol, containing mainly long-chain bnt also some short-chain fatty acids. The stractnre, digestion, absorption and eventual fate of the products of absorption are described in Chapters 4 and 5 and the metabolism of fat is discussed in Chapter 7. 

Lipids are insoluble in water and this profoundly affects the particular phenomena associated with their digestion, absorption, transport in the blood, and metabolism at the cellular level. Fat is easily recognized when it accumulates within the body, but the chemical and technical definition of fats and hpids are more difficult. Of nutritional interest are triacylglycerols (TAG), phospholipids (PL), sterols, and some derived lipids — results of the hydrolysis or enzymatic breakdown of simple and compound lipids. 

J. E. Staggers, G. J. P. Femando-Wamakulasuriya, M. A. Wells. Studies on fat digestion, absorption and transport in the suckling rat. 11. Triacylglycerols molecular species. slereospeciftc analysis and specificity of hydrolyses oflingual lipase, i Lipid Res 22 675-679, 1981. 

Four major groups of lipoproteins are recognized Chylomicrons transport lipids resulting from digestion and absorption. Very low density lipoproteins (VLDL) transport triacylglycerol from the liver. Low-density lipoproteins (LDL) deliver cholesterol to the tissues, and high-density lipoproteins (HDL) remove cholesterol from the tissues in the process known as reverse cholesterol transport. 

Figure 44-2. Digestion and absorption of triacylglycerols.The values given for percentage uptake may vary widely but indicate the relative importance of the three routes shown.

The absorptive state continues for 2-4 hours after food intake. As a result of food digestion, the plasma levels of glucose, amino acids, and fats (triacylglycerols) temporarily increase. 

FIGURE 17-1 Processing of dietary lipids in vertebrates Digestion and absorption of dietary lipids occur in the small intestine, and the fatty acids released from triacylglycerols are packaged and delivered to muscle and adipose tissues. The eight steps are discussed in the text. 

People of western culture ingest about 100 g of triacylglycerol per day. The digestion and absorption of this lipid, together with the ingested phospholipids, depend on secretions from the pancreas (exocrine) and a flow of bile from the gall-bladder. The important constituents of the pancreatic secretions are enzymes, and those of the bile are the bile salts (Chap. 6). 

In neonate, suckling mammals, short- and medium-chain fatty acids are preferentially split at the sn-3 triacylglycerol position by oral and gastric lipases and are absorbed in the stomach, while the long-chain fatty acids are hydrolyzed at the sn- and sn-2 positions and by pancreatic lipases and are absorbed in the small intestine (50, 51). With growth, the neonate fat digestion system becomes less active, and is replaced by the small intestine-pancreatic lipase pathway. But residual oral and gastric lipase activities and direct absorption of short-chain fatty acids in the... 

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. 

Digestion and absorption of triacylglycerols with medium-chain fatty acids 12 carbons) proceed by a different pathway. Medium-chain triacylglycerols are partly water-soluble, are rapidly hydrolyzed by lingual and pancreatic lipases, and do not require the participation of bile acids. Some are absorbed intact and hydrolyzed inside the absorptive cell. Medium-chain fatty acids enter the portal... 

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. 

Lie, O. and Lambertsen, G., 1991a, Lipid digestion and absorption in cod (Gadus morhua) comparing triacylglycerols, wax esters and diacylalkylglycerols, Comp. Biochem. Physiol, 98A, 159. 

In the fasting state (sometimes known as the post-absorptive state, as it begins about 4-5 hours after a meal, when the products of digestion have been absorbed) metabolic fuels enter the circulation from the reserves of glycogen, triacylglycerol and protein... 

The digestion products of triacylglycerols retain their association with bile acids and lipase and combine with other lipids to form small aggregates called micelles that are suspended within the ingesta. The micelles bump into the brush border of the enterocytes (columnar epithelial absorptive cells of the intestinal wall) into which the digestion products pass by diffusion. 

Previous studies from other laboratories have demonstrated that the hypocholesterolemic effect of tea is due to tea catechins by decreasing the micellar solubility and intestinal absorption of dietaiy cholesterol (5). EGCG is the major catechin in tea leaves to inhibit the absorption of exogenous cholesterol from digestive tract. Moderate enhancement in the elimination of endogenous cholesterol by tea catechins has been reported (9). A previous study has demonstrated that Pu-Erh tea reduces plasma cholesterol and triacylglycerol... 

Trans fatty acids are present in the diet in esterified form, mainly in triacylglycerols but those from ruminant sources may also be present in phospholipids. Before absorption into the body, triacylglycerols must be digested by pancreatic lipase in the upper small intestine. There is no evidence of differences in the hydrolysis and absorption of trans fatty acids, in comparison with that of cis fatty acids. Trans fatty acids are transported from the intestine mainly in chylomicrons, but some are also incorporated into cholesteryl esters and phospholipids. 

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