Digestion of dietary lipid

Note The role of lingual lipase in the digestion of dietary lipids is minor because it accounts for less than 10% of the enzymatic breakdown of triglycerides. 

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. 

An adult ingests about 60 to 150 g of lipids per day, of which more than n nety percent is normally triacylglycerol (formerly called triglyceride). Uhe remainder of the dietary lipids consists primarily of cholesterol, cholesteryl esters, phospholipids, and unesterified ("free") fatty acids. "The digestion of dietary lipids is summarized in Figure 15.2. 

Digestion of dietary lipids Dietary lipids DIGESTION OF DIETARY LIPIDS (p. 171) Dietary lipids consist primarily of triacylglycerol, with some cholesterol, cholesteryl esters, phospholipids, and free (nonesterified) fatty acids. 

Which of the following statements concerning digestion of dietary lipids is TRUE ... 

A. Pancreatic lipase catalyzes the breakdown of dietary triacylglycerols into free fatty acids and 2-monoacylglycerols, an essential step in the digestion of dietary lipids. Since prostaglandins are produced from linoleate, an essential fatty add, a deficiency of pancreatic lipase would eventually cause a prostaglandin defidency. 

Fredrikzon, B. and Blackberg, L. (1980) Lingual lipase an important lipase in the digestion of dietary lipids in cystic fibrosis Pediatr. Res. 14, 1387-1390. 

Hamosh M, Scow RO. Lingual lipase and its role in the digestion of dietary lipid. 

Much of our information about lipases comes from studies on the enzyme isolated from pancreatic juice this is the enzyme primarily responsible for the digestion of dietary lipids in the animal digestive tract, a process that is described in detail in section 5.3.1. Pancreatic lipase is a glycoprotein of molecular mass 50kDa. The first step in the catalytic process is the adsorption of the enzyme on to the hydrophobic interface of the substrate micelles. Detergent molecules, like bile salts, tend to compete with the lipase for binding sites and the adsorption of the enzyme is assisted by a helper molecule called co-lipase, which is a small protein. In vitro, it can be shown that the activity of lipase is inhibited by high concentrations of bile salts and that this inhibition can be overcome by the addition of co-lipase. 

The nature of the diet sets the basic pattern of metabohsm. There is a need to process the products of digestion of dietary carbohydrate, lipid, and protein. These are mainly glucose, fatty acids and glycerol, and amino acids, respectively. In ruminants (and to a lesser extent in other herbivores), dietary cellulose is fermented by symbiotic microorganisms to short-chain fatty acids (acetic, propionic, butyric), and metabohsm in these animals is adapted to use these fatty acids as major substrates. All the products of digestion are metabohzed to a common product, acetyl-CoA, which is then oxidized by the citric acid cycle (Figure 15-1). 

The regular digestion of dietary fats and fat-soluble substances and/or their absorption via the lipid route is compromised in the frame of various diseases such as cystic fibrosis, short bowel syndrome, cholestasis, and chronic inflammatory bowel diseases. 

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. 

II. DIGESTION, ABSORPTION, SECRETION, AND UTILIZATION OF DIETARY LIPIDS ... 

The critical process of emulsification of dietary lipids occurs in the duodenum. Emulsification increases the surface area of the hydrophobic lipid droplets so that the digestive enzymes, which... 

II. Digestion, Absorption, Secretion, and Utilization of Dietary Lipids... 

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. 

The digestion and absorption of dietary lipid can be completed only in the presence of adequate amounts of bile salts that are synthesized in the liver and pass, via the bile duct, into the duodenum and thence into the jejunum. Reabsorption of the bile salt micelles occurs in the ileum, from which a large proportion return via the blood to the liver. The bile ducts carry bile salts from the liver to the gallbladder, where they are stored excreted (excess) cholesterol is dissolved in the bile salt micelles. Overall, 90 percent of the bile salts involved in absorption of lipid in the jejunum are recycled, in a process called the enterohepatic circulation, and 10 percent are lost in the feces. Replacement of this amount necessitates conversion from cholesterol. Thus, de novo synthesis of cholesterol itself plays an important part in maintaining the supply of bile salts. 

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. 

Nutritional Uses of Lecithin in Animal Feeds The functional aspects of lecithin use as an emulsifier are referenced above. Emulsification of dietary lipid is also nutritionally important for digestion. Immature animals of all species (e.g., calves, dogs, fish, pigs, and poultry) have limited production of digestive emulsifiers (i.e., bile) and enzymes. Providing lecithin as a dietary emulsifier allows improved utilization of feed energy. 

Blockage of the bile duct caused by problems such as cholesterol-containing gallstones or duodenal or pancreatic tumors can lead to an inadequate concentration of bile salts in the intestine. Digestion and absorption of dietary lipids is diminished. Certain diseases that affect the pancreas can lead to a decrease in bicarbonate and digestive enzymes in the intestinal lumen. (Bicarbonate is required to raise the intestinal pH so that bile salts and digestive enzymes can function.) If dietary fats are not adequately digested, steatorrhea may result. Malabsorption of fats can lead to caloric deficiencies and lack of fat-soluble vitamins and essential fatty acids. 

C. The pancreas produces bicarbonate (which neutralizes stomach acid) and digestive enzymes (including the lipase that degrades dietary lipids). Decreased bicarbonate will lead to a decrease of intestinal pH. Decreased digestion of dietary triacylglycerols will lead to formation of fewer bile salt micelles. Intestinal cells will have less substrate for chylomicron formation, and less fat-soluble vitamins will be absorbed. More dietary fat will be excreted in the feces. 

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