Chapter Summary, Questions Answers - Dietary Lipid Metabolism

CHAPTER SUMMARY

The digestion of dietary lipids begins in the stomach and continues in the small intestine (Figure 15.8). The hydrophobic nature of lipids requires that the dietary lipids, particularly those that contain long-chain length fatty acids (LCFAs), be emulsified for efficient degradation. Triacylglycerols (TAG) obtained from milk contain short- to medium-chain length fatty acids that can be degraded in the stomach by the acid lipases (lingual lipase and gastric lipase). Cholesteryl esters (CEs), phospholipids (PLs), and TAG containing LCFAs are degraded in the small intestine by enzymes secreted by the pancreas. The most important of these enzymes are pancreatic lipase, phospholipase A2, and cholesterol esterase. The dietary lipids are emulsified in the small intestine using peristaltic action and bile salts, which serve as detergents. The primary products resulting from enzymatic degradation of dietary lipid are 2-monoacylglycerol, unesterified cholesterol, and free fatty acids. These compounds, plus the fat-soluble vitamins, form mixed micelles that facilitate the absorption of dietary lipids by intestinal mucosal cells (enterocytes). These cells resynthesize TAG, CE, and PL using LCFAs and also synthesize protein (apolipoprotein B-48), all of which are then assembled with the fat-soluble vitamins into lipoprotein particles called chylomicrons. Short- and medium-chain fatty acids enter blood directly. Chylomicrons are released into the lymph, which carries them to the blood, where their lipid core is degraded by lipoprotein lipase (with apolipoprotein C-II as the coenzyme) in muscle and adipose tissues. Thus, dietary lipids are made available to the peripheral tissues. Problems with fat absorption cause steatorrhea. A deficiency in the ability to degrade chylomicron components, or remove their remnants after TAG has been removed, results in accumulation of these particles in blood.

Study Questions

Choose the ONE best answer.

15.1 Which one of the following statements about the digestion of lipids is correct?

A. Large lipid droplets are emulsified (have their surface area increased) in the mouth through the act of chewing (mastication).

B. The enzyme colipase facilitates the binding of bile salts to mixed micelles, maximizing the activity of pancreatic lipase.

C. The peptide hormone secretin causes the gallbladder to contract and release bile.

D. Patients with cystic fibrosis have difficulties with digestion because their thickened pancreatic secretions are less able to reach the small intestine, the primary site of lipid digestion.

E. Formation of triacylglycerol-rich chylomicrons is independent of protein synthesis in the intestinal mucosa.

Correct answer = D. Patients with cystic fibrosis, a genetic disease due to a deficiency of a functional chloride transporter, have thickened secretions that impede the flow of pancreatic enzymes into the duodenum. Emulsification occurs through peristalsis, which provides mechanical mixing, and bile salts that function as detergents. Colipase restores activity to pancreatic lipase in the presence of inhibitory bile salts that bind the micelles. Cholecystokinin is the hormone that causes contraction of the gallbladder and release of stored bile, and secretin causes release of bicarbonate. Chylomicron formation requires synthesis of apolipoprotein B-48.

15.2 Which one of the following statements about the absorption of lipids from the intestine is correct?

A. Dietary triacylglycerol must be completely hydrolyzed to free fatty acids and glycerol before absorption.

B. The triacylglycerol carried by chylomicrons is degraded by lipoprotein lipase to fatty acids that are taken up by muscle and adipose tissues and glycerol that is taken up by the liver.

C. Fatty acids that contain fewer than 12 carbon atoms are absorbed and enter the circulation primarily via the lymphatic system.

D. Deficiencies in the ability to absorb fat result in excessive amounts of chylomicrons in the blood.

Correct answer = B. The triacylglycerols (TAGs) in chylomicrons are degraded to fatty acids and glycerol by lipoprotein lipase on the endothelial surface of capillaries in muscle and adipose, thus providing a source of fatty acids to these tissues for degradation or storage and providing glycerol for hepatic metabolism. In the duodenum, TAG are degraded to one 2-monoacyl-glycerol + two free fatty acids that get absorbed. Medium- and short-chain fatty acids enter directly into blood (not lymph), and they neither require micelles nor get packaged into chylomicrons. Because chylomicrons contain dietary lipids that were digested and absorbed, a defect in fat absorption would result in decreased production of chylomicrons.