The digestion of dietary lipids begins in the stomach and continues in the small intestine .
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.
Related Topics
TH 2019 - 2023 pharmacy180.com; Developed by Therithal info.