Lipid Digestion and Transport: Objectives
1. Trace the transfer (digestion, assimilation, transport) of lipids (triacylglycerol, fatty acids, and
cholesterol) from food to their site of storage and/or utilization.
a. Triacylglycerol (TG)
i. Digestion:
1. 98% of TGs are reabsorbed
2. Broken into FAs and diglycerides, hydrolyzed by esterase
3. In Duodenum, Pancreatic Lipase hydrolyzes TG (and CEs) to form
glycerol + 3FAs or (more often) 2-Monoacylglycerol +2 FA
ii. Assimilation: Stored in adipose tissue (and a little bit in muscle)
1. Can be synthesized primarily in liver and adipose tissues
iii. Transport: via chylomicrons (along with CEs) or VLDL mostly.
b. Fatty Acids (FA)
i. Digestion:
1. Short and medium chain FAs (<11C) are directly absorbed by gastric or
intestinal mucosa
ii. Assimilation:
1. FAs go from chylomicron into adipose tissue (from TG to FAs)
a. Then are repackaged into TGs with Glycerol-3-P
b. Stored as fat for energy use later
iii. Transport:
1. Short and medium chain FAs are transported to the liver via the portal
vein (NOT via the chylomicrons!)
2. Transported as micelles (along with Cholesterol)
3. Lipoprotein Lipase (LPL) hydrolyzes FAs from TGs
a. Which allows FAs to be transported into fat cell and
consequently re-esterified with glycerol to re-make TG
c. Cholesterol (C)
i. Digestion:
1. 50% is excreted into stool; 50% of cholesterol reabsorbed in the small
intestine
2. Hydrolyzed by esterase
3. Incorporate into micelles, which diffuse thru intestinal epithelial cells
a. (more info in #2)
ii. Assimilation:
1. Choelsterol Esters (CE): Formed by esterification with FAs, the storage
form of cholesterol
2. 90% synthesized by non-hepatic cells (and utilized by cell which made it)
3. 10% synthesized by hepatic cells (or less, depending on dietary intake of
Cholesterol)
a. Transported from liver into the bloodstream
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Lipid Digestion and Transport: Objectives
iii. Transport:
1. As CE, transport is via LDL mostly (and also HDL)
2. As Free C, transport is via
a. LDL (10% of total lipid carried in LDL)
b. HDL2 (10% of total lipid carried in HDL2)
c. IDL (intermediate density lipoprotein) (9%)
d. VLDL (8% of total lipid carried in VLDL)
2. Explain how bile salts and other amphipathic lipids solubilize lipids in the intestine.
a. Bile acids are formed in the liver and stored in the gall bladder
b. The presence of fat stimulates the release of cholecystokinin
i. Which in turn stimulates contraction of gall bladder, and release of bile into
duodenum
ii. The TG’s and cholesterol esters (CEs) are emulsified with bile acids and a bit of
Pancreatic Lipase
c. Amphipathic agents:
i. 2-Monoglyceride (MG)
ii. Bile acids (BA)
iii. Phospholipids (PL)
iv. They work by incorporating FAs into micelles, which is essential for lipid
absorption
d. Absorption of lipids occurs in the ileum (90%)
i. Micelles diffuse through the intestinal wall and deliver the lipids to the “brush
border” (microvilli)
ii. Transported into intestinal epithelial cells, re-esterified there.
iii. TG’s and CE are Packaged into chylomicrons
3. Describe the composition, origin, metabolic function, and degradation of the lipoproteins.
a. Lipoprotein Composition:
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Lipid Digestion and Transport: Objectives
b. Origin of lipoproteins: the apoproteins are mostly synthesized in the liver
i. Many lipoproteins begin their function in the small intestine
c. Metabolic function of lipoprotein:
i. Function in the transport and disposition of post-meal lipids
d. Degradation of lipoproteins:
i. Sent to the liver to be degraded
ii. Hepatic Lipase hydrolyzes remaining TG remnants
4. Know the major apoproteins of the lipoproteins and their function:
a. ApoA: HDL
b. ApoB-100: LDL
c. ApoB-48: Chylomicrons
i. Stay on chylomicrons throughout transport of TG and C
d. ApoC-II: Chylomicrons
i. Binds CM to LPL (chylomicron to Lipoprotein Lipase), so the TG can be
hydrolyzed into FAs
ii. Transfer from HDL to CM (chylomicron) and also from CM to peripheral tissue
e. ApoC-III: Chylomicrons
i. Is antagonistic with ApoC-II
ii. Transfers from peripheral tissue to CM and also from CM to HDL
f. ApoE: Chylomicrons
i. Transfer from HDL to CM (chylomicron) and also from CM to peripheral tissue
5. Describe the functions of these proteins in lipid digestion and assimilation:
a. Gastric Lipase: secreted in the stomach
i. Acts on TGs to produce FAs and diglycerides
ii. Responsible for 10-30% of overall lipolysis
iii. “Gets the ball running” in lipid digestion, and allows for further lipolysis
b. Pancreatic Lipase: secreted in the duodenum
i. Emulsifies Dietary Lipids (TG &CEs) along with colipase
ii. TG  2-Monoacylglycerol +2 FAs
(78%)
iii. TG  Glycerol +3FAs
(22%)
c. Colipase: secreted in the duodenum
i. Emulsifies Dietary Lipids (TG & CEs) in conjunction with pancreatic lipase
1. Binds to the surface of lipid emulsion
d. Cholesterol Esterase: secreted in the duodenum
i. Cholesterol Ester Cholesterol + FA
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