LIPID ABSORPTION

1. MUHAMMAD MUSTANSAR

2. Absorption of lipids
The end products of lipid digestion are
 Fatty acids
 Glycerol
 2-Monoacylglycerol
 1- Mono acylglycerol
 Cholesterol
 Lysophonpholipid

3. Stomach
gastric
mobility -
Small
Hormonal control
intesti cholecytokinin
ne
+
Gut
Endocrine
cells
Dietary (enlarged)
lipids +
bicarbonate Secretin
sec (in blood)
re
Pancreatic tes
lipase secret +
e s
pancreas
bile secret +
e s
Degradation of
dietary lipids Gall bladder

4.  As food enters the intestine, cholecystokinin hormon is
released which signals (1) the gallbladder to release bile salts
and; (2) the exocrine (pancreas) to release digestive
enzymes.
 Within the intestine, bile salts emulsify fats, which increase
their accessibility to pancreatic lipase.
4

5. Figure: Bile salts.
5

6.  Bile salts are synthesized in the liver, stored in the gallbladder,
secreted into the small intestine, resorbed in the ileum, and
returned to the liver via the enterohepatic circulation.
 Under normal circumstances, 5% or less of luminal bile salts
are excreted in the stool.
6

7. Figure: Enterohepatic
circulation.
 The bile salts
are reabsorbed
farther down the
intestinal tract and
returned to the
liver by the
enterohepatic
circulation.
7

8. Fate of dietary lipids:
 TGs: FFA + glycerol
 FFA:
- FFA from TGs muscle (energy production)
adipocytes( re esterified to TGs)
 Glycerol : Glycerol from TGs in liver forms glycerol 3 phosphate ( glycolysis,
gluconeogenesis)
 Chylomicron remnants:
Endocytosed into liver and are hydrolysed to their component parts and recycled by
the body.
- If this process is decreased due to impaired binding to the receptor on liver, they
accumulate in the plasma leading to type III hyperlipoproteinemia

9.  2-monoacylglycerols are reacylated to TAG via the
monoacylglycerol pathway.
 (1) Glycerol released in the intestinal lumen is not reutilized
but passes into the portal vein; (2) glycerol released within the
epithelium is reutilized for TAG synthesis via the normal
phosphatidic acid pathway.
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10.  Monoacylglycerols are poor substrates for hydrolysis, so it
mean less than 25% of ingested TAG is completely hydrolyzed
to glycerol and FAs.
 Because the micelles are soluble, they allow the products of
digestion to be transported through the aqueous environment of
the intestinal lumen and permit close contact with the brush
border of the mucosal cells, allowing uptake into the
epithelium.
10

11.  TAG are further degraded to 2-monoacylglycerols and FAs as
the major end products of luminal TAG digestion.
 Bile salts formed in the liver and secreted into the bile, permit
emulsification of the products of lipid digestion into micelles
together with phospholipids and cholesterol from the bile.
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12.  Micelles are formed within the intestinal lumen and interact
with the enterocyte (intestinal absorptive cells) membrane.
 Lipid-soluble components diffuse from the micelle into the
enterocytes.
12

13. MICELLE

14. Figure: Diffusion of FFA and
2-monoglycerides into the enterocytes.
14

15.  The intestinal epithelial cells resynthesize TAG and package
them into nascent (new) chylomicrons for release into the
circulation.
 Long-chain FAs are esterified to yield to TAG in the mucosal
cells and together with the other products of lipid digestion,
secreted as nascent chylomicrons into the lymphatics, entering
the bloodstream via the thoracic duct.
15

16.  Short- and medium-chain FAs are mainly absorbed into the
hepatic portal vein as FAs.
 Cholesterol is absorbed and dissolved in lipid micelles, and is
mainly esterified in the intestinal mucosa before being
incorporated into chylomicrons.
 Unesterified cholesterol and other sterols are actively
transported out of the mucosal cells into the intestinal lumen.
16

17. Figure: Synthesis of TAG in ER and
formation of chylomicrons.
17

18. Formation and secretion of (A) chylomicron in intestinal and
(B) VLDL in Hepatic cell.

19. Chylomicron Formation
Intestinal
Lymph Enterocyte Lumen
Phospholipid
Triglyceride
With
apoB48 Cholesteryl
Ester

20. Packaging for transport
Lehninger et al., apolipo-
3rd ed., Fig. 17-2 proteins
chylomicrons
 Particles for
transport of lipids
PL
to liver &
adipocytes
 Size: 0.1–1 µm
 Average
composition:
TG (84%)
chol (2%)
cholE (4%)
PL (8%) apolipoproteins (2%)
chol PL
cholE, TG

21. Summary of lipid
oil
digestion & absorption
drop
Enterocyte
lipase-
colipase MG 4ATPs/TG
FA TG
emulsion (>10C) apolipoproteins
TG
droplet MG phospholipids
FA
BILE
SALTS
lipase-
colipase chylomicron chylomicron
FA
mixed (<12C) FA
BILE albumin
micelle BILE
SALTS
SALTS
18

22. Lipids Are Transported in the Plasma as
Lipoproteins
Four Major Lipid Classes Are Present in
Lipoproteins.
 Plasma lipids consist of TAG (16%), phospholipids (30%),
cholesterol (14%), and cholesteryl esters (CE) (36%) and a
much smaller fraction of unesterified long-chain FAs (4%).
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23. Figure: Lipoproteins.
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24. Figure: Lipoprotein.
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