3. Introduction
Lipids:
 Water-insoluble organic compounds, but
soluble in polar solvents, e.g. fats, oils,
steroids, waxes, etc.
 Importance:
o high energy value.
o fat-soluble vitamins.
o essential FAs.

4. o thermal insulator in adipose tissue.
o electrical insulators in nerves.
o Phospholipids = constituents of cell
membrane & mitochondria.
o Lipoproteins transport lipids in blood.
o Involved in pathogenesis of: obesity, DM,
atherosclerosis, etc.

5.  Classified into:
 Simple: esters of FAs with alcohols, e.g. fats,
oils, waxes.
 Complex: esters of FAs with alcohols +
Group, e.g. Phospholipids, Glycolipids ,
Sulfolipids , Aminolipids, etc.
 Derived: e.g. steroids, fatty aldehydes, ketone
bodies, hydrocarbons, lipid-soluble vitamins,
hormones, etc.

6. Dietary lipids
• TAGs & Phospholipids:
– Major ones.
– Since hydrophobic, they need to be
hydrolyzed & emulsified to “micelles” before
absorption.
• Fat-soluble Vits (A, D, E, & K) & cholesterol
absorbed dissolved in lipid micelles.

7. Digestion & Absorption of TAGs
• Initiated by lingual & gastric lipases attacking sn-3
ester bond, forming 1,2- DAGs & FFAs.
Lipase
H2O
H
+
FFA
• In intestine, further hydrolysis is by pancreatic lipase
which:
– Needs colipase, for activity.
– Specific for sn-1 & sn-3 ester bonds → 2-monoacylglycerols
– major end-products of TAG digestion – & FFAs.
• N.B: <25% of ingested TAG is completely hydrolyzed
to glycerol & FFAs.

8. Intestinal Lumen
Acyl1
Acyl2
Acyl3
TAG
Lymphatics
Intestinal Cell
Acyl1
Pan. Lipase
FA
100%
Acyl2
1,2-DAG
OH
Acyl1
OH
Acyl2 Monoacylglycerol pathway
Acyl2
Acyl3
OH
2-MAG
Pan. Lipase
FA
TAG
72%
Acyl1
OH
Bile 2
Acyl
Salts
Enterohepatic circulation
To ileum
Acyl3
OH
2-MAG*
Isomerase
28%
FA
Acyl CoA
Synthetase
AcylCoA
ATP
Acyl1
Phosphatidic acid
pathway
Acyl3
Acyl CoA
Synthetase
OH
OH
1-MAG
Pan. Lipase
22%
OH
TAG
ATP
FA
6%
Acyl1
OH
OH
OH ATP
Thoracic
Duct
OH
OH
OH
1-MAG
Int.
Lipase
OH
Glycerol
OH
OH
Glycerol
Chylomicrones
Acyl2
Acyl1
FA
Liver
+ Fatsoluble Vits 2
&Acyl
Cholesterol
Glycerol
Kinase
P
Glycerol-3
phosphate
Portal Vein
Glycolysis
Glycerol

9. • 2-monoacylglycerols, Fat soluble Vits,
cholesterol & phospholipids are emulsified by
bile salts into micelles and liposomes.
• Water soluble micelles are transported through
the intestinal lumen and come into close contact
with the brush border of jejunal mucosal cells,
allowing cellular uptake of enclosed lipids.

10. • The released bile salts pass on to the ileum,
where absorbed into the enterohepatic
circulation returning to the liver.
• Glycerol released in the intestinal lumen is not
reutilized but passes via portal vein to the liver;
where it is used for esterification of FAs
synthesized in lipogenesis.

11. • Glycerol released within the epithelium is
reutilized for TAG synthesis via the normal
phosphatidic acid pathway.
• All long-chain fatty acids absorbed are converted
to TAG in the mucosal cells and, together with
the other products of lipid digestion, secreted as
chylomicrons into the lymphatics, entering the
blood stream via the thoracic duct.