DIGESTION & ABSORPTION OF LIPIDS

بسم الله الرحمن الرحيم

By: Ali M. Osman

IntroductionLipids: 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.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.

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.

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.

Digestion & Absorption of TAGs• Initiated by lingual & gastric lipases attacking sn-3

ester bond, forming 1,2- DAGs & FFAs.

• 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.

H

Lipase

H2O

FFA+

BileSalts

Acyl1

Acyl2

Acyl3

TAG 100%

Intestinal Lumen Intestinal Cell Lymphatics

Pan. Lipase

FA

FA

Acyl1

Acyl2

OH1,2-DAG

OH

Acyl2

OH

2-MAG*

Acyl1

OH

OH1-MAG

FA

Pan. Lipase

Isomerase

OH

OH

OHGlycerol

FA

Pan. Lipase

OH

Acyl2

OH

Acyl-CoA

Acyl1

Acyl2

Acyl3

Acyl1

Acyl2

Acyl3

Acyl1

Acyl2

Acyl3

Acyl1

OH

OH

OH

OH

OHGlycerol

FA

Int.Lipase

GlycerolKinase

ATP

ATPAcyl CoASynthetase

OH

OH

PGlycerol-3phosphate

Ente

rohe

patic

circ

ulati

on

Liver

Glycolysis

Monoacylglycerol pathway

Phosphatidic acid pathway

Portal Vein

Glycerol

6%

72%

Acyl CoASynthetase

+ Fat-soluble Vits

& Cholesterol

Chylomicrones

ATP

To ileum

22%

28%

TAG

TAG

1-MAG

Thoracic Duct

2-MAG

• 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.

• 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.

• 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.

Thank U