Digestion & Absorption of lipids

DR ROHINI C SANE

Digestion & absorption of lipids

• Daily intake of lipids is 60-150 gm (phospholipids ,cholesterol, cholesterol esters, free fatty acids )

• Lipids insoluble, digestive enzymes in aqueous solution --therefore problem for digestion and absorption of lipids

• Specialized machinery needed to

(1) increase surface area of lipid for digestion

(2) Solubilize the digested products for absorption

Digestion & absorption of lipids in stomach• Minor digestion of lipids in stomach by acid lipase (also called lingual

lipase )

• Gastric lipase act on fat containing short chain fatty acid at neutral pH

• Digestion of lipids in stomach is negligible

• LOW pH in stomach –UNFAVOURABLE for action of gastric lipase

• infants –milk fat with short chain fatty acid digested at neutral pH

Digestion & absorption of lipids in intestine

• Emulsification of lipids is dispersion of lipids into smaller droplets due to reduction in surface tension ( increase in surface area of oil droplets)

• Emulsification is essential as lipases act only on surface of lipid droplets

Three mechanism of emulsification of lipids

A- detergent action of bile salts

B –surfactant action of degraded lipids

C –mechanical mixing by peristalsis help in Emulsification –smaller droplets are good substrate for digestion.

Degradation of Tri acyl glycerol by pancreatic lipase

Role of Bile salts in lipid digestion

Bile salts :

• Sodium Glycocholate ( derived from Glycine )& Sodium Taurocholate(derived from Taurine)

• Synthesized from Cholesterol

• Help in emulsification ie interact with lipid particles & aqueous duodenal content & convert into them into smaller particles

• Stabilize smaller particles & prevent them from coalescing

Enzymatic cleavage of dietary fat

Enzymatic cleavage of cholesterol ester

Overview of Digestion of lipids

Mechanism of absorption of lipids

• Mixed micelles serve as the major vehicle for transport of lipid from intestinal lumen to membrane of the intestinal mucosal cells ,(site of lipid absorption)

• The lipid components pass unstirred fluid layer & are absorbed through plasma membrane by diffusion.

• Absorption is complete for free fatty acids& Monoacyl Glycerol(water soluble )

• Micelle formation essential for absorption of fat soluble vitamins A ,D,E&K.

• BILE SALTS SOLUBILIZE DIGESTED LIPIDS IN THE MIXED MICELLES.(SHORT & MEDIUM CHAIN Free fatty acids- FFA NOT DEPENDENT ON MICELLE FOR ABSORPTION )

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Lipid Bilayer

Absorption of lipids by intestinal mucosal cells

Formation and secretion of chylomicrons by intestinal mucosal cells

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Fatty AcidsStearic Acid – C18H36O2 a saturated fatty acid

CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 C

O

OHCH2CH2CH2CH2CH2CH3

Oleic Acid – C18H36O2 a monounsaturated fatty acid

CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH2 C

O

OHCH2CH2CH2CH2CH2CH3

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Fats and Oils: Triglycerides• Fats are solid at room temperature, oils are liquids

• Triglycerides are triesters of glycerol with fatty acids • the bonds that join glycerol to the fatty acids are called

ester linkages

Glycerol

CH2

CH2

CH2

OH

OH

OH

CH2

CH2

CH2

O

O

O

C

C

C

O

O

O

CH2

CH2

CH2

CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

CH2

CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

CH3

CH3

CH3

ester linkage

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Phospholipids

• Esters of glycerol

• Glycerol attached to 2 fatty acids and 1 phosphate group

• Phospholipids have a hydrophilic head due to phosphate group, and a hydrophobic tail from the fatty acid hydrocarbon chain

• part of lipid bilayer found in animal cell membranes

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Phosphatidyl Choline

ABNORMALITIES OF LIPID DIGESTION & ABSORPTION

• Gastro intestinal tract (GIT) handles four times normal intake of lipids.

STEATORRHEA

1. is defect in secretion of bile or pancreatic juice into intestine

2. Impairment in lipid absorption by intestinal cells

3. Associated with disorders pancreas, liver dysfunction ,biliary obstruction

Cholesterol stones—in gall bladder (gall stone) more common in female than male associated with obesity

cholesterol stones—formation takes when liver secretes bile supersaturated with cholesterol

OBESITY & FAT ABSORPTION

Obesity is Overeating (excessive intake )without physical exercise

Pharmacological approaches for obesity

• Orlistat ---non hydrolysable analog of Triglycerol—inhibitor of pancreatic lipase, hence prevent digestion & absorption

• Olestra –synthetic lipid –esterification of natural fatty acid with sucrose– cannot be hydrolyzed & gets excreted.