Digestion of fat

digestion of fats made brief in the pptx file and hope it will help to cover whole topic

1. 11
The Digestion of FatsThe Digestion of Fats

2. 22
Digestion Digestion

HydrolysisHydrolysis of the fat is of the fat is initiatedinitiated by the by the lingual lingual
lipaselipase (Ebner’s glands) & by the (Ebner’s glands) & by the gastric lipasegastric lipase..

Up to 30%Up to 30% hydrolysis of ingested fats takes hydrolysis of ingested fats takes
place in place in stomachstomach..

The fat entering the The fat entering the duodenumduodenum is still is still mostly mostly
un-hydrolyzedun-hydrolyzed. .

One One significant changesignificant change in duodenum is the in duodenum is the
emulsificationemulsification of fats due to the of fats due to the shearing shearing
actionaction of gastric contractions. of gastric contractions.

3. 33

Further emulsificationFurther emulsification occurs in the duodenum occurs in the duodenum
due the presence of due the presence of bile saltsbile salts. .

The The pancreaticpancreatic juice contains a very powerful juice contains a very powerful
lipase (lipase (Pancreatic lipasePancreatic lipase) it brings about ) it brings about
hydrolysis of hydrolysis of triacyltriacylglycerol & glycerol & diacyldiacylglycerols glycerols
to to 2-monoacylglycerols & FAs2-monoacylglycerols & FAs. .

4. 44

2-monoglyceride isomerize to 1-2-monoglyceride isomerize to 1-
monoglyceride & hydrolyzed to monoglyceride & hydrolyzed to gylcerol & gylcerol &
fatty acidfatty acid. .

The released glycerol The released glycerol enters the enterocytesenters the enterocytes
from which it is transported out into the from which it is transported out into the portal portal
veinvein. .

The The fatty acidsfatty acids in the intestinal lumen exist as in the intestinal lumen exist as
salts (salts (soapssoaps), because they are neutralized by ), because they are neutralized by
sodium bicarbonatesodium bicarbonate present in the pancreatic present in the pancreatic
juice. juice.

5. 55
The absorption of digested fatsThe absorption of digested fats

Normally about 95% of the fat taken in the Normally about 95% of the fat taken in the
diet is absorbed. diet is absorbed.

The absorption of fats from the The absorption of fats from the intestinal intestinal
lumenlumen to to lactealslacteals occurs in four stages. occurs in four stages.

1-1- Formation of micelles within the intestinal Formation of micelles within the intestinal
lumen:lumen:

The The ratioratio of 2-monoacylglycerol to 1- of 2-monoacylglycerol to 1-
monoacylglycerol is monoacylglycerol is quite highquite high in the in the intestinal intestinal
lumenlumen may be up-to may be up-to 12:112:1..

6. 66

The products of The products of fatfat digestion except glycerol digestion except glycerol
now now form micellesform micelles with the help of with the help of bile saltbile salt..

The bile salts are The bile salts are ampholytesampholytes, i.e molecules , i.e molecules
with a with a polar grouppolar group at the end of a at the end of a long non-long non-
polarpolar hydrocarbon hydrocarbon chainchain. .
These substances when These substances when mixed with watermixed with water to to
arrange into arrange into small sphericalsmall spherical called called micellesmicelles, ,
the the polarpolar groups are arranged to groups are arranged to face the waterface the water
phase, while phase, while non-polarnon-polar tails are directed to tails are directed to
towards the centertowards the center. .

7. 77

Micelles are formed when Micelles are formed when bile salt concbile salt conc is is
minimumminimum (2millimolar).(2millimolar).

The fat digestion products i.e. The fat digestion products i.e. monoglyceridesmonoglycerides
& & FAsFAs are held are held withinwithin the the central non-polarcentral non-polar
region of micelles – stabilized micelles region of micelles – stabilized micelles
(0.5mm). (0.5mm).

CholesterolCholesterol & & fat soluble vitaminsfat soluble vitamins are also are also
incorporated in the center. incorporated in the center.

8. 88
MicellesMicelles
polar
nonpolar
examples:examples:
monomer unit:
simple micelle:simple micelle: contains only one substance contains only one substance
mixed micelle:mixed micelle: more than one substance more than one substance
↔↔
33

9. 99

Further hydrolysisFurther hydrolysis of fats occurs of fats occurs within within
micellesmicelles due to the presence of due to the presence of lipaselipase. .

Micellar formation Micellar formation facilitatefacilitate the the lipids to be
transportedtransported to the to the enterocytesenterocytes. .

10. 1010

2-2- Absorption of products of fat digestion Absorption of products of fat digestion
from the intestinal lumen into the from the intestinal lumen into the
enterocytes.enterocytes.

The micelles are The micelles are not absorbed intactnot absorbed intact but only but only
bringbring the FAs & monoglycerides the FAs & monoglycerides near the near the
brush borderbrush border of the enterocytes. of the enterocytes.

Here the lipidsHere the lipids diffuse outdiffuse out of the micelles & of the micelles &
are readily are readily transported acrosstransported across the cell the cell
membrane mostly by membrane mostly by passive diffusion.passive diffusion.

11. 1111

Fat absorptionFat absorption occurs mainly in the occurs mainly in the proximal proximal
partpart of the small intestine ( of the small intestine (jejunumjejunum). ).

The bile salts giving rise to the The bile salts giving rise to the micellesmicelles are are
turned backturned back to the lumen of the gut to be to the lumen of the gut to be
reusedreused in the formation of in the formation of new micellesnew micelles. .

12. 1212

3-3- Changes within the enterocytes – formation Changes within the enterocytes – formation
of chylomicrons:of chylomicrons:

The enterocytes are not only take part in The enterocytes are not only take part in
absorbingabsorbing & & transportingtransporting lipids but also lipids but also
actively involved in actively involved in lipid metabolismlipid metabolism - -
resynthesisresynthesis of triglycerides from of triglycerides from
monoglycerides, glecerol & free FAs.monoglycerides, glecerol & free FAs.

This causes a fallThis causes a fall in the conc of FAs & in the conc of FAs &
monoglycerides within the enterocytes & monoglycerides within the enterocytes &
favorsfavors continued continued absroption absroption of these into the of these into the
enterocytes. enterocytes.

13. 1313

ResynthesisResynthesis of triglycerides takes place by the of triglycerides takes place by the
acylationacylation of absorbed monoglycerides. of absorbed monoglycerides.

Fatty acids neededFatty acids needed for the synthesis of for the synthesis of
triacylglycerol are either from triacylglycerol are either from intestinal lumenintestinal lumen
or by or by hydrolysishydrolysis of monoacylglycerols. of monoacylglycerols.

After the synthesis of triglycerides in the After the synthesis of triglycerides in the
mucosal cells, mucosal cells, an aggregationan aggregation of of triglyceridestriglycerides
takes place; takes place; somesome absorbed absorbed cholesterolcholesterol is also is also
esterifiedesterified with FAs. with FAs.

14. 1414

These These aggregatesaggregates are then are then coatedcoated with with
cholesterol , phospholipids & apolipoprotein cholesterol , phospholipids & apolipoprotein
synthesized within enterocytes. synthesized within enterocytes.

The The whole complexwhole complex is called a is called a chylomicronchylomicron. .

They They containcontain about 80% about 80% triglyceridestriglycerides, 10% , 10%
cholesterolcholesterol, 7% , 7% phospholipidsphospholipids & 2% & 2% proteinprotein. .

15. 1515
Chylomicron structureChylomicron structure

ApoA, ApoB, ApoC, ApoEApoA, ApoB, ApoC, ApoE ( (apolipoproteinsapolipoproteins); ); TT ( (triacylglyceroltriacylglycerol); );
CC ( (cholesterolcholesterol); green (); green (phospholipidsphospholipids))

16. 1616

4-4- Secretion of chylomicron from the Secretion of chylomicron from the
enterocytes into lacteals.enterocytes into lacteals.

The chylomicrons (within intestinal The chylomicrons (within intestinal
epithelium) are now epithelium) are now secretedsecreted into the into the central central
lactealslacteals by exocytosis, from where they pass by exocytosis, from where they pass
into the into the lymphaticslymphatics to be taken to the to be taken to the blood blood
streamstream. .

CollectionCollection of chylomicrons in large numbers of chylomicrons in large numbers
gives rise to gives rise to a milky fluida milky fluid called called chylechyle in the in the
lymphatics. lymphatics.

17. 1717

Tissues likeTissues like adipose tissue, muscle cells, heart, adipose tissue, muscle cells, heart,
lungs, kidney & liver possess an enzyme lungs, kidney & liver possess an enzyme
lipoprotein-lipaselipoprotein-lipase that that releasesreleases FAs from the FAs from the
chyolmicronchyolmicron which are which are takentaken up by up by tissue cellstissue cells
& chylomicrons & chylomicrons left behindleft behind are called are called
chylomicron remnantschylomicron remnants..


18. 1818

19. 1919

Lacteals = a Lacteals = a lymphatic capillarylymphatic capillary that absorbs dietary fats in the that absorbs dietary fats in the
villi of the small intestine, relating to milkvilli of the small intestine, relating to milk

Acylation = addition of acyl group (RCO-) Acylation = addition of acyl group (RCO-)

Enterocyte = Enterocytes, or intestinal absorptive cells, are Enterocyte = Enterocytes, or intestinal absorptive cells, are
simple columnare simple columnar epithelial cells found in the simple columnare simple columnar epithelial cells found in the
small intestines and colon.small intestines and colon.

Mushtaq ahmed volume IMushtaq ahmed volume I