Digestion and absorption of lipids

1. DIGESTION AND
ABSORPTION OF
NUTRITIONAL
CONSTITUENTS
Dr Ifat Ara Begum
Assistant Professor
Dept Of Biochemistry
Dhaka Medical College
Dhaka

2. DAY 3
Digestion and
absorption of lipids

3. DIETARY LIPIDS
Average adult intake: 60-150
gm/Day
90% is TAG
Remainder consists primarily of
free cholesterol, cholesteryl ester,
PL & FFA

4. DIETARY SOURCES OF LIPIDS
Animal source: Dairy products
(milk, butter, ghee, etc), meat, fish,
eggs.
Vegetable source: Cooking oils
(sunflower/mustard oil, etc), fats
from other vegetable sources

5. PHYSIOLOGICALLY IMPORTANT
LIPASES
Enzyme Source Substrate Product
Lingual lipase Lingual
glands
Dietary TAG Diglyceride +
FFA
Gastric lipase Stomach Dietary TAG Diglyceride +
FFA
Pancreatic
lipase
Pancreas Dietary and
other TAG &
DAG
2-MG + FFA
Hepatic lipase Hepatocyte TAG of IDL,
LDL & HDL
Glycerol
+FFA

6. CONTD
Enzyme Source Substrate Product
Acid lipase Most tissues TAG of LP in
phagocytes
Glycerol
+FFA
HSL Adipocytes TAG stored in
adipose cell
Glycerol+ FFA
LPL Extra
hepatic
tissues
TAG of CM &
VLDL
Glycerol+ FFA
Cholesterol
esterase
Pancreas Cholesterol
ester
Cholesterol
+FA
Phospholipase
A2
Pancreas PL LysoPL+FA

7. A) DIGESTION IN MOUTH
No digestion of fat occurs here.
Lingual lipase can’t start lipid
digestion in mouth due to shorter
stay of food in mouth and
differences in optimum pH for the
enzyme function (pH of saliva 7.0
but its opt pH is 4.0-4.5)

8. B) DIGESTION IN STOMACH
Lipid digestion begins in stomach
by acid stable lingual lipase
As gastric lipase is active only at
near neutral pH (which is true
only in neonates in infants), it is of
little use in adult.

9. CONTD
Lingual lipase:
 Secreted by dorsal surface of tongue
 Active at low pH
 Optimum pH: 4.0-4.5
 Ideal substrate: TAG with short chain
FA (e.g. milk fat)
 Rate of digestion is slow because fat is
not yet emulsified
 Short chain FAs are released as end
products of digestion, which are
absorbed directly from stomach wall and
enters portal vein.

10. CONTD
Gastric lipase:
 Secreted in small quantity
 Effective at alkaline pH
 Optimum pH: 7.8
 Less effective in stomach due to
acidic pH except when intestinal
contents are regurgitated in to gastric
lumen.
 Substrate: TAG with short chain FA
( e.g. milk fat)

11. CONTD
In spite of limitations, lingual
lipase and gastric lipase account
for about 30% of total fat digestion.
Play important role in lipid
digestion in neonates as milk is the
main source of their energy.
These lipases are esp. important
for fat digestion in patients with
pancreatic insufficiency (e.g. cystic
fibrosis) with near /complete
absence of pancreatic lipases.

12. C) DIGESTION IN SMALL
INTESTINE
Major site of fat digestion
Effective digestion due to presence
of bile salts (via emulsification)
and pancreatic lipase
Bile salts act as effective
emulsifying agents for fat.

13. I)EMULSIFICATION OF LIPIDS IN
DUODENUM
The breaking up of fat globules in to
much smaller emulsion droplets.
This process increases the surface
area of fat/oil, so that digestive
enzymes can act effectively.
2 mechanisms:
I. Reduction of surface tension of fat
droplets by detergent action of bile
salts & lecithin
II.Mechanical mixing by gut peristalsis

16. II) ACTION OF PANCREATIC
LIPASE
Secretion of pancreatic juice is
stimulated by:
 Passage of acidic gastric contents
in to the duodenum
 By secretion of secretin & CCK

17. CONTD
Pancreatic lipase with the help of
colipase digests TAG further.
It removes FA from C1 & C3 of
glycerol and produces 2-MAG (2-
monoglyceride) & 2 FFA (major
end product of TGL digestion)

18. CONTD
Primary end product: 2-MAG &
glycerol
2-MAG represent 70% of total end
product
FA & glycerol together represent
30% of total end product.

19. CONTD
Bile salts are required for proper
functioning of pancreatic lipase
enzyme
Bile salts help in combination of
lipase with 2 molecules of small
protein called as colipase . This
combination enhances the lipase
activity

20. CONTD
An isomerase shift the ester bond
from position 2 to 1
In other words, isomerization of
some of the 2-MAG occurs to
produce 1-MAG
1-MAG is then hydrolyzed to
glycerol & FFA (action of
pancreatic lipase)

22. CONTD
10-15% of total dietary cholesterol
is in ester form which is digested
by pancreatic cholesterol esterase
to form cholesterol & FA.
Bile salt enhance the activity of
cholesterol esterase

24. CONTD
Dietary PL is digested by
pancreatic phospholipase A2.
Phospholipase A2 removes FA
from C2 of glycerol moiety and
produce lysoPL.
Bile salt enhance the activity of
phospholipase A2 enzyme.

26. END PRODUCT OF LIPID
DIGESTION
FA
Glycerol
2-MAG
Cholesterol
LysoPL
All these are of amphipathic nature
having hydrophilic and
hydrophobic group

27. ABSORPTION OF LIPID
Short & medium chain FA (chain
length less than 14) and glycerol
are directly absorbed from the
intestinal lumen in to the portal
vein & taken to liver for further
utilization.
Long chain FA, cholesterol, 2-MAG
& lysoPL together interact with
bile salts & form micelle.

28. I) MICELLE FORMATION
Disk shaped clusters of
amphipathic lipids that coalesce
with their hydrophobic groups on
the inside and their hydrophilic
groups on the outside of clusters

29. CONTD
Mixed micelles are soluble in the
aqueous environment of the
intestinal lumen.
Micelle approach the brush border
membrane of the enterocytes
Micelles continuous break down
and reform
It is the monoglycerides & FA that
are free in solution , are absorbed
by diffusion. Micelles ferry them to
the intestinal mucosa

30. II) EVENTS INSIDE ENTEROCYTES
Fatty acids and monoglycerides
are converted back into
triglycerides . TG aggregates with
the cholesterol, protein & PL to
form chylomicrons
Chylomicrons move in to a lymph
capillary which transports them to
the rest of the body

32. ABSORPTION AT A GLANCE

33. SUMMARY