Lipids undergo a three phase process for digestion and absorption. In the preparatory phase, bile salts emulsify lipids in the small intestine. Pancreatic lipase then breaks triglycerides into fatty acids and monoglycerides. These products are absorbed into intestinal cells by simple diffusion and reassembled into triglycerides for transport through the lymphatic system and bloodstream. Bile salts and pancreatic enzymes play crucial roles in emulsifying and breaking down lipids to allow for absorption within the small intestine.

2.  DIGESTION AND ABSORPTION OF LIPID
 By Asmat Ali
 Lecturer in Botany
 Gc Peshawar

3.  Organic substances relatively insoluble in water, soluble in
organic solvents ( alcohol, ether etc), actually or potentially
related to fatty acids and utilized by the living cells
 They are not polymer unlike proteins and nucleic acid
 They are mostly small molecules
 A class of compounds distinguished by their insolubility in
water and solubility in non-polar solvent.

4. • Triglyceride is the simplest lipid.
• consist of one glycerol and three fatty acid.
H2C------OH
HC-------OH + 3 R----COOH
H2C------OH

5. A short view of lipids in biological system is that, they help in;
 Providing mechanical barrier by formation of cell membrane.
 Several essential vitamins are lipids.
 Provide energy for life.
 Also form essential hormone like testosterone, steroid, etc.
 Building block

6.  Like carbohydrate and protein lipids are also broken into small
component for absorption.
 Poses a special problem because of the insolubility of fats in
water and because of lipolytic enzyme (these are a number of
enzyme present in the pancreatic juice).

7.  These are solved in the gut by emulsification
(a process of dispersion of lipid into small droplet by reducing
surface tension.) , particularly by bile salt present in bile and
pancreatic lipase.
 With the emulsification the rate of digestion is proportionally
increase.

8.  The whole process of digestion of dietary lipids and its
subsequent absorption may be arbitrarily divided into three
phases;
1. Preparatory phase
2. Transport phase
3. Transportation phase

9.  During the preparatory phase , the overall digestive system of
body prepare to digest and absorb the lipid in the body.
 Its digestion is divided on the basis of different parts of the
body
i. Digestion in mouth and stomach
ii. Digestion in small intestine

10.  It was believed that little or no fat digestion take place in the
mouth, but recently a lipase has been detected called lingual
lipase which is secreted by the dorsal surface of the tongue (
Ebner’s gland)

11.  Its pH is 2.0-7.5
 Its activity is continued in stomach, and also where the pH
value is low. Due to retention of food bolus for 2-3 hours,
about 30% dietary TG( Triacyl glycerol)
 More active on TG and more specific for ester linkage at
position-3 rather than position-1.
 Milk fat contain short and medium fatty acid . So milk fat
appears to be the best substrate for this enzyme.

12.  More soluble, this short fatty acid and can be absorbed directly
from the stomach wall and enter the portal vein.
 After the action of lingual lipase it pass to the stomach and the
action of gastric lipase start.

13.  There is evidence of presence of small amount of gastric lipase
in gastric secretion.
 The overall digestion of fats , brought about by gastric lipase
is negligible because;
 No emulsification occur.
 Small quantity of enzyme.
 It need ca++.
 pH (not conductive which is highly acidic)

14.  Liquid substance released by lever from gall bladder to main
pancreatic duct and then goes to duodenum.
 It contain bile salt.
 Store in the gall bladder and when necessary move to the
duodenum.
 The bile salt break down the large lipid molecule into smaller
through emulsification process.

15.  They are not enzymes but there are some special compounds
which helps in digestion and absorption of lipid.
 Phasphoglycerolipids (emulsification).
 Bile acid (colic acid) which help to prevent the enzymatic
activity (reverse) and form micelles.

16.  Some fatty acid which are not absorbed into the cell freely,
aggregate with bile salt, cholesterol and lipid soluble
substances to create a substance called micelles.

17.  Small intestine is the major site of fat digestion. Its due to the
presence of;
 Lipase ( Steapsin).
 Bile salts.
 These two inter the intestine through pancreatic and bile ducts
to duodenum.

18.  For the stimulation of pancreatic juice
secretion is
 Passage of an acid gastric contents (acid chyme) into
duodenum.
 By secretion of GI hormones, secretin and CCK_PZ.

19.  The function of secretion in the small intestine is to increase
the secretion of electrolytes and fluid component of pancreatic
juice.
HEPATOCRININ:
 Function of Hepatocrinin is to released by the intestinal
mucosa stimulates more bile formation which is relatively
poor in the bile salt content.

20.  To stimulate the secretion of pancreatic enzyme.
cholecystokinin of CCK-PZ:
 To cause contraction of the gallbladder and discharge of bile is
also stimulated by secretin and bile salt themselves.
the above sequence of events prepares the small intestine
for the digestion of fats.

21.  These are a number of enzyme present in the pancreatic juice.
 Pancreatic lipase
 Phospholipase A2
 Cholesterol esterase.
Pancreatic lipase is most important which hydrolyze TG containing
short and long chain fatty acid.

22.  TG hydrolyze through lipase to remove one fatty acid at the
one terminal to form α,β-diglyceride.
 At the other step the other fatty acid is removed and α,β-
diglycerid is converted to β-diglycerid.
 the middle fatty acid is not easily removable so first it will
convert into 1 or 3 position through isomarase and then
remove to form glycerol and fatty acid.

23.  The product of preparatory phase ( fatty acid and
monoglyceride) enter the microvilli( enterosytes cell )and
apical pole of absorptive epithelial cells by “simple diffusion”
through cell membrane.
 in this phase short , medium and unsaturated fatty acid are
more absorbed than the long chain fatty acid.
 The product of fat digestion next appear to be taken up by the
SER and resynthesised into TG again by enzymatic action.
.

24.  The rapid removal of product and their synthesis into TG in
intestinal epithelial cell, maintains a sharp gradient of
concentration with in the mucosal cell that favors the
continued rapid diffusion into the cell from intestinal lumen.

25. • Sequence of events that occur inside the intestinal mucosal
cell are; with in the intestinal cell, α-monoglycerid are further
hydrolyzed by intestinal lipase to produce free fatty acid and
glycerol.
• Intestinal lipase: A lipase distinct from that of the pancreatic
lipase is present in intestinal mucosal cell.

26. • Fatty acid absorbed from intestinal lumen
and fatty acid formed from hydrolysis of
α-monoglycerid are activated to “Acyl-CoA”. An
ATP-dependent thiokinase .

27.  Pancreatic juice contain a type of enzyme called phaspholipase
A2( Lecithinase ).
 It is an esterase, and secreted as in inactive zymogene
proenzyme, which is changed to active form, by hydrolysis of
peptide molecule with the help of trypsin.

28.  In the presence of bile salt and Ca++, the active phaspholipase
A2 hydrolyses the ester linkage between fatty acid and
secondary alcohol group of position-2 of glycerol in
phospholipids molecule so that free fatty acid and
lysophospholipid are formed and are absorbed.

29.  Pancreatic juice contain an enzyme cholesterol esterase, which
may either catalyze the estrification of free cholesterol with
fatty acid or it may catalyze opposite reaction.
 The cholesterol appears to be absorbed from the intestine
almost entirely in free form.

30.  Nevertheless , 85 to 90% of cholesterol in the lymph is found
to be in estrified form, indicating that estrification of
cholesterol must take place with in the mucosal cell of
intestine.
 Absorption of cholesterol has been reported to be facilitated
by presence of unsaturated fatty acid and bile are necessary
for absorption of cholesterol.
 Certain plant sterol like sitosterol and stigmasterol are not
absorbed, rather their presence can inhibit cholesterol
absorption.