The document summarizes the digestion and absorption of lipids in the human body. It describes how lipids are broken down by lipases in the mouth, stomach, and small intestine. In the small intestine, bile salts emulsify lipids and pancreatic lipases further digest triglycerides into fatty acids and monoacylglycerols. These products form mixed micelles that are absorbed via enterocytes and re-esterified into triglycerides. The triglycerides are then packaged into chylomicrons and secreted into lymphatic vessels for transport to other tissues. Short and medium chain fatty acids do not require this processing and are absorbed directly into the bloodstream. Abnormalities can occur if pancreatic function is impaired

1. DIGESTION & ABSORPTION
OF LIPIDS
Dr Sana Parveen
MD Biochemistry

2. DIGESTION OF LIPIDS
• Dietary consumption of lipids is variable
across globe based on economic status &
dietary habits.
• An average Indian diet contains 20–50 gm of
lipids per day(western diet-2-3 times more)
• 90% fat intake in the diet is as triacylglycerols,
rest consisting of cholesterol, cholesterol
ester, phospholipid and free fatty acid.

3. Dietary
sources of
lipids
Animal derived foods:
milk,butter,ghee,cheese,meat ,fish
&eggs
Vegetable based foods: various
vegatable cooking oils
(coconut,sunflower,groundnut,palm,rice
bran)

5. Lipid
Digestion
in Mouth
&Stomach
• Very little digestion
• No significant amount of
lipase is present in the
secretion of mouth or
stomach
 No mechanism for
emulsification of lipid exists
here
 Acid ph of gastric secretion
is not helpful to lipid
digestion.

7. • In mouth:
• The lingual lipase (secreted from ebner’s
gland)from the mouth enters stomach along
with the food.
• It has an optimum pH of 2.5-5 (acid stable
lipase)
• it acts on triglycerides with short chain fatty
acids & medium chain fatty acids.

8. • In stomach:
• Gastric lipase is also acid stable, with an optimum
pH around 5.4.
• It is secreted by chief cells; the secretion is
stimulated by gastrin.
• It acts on dietary triacylglycerols mainly composed
of short & medium chain FA.
• The action of lingual & gastric lipase most active in
infants & young children whose stomach ph is
nearer to neutrality & diet contain milk lipids-
short & medium chain FA.

9. Digestion in small Intestines
• The acidic stomach contents containing dietary fat
upon entering the duodenum will stimulate
secretion of enteric hormones secretin &
cholecystokinin.
• Cholecystokinin acts on gall bladder and causes
release of bile into small intestine & also acts on
pancreas causing them to release lipase.
• Secretin stimulates pancreas to release pancreatic
juice rich in bicarbonate which changes ph
necessary for activity of pancreatic & intestinal
enzymes.

10. Emulsification of lipids
• Emulsification is a prerequisite for digestion of
lipids.
• This process is favored by:
1. Bile salts(detergent action)
2. Peristalsis (mechanical mixing)
3. Phospholipids

11. • The bile salts : lower the surface tension causing
dispersion of lipids into smaller droplets exposing
more surface area for enzymatic action.
• This phenomenon is known as emulsification.
• PL (lecithin) constituent of bile act as surfactant
along with partially digested lipid particles and
get absorbed into water lipid interface increasing
the surface area for enzymatic action
• Peristaltic movements of GI tract help in
emulsification by mixing of the dietary lipid
particles.

12. Action of pancreatic Enzymes on
dietary lipids in small Intestines
• The three lipid digestive enzymes secreted by
pancreas are:
• 1. Pancreatic lipase with Co-lipase
• 2. Cholesterol esterase
• 3. Phospholipase A2.

14. Digestion of dietary triacylglycerol
• Emulsified TAG are readily attacked by
pancreatic lipase.
• Pancreatic lipase as such is not functional it
becomes functional by combining to co-lipase.
• The co-lipase is secreted by the pancreas as an
inactive zymogen It is activated by trypsin.
• The binding of co-lipase to the triacylglycerol
molecules at the oil water interface is
obligatory for the action of lipase.

15. • Pancreatic lipase acts on long chain triglycerides.
• Pancreatic lipase can easily hydrolyse the fatty
acids esterified to the 1st and 3rd carbon atoms
of glycerol forming 2-monoacylglycerol and two
molecules of fatty acid.
• Certain quantity of 2MAG is then acted upon by
another enzyme isomerase which shifts the ester
bond from position 2 to 1, then hydrolysed by the
lipase to form free glycerol and fatty acid.
• The major end products of the digestion of TAG
are 2-MAG (78%), 1-MAG (6%), glycerol and fatty
acids (14%).
• Thus digestion of TAG with LCFA is partial.

16. Digestion of cholesterol ester
• Cholesterol esters are hydrolyzed by
pancreatic cholesterol ester hydrolase
(cholesterol esterase), which produces
cholesterol plus free fatty acid.

17. Digestion of phospholipids
• Phospholipases are enzymes which hydrolyze
phospholipids.
• Dietary & biliary origin glycerophospholipids
are digested by pancreatic phospholipase-A2.
• This enzyme catalyzes the hydrolysis of fatty
acid residues at the 2-position of the
phospholipid, leaving lysophospholipids and a
molecule of fatty acid

18. • INTESTINAL LIPASE:
• Lipase produced by ileum which also
hydrolyzes TAG .
• But its action is very limited because TAG are
digested completely by time they reach lower
intestine.

19. Absorption of Long Chain Fatty Acids
• The major end products of lipid digestion include
 2MAG
 FFA
 Cholesterol
 Lysophospholipids
• These, together with bile salts, form mixed micelles.
• Bile salts beside their role in digestion also play crucial
role in absorption.
• Fat soluble vitamins A, D, E and K are also packaged in
these micelles and are absorbed from the micelles along
with the products of dietary lipid digestion

21. Bergstorm
theory
• The theory proposed by Bergstrom
has the following steps.
1. Mixed Micelle Formation
• the end product of lipid digestion
form tiny spherical particles known
as micelles in combination with
bile salts.
• The micelles are organized in a
manner that end products of lipid
digestion occupy hydrophobic
interior core & bile salts take up
exterior
• These are known as mixed micelles
because they poses mixed variety
of lipid molecules.

22. • The mixed micelles approach the brush border
membrane of the intestinal mucosal cells.
• There the lipid components from mixed
micelles are absorbed into the mucosal cells
by diffusion.
• The net result is the transfer of
monoacylglycerol, fatty acids, cholesterol, and
lysophospholipid molecules into the mucosal
cell.
• About 98% of dietary lipids is normally
absorbed.

23. Enterohepatic Circulation of Bile Salts:
• The bile salts are left behind which are mostly
reabsorbed from the ileum and returned to
the liver to be re-excreted (enterohepatic
circulation).

24. 2. Re-esterification Inside the
Mucosal Cell
• inside the intestinal mucosal cell, the long chain
fatty acids are re-esterified to form Triglycerides.
• The fatty acids are first activated to fatty acyl CoA
by the enzyme, acyl CoA synthetase or
thiokinase.
• Two such activated fatty acids react with
monoacylglycerol to form the triglyceride.
• The absorbed lysophospholipids and cholesterol
are also reconverted to phospholipids and
cholesterol esters

25. • Free glycerol absorbed from intestinal lumen
directly enters into the bloodstream.
• But the intestinal cells convert glucose to
glycerol -3-phosphate, and then add 3
molecules of acylgroups to synthesize TAG.

26. 3.Secretion of lipids from intestinal
mucosal cells
• Triacylglycerol, phospholipid, cholesterol
esters resynthesized in the intestinal mucosa
and absorbed fat soluble vitamins along with
apoprotein B-48&apolipoprotein –A I(APO –A)
are are incorporated as chylomicrons.
• chylomicrons migrate to the plasma
membrane of intestinal mucosal cells are
secreted into lymphatic vessels by exocytosis.

27. • The chyle from intestinal cells loaded with
chylomicrons are transported into thoracic
duct and then into blood circulation.
• After a fatty meal, the plasma is milky in
appearance due to the presence of
chylomicrons.
• Blood carries chylomicrons to various
peripheral tissues including muscle and
adipose tissue and finally to liver.
• Adipose tissue take up large quantity of lipids
from chylomicrons for storage.

28. SCFA &MCFA Absorption
• Short chain fatty acids (seen in milk, butter, ghee)
and medium chain fatty acids (in coconut oil and
mother's milk) do not need re-esterification.
• They can directly enter into blood vessels, then to
portal vein, finally to liver where they are
immediately utilized for energy.
• Their absorption is rapid.
• They are better absorbed than long chain fatty
acids

29. Abnormalities in Digestion &
Absorption of Lipids
Defective digestion:
• Daily excretion of fat in feces is more than 6 g
per day is called as steatorrhea.
• It is due to chronic diseases of pancreas.
• In such cases, unsplit fat is seen in feces

30. • Defective absorption: the absorption is defective,
most of the fat in feces are split fat, i.e. fatty acids and
monoglycerides .
• Causes:
1.Celiac disease
2.Surgical removal of intestine.
3.Obstruction of bile duct: due to gall stones, tumors of
head of pancreas, enlarged lymph glands, etc
due to obstruction there is deficiency of bile salts.
In such cases, triglycerides with short chain and medium
chain fatty acids are digested and absorbed properly,
because they do not require micellerisation for
absorption.

31. • Chyluria:
• There is an abnormal connection between the
urinary tract and lymphatic drainage system of
the intestine.
• Urine appears milky due to lipid droplets.
• Chylothorax: can result from an abnormal
connection between the pleural cavity and
thoracic duct

32. References
• Textbook of Biochemistry for medical students-DM Vasudevan
• Textbook of Biochemistry for MBBS-Dr S.K. Gupta
• Harper's Illustrated Biochemistry