Lipids from the diet and those synthesized in the body are digested and absorbed through a multi-step process. Lingual and gastric lipases initiate digestion in the mouth and stomach. In the small intestine, pancreatic lipases and bile salts emulsify lipids into small droplets to allow further hydrolysis. The products of digestion are incorporated into micelles for absorption by enterocytes. Absorbed lipids are reassembled into chylomicrons and transported via the lymphatic system. Defects in digestion, absorption, or transport can cause conditions like steatorrhea or accumulation of lipids.

1. LIPID
DIGESTION AND ABSORPTION
AnupShamshsher Budhathoki
National Medical College , Birgunj, Nepal

2. Composition different lipids in diet
• Triglycerides (> 90%)
• Cholesterol
• Cholesteryl esters
• Phospholipids, and
• Unesterified fatty
All of the above varieties of lipids are also
synthesized in body

3. Overview of lipid digestion

4. Overview of lipid digestion

5. Summary of digestion and absorption of lipids

6. Lipid digestion in mouth
• Digestion in Mouth
• Hydrolysis of triacylglycerols is initiated by lingual which
attack the sn-3 ester bond forming 1,2-diacylglycerols and
free fatty acids, aiding emulsification.
• Lingual lipase:
• Secreted by Ebner’s gland present on dorsal surface of
tongue
• Active at low pH (pH 2.0 – 7.5)
• optimum pH 4.0-4.5
• Ideal substrate-Short chain TGS.
• Milk fat contains short chain fatty acids which are esterified at
-3 position, thus it is the best substrate for lingual lipase
• Enzymatic action continues in stomach
• Short chain fatty acids, released are absorbed directly from
the stomach wall and enter the portal vein.

7. Lipid digestion in stomach
• Gastric Lipase- secreted by chief cells and secretion is
stimulated by gastrin.
• Hydrolysis of triacylglycerols by gastric lipase attack the
sn-3 ester bond forming 1,2-diacylglycerols and free
fatty acids
• Optimum pH is 5.5 and therefore it is more significant in
neonates
• Up to 30% TGs is digested in stomach
• Ideal substrates: Short and medium chain fatty acid
containing TGs

8. Significance of lingual lipase
• Play important role in lipid digestion in neonates
since milk is the main source of energy and pH of
neonatal stomach also favors the optimal activity of
enzymes
• Important digestive enzymes in pancreatic
insufficiency such as Cystic fibrosis or other
pancreatic disorders

9. Role of emulsification in digestion
• Emulsification is the process of breaking down of
large fat globules into uniform tiny droplets.
• It is accomplished by bile salts and peristalsis
emulsification
• it occurs in duodenum
• Emulsification decreases the surface tension and
increases the surface area of fat globules and
thereby lipolytic enzymes can hydrolyze lipids

10. Emulsification

11. Lipid digestion in small intestine
• Major site of fat digestion
• Effective digestion due to the presence of Pancreatic
enzymes and bile salts.
• Bile salts act as effective emulsifying agents for fats
Secretion of pancreatic juice is stimulated by-
• Passage of acid gastric contents in to the duodenum
• By secretion of Secretin, Cholecystokinin
Pancreozymin (CCK-PZ), the gastro intestinal
hormones

12. Hormonal regulation of digestion

13. Pancreatic enzymes and their catalysis

14. Physiologically important role of
lipases

15. Absorption of lipid
• Lipolytic theory put forth by Verzar:
According to this, fats are completely hydrolysed to
glycerol & FFA
These are absorbed either as soaps or in association with
bile salts
• Partition theory proposed by Frazer:
 digestion of TAGs is partial & not complete
 partially digested TAGs in association with bile salts,
form emulsion droplets
 Lipids are taken up by the intestinal mucosal cells

16. Absorption of Lipids
• Bergstrom theory:
Long chain fatty acids (>14 carbons) are
absorbed to the
lymph & not directly to the blood
• Mixed Micelle Formation:
The products of digestion are 2-
monoglycerides, long chain
fatty acids, cholesterol, phospholipids
and lysophospholipids
are incorporated into molecular
aggregates to form mixed
Micelle
Micelles are spherical particles with a
hydrophilic exterior &
hydrophobic interior core

17. Absorption of Lipid
• micelle formation is important of absorption of lipid
contents in intestine
• MAGs, free fatty acids , phospholipids, cholesterol are
absorbed passively and to some extent actively in
enterocytes of duodenum and jejunum
• Medium and short chain fatty acids are directly enter
portal circulation and are transported to liver
• Long chain fatty acids are activated and esterified with
MAGs ,cholesterol and phospholipids and are packed in
chylomicrons.

18. Absorption of Lipid

19. Re-synthesis of lipids in enterocytes

20. Transport of dietary lipids via
chylomicrons

21. Clinical conditions
• Defective synthesis- Due to deficiency of apo-B 48 protein.
The triglyceride may accumulate in intestinal cells.
• Defective digestion: In steatorrhea, daily excretion of fat in feces is more
than 6 g per day. (Greek word, "stear", means fat).
• Defective absorption: Celiac disease, surgical removal of intestine and
obstruction in bile duct
Chyluria- Due to an abnormal connection between urinary tract& lymphatic
drainage system of the intestines, Characterized by passage of Milky
urine.
Chylothorax -can result from an abnormal connection between the pleural
cavity and thoracic duct. It is characterized by accumulation of lymph in
pleural cavity