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Lipids are a heterogeneous group of water insoluble organic molecules that can be extracted from tissues by non polar solvents.They are the major source of energy for the body.

Dietary fat Composition

More than 95% are triglycerides, the other are – Cholesteryl esters, phospholipids  and unesterified fatty acids.

Dietary sources of Lipids

a)  Animal Sources

  • Dairy products -Milk , butter, ghee
  • Meat , fish, pork, and eggs

b) Vegetable Sources

  • Cooking oils -Sun flower oil, Mustard oil, Ground nut oil
  • Fats from other vegetable sources.

 Enzymes of lipid digestion

  • Lipases- For the digestion of triglycerides
  • Phospholipase A2- for the digestion of Phospholipids
  • Cholesterol esterase-For the digestion of Cholesteryl esters

Lipases- Three main lipases

  • Lingual
  • Gastric and
  • Pancreatic

 General reaction catalyzed by lipase

Triglycerides are degraded by lipases to form free fatty acids and glycerol. The reaction proceeds in a step wise manner (Figure-1).

 Action of lipase

Figure-1-Triglycerides are degraded by lipases to form free fatty acids and glycerol

A) Digestion in Mouth

Hydrolysis of triacylglycerols is initiated by lingual and gastric lipases, which attack the sn-3 ester bond forming 1,2-diacylglycerols and free fatty acids, aiding emulsification.

Lingual lipase:


  • Secreted by 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.

B) Digestion in Stomach


  • Gastric Lipase is secreted in small quantities
  • More effective at alkaline p H (Average p H 7.8)
  • Requires the presence of Ca++
  • Less effective  in stomach due to acidic pH except when intestinal contents are regurgitated in to the gastric lumen
  • Not effective for long chain fatty acids, most effective for short and medium chain fatty acids
  • Milk, egg yolk and fats containing short chain fatty acids are suitable substrates for its action

 Role of fats in gastric emptying

  • Fats delay the  rate of emptying of stomach
  • Action is brought about by secretion of Enterogastrone
  • Enterogastrone inhibits gastric motility and retards the discharge  of bolus of food from the stomach.
  • Thus fats have a  high satiety value.

Significance of Lingual and Gastric Lipases

  • Play important role in lipid digestion in neonates since milk is the main source of energy
  • Important digestive enzymes in pancreatic insufficiency such as Cystic fibrosis or other pancreatic disorders
  • Lingual and gastric lipases can degrade triglycerides with short and medium chain fatty acids in patients with pancreatic disorders despite a near or complete absence of pancreatic lipase

Emulsification and digestion

  • Lipids are hydrophobic, and thus are poorly soluble in the aqueous environment of the digestive tract. 
  • The digestive enzyme, lipase, is water soluble and can only work at the surface of fat globules. 
  • Digestion is greatly aided by emulsification, the breaking up of fat globules into much smaller emulsion droplets.
  • Triacylglycerol digestion occurs at lipid-water interfaces
  • Rate of TAG digestion depends on surface area of this interface which is increased by churning peristaltic movements of the intestine ,
  • combined with the emulsifying action of bile salts
  • The critical process of emulsification takes place in the duodenum.

C) Digestion in small intestine

  • Major site of fat digestion
  • Effective digestion due to the presence of Pancreatic lipase and bile salts.
  • Bile salts act as effective emulsifying agents for fats (figure-4)
  • Secretion of pancreatic juice is stimulated by-
  • Passage of acid gastric contents in to the duodenum
  • By secretion of Secretin, Cholecystokinin and Pancreozymin, the gastro intestinal hormones

Gastro Intestinal hormones

  • Secretin- Increases the secretion of electrolytes and fluid components of pancreatic juice
  • Pancreozymin of CCK -PZ stimulates the secretion of the pancreatic enzymes
  • Cholecystokinin of CCK-PZ- causes the contraction of the gall bladder and discharges the bile in to the duodenum.
  • Hepatocrinin- Released by intestinal mucosa, stimulates more bile formation which is relatively poor in bile acid content .

Contents of Pancreatic Juice

  • Pancreatic Lipase- For the digestion of triglycerides (figure-5)
  • Cholesterol esterase-For the digestion of Cholesteryl esters (figure-7)
  • Phospholipase A2- for the digestion of Phospholipids (figure-8)

 Role of Bile Salts

  • Bile salts are required for the proper functioning of the pancreatic lipase enzyme
  • They help in combination of  lipase with two molecules of a small protein called as Colipase. This combination enhances the lipase activity.
  • Bile salts also help in the emulsification of fats (figure-4)
  • They are synthesized in the liver and are stored in the gall bladder
  • They are derivatives of cholesterol
  • They consist of a sterol ring structure  with a side chain to which a molecule of glycine or taurine is covalently attached by an amide linkage (figure-2).

 Structure of bile salts

Figure-2- Bile salts are derivatives of cholesterol containing a sterol ring structure  with a side chain to which a molecule of glycine or taurine is covalently attached by an amide linkage.

  • Bile salts are formed from bile acids
  • The primary bile acids are cholic acid (found in the largest amount) and chenodeoxycholic acid .
  • The primary bile acids enter the bile as glycine or taurine conjugates.
  • In the alkaline bile, the bile acids and their conjugates are assumed to be in a salt form—hence the term “bile salts.“

Enterohepatic circulation of Bile salts

The bile salts present in the body are not sufficient to fully process the fats in a typical meal, thus they need to be recycled. This is achieved by the enterohepatic circulation. Specific transporters in the terminal ileum move bile salts from the lumen of the digestive tract to the intestinal capillaries. They are then transported directly to the liver via the hepatic portal vein. Hepatocytes take up bile salts from the blood, and increase the secretion of bile salts into the bile canaliculi, small passage ways that convey bile into the larger bile ducts. 95% of the bile that is released to the small intestine is recycled via the enterohepatic circulation, while 5% of the bile salts are lost in the feces. (Figure-3).

Emulsification by bile salts

Bile salts as emulsifying agents interact with the dietary lipid particles and the aqueous duodenal contents, thereby stabilizing the lipid particles as they become smaller, and preventing them from coalescing (Figure-4) .

Triacyl glycerol degradation by pancreatic lipase (Figure-5)

  • Pancreatic lipase is specific for the hydrolysis of primary ester linkages (Fatty acids present at position 1 and 3)
  • It cannot hydrolyze the ester linkages of position -2
  • Digestion of triglycerides  proceeds by removal of a terminal fatty acid to produce  an α,β diglyceride.
  • The other terminal fatty acid is then removed to produce β mono glyceride.
  • The last fatty acid is linked by secondary ester group, hence cannot be hydrolyzed by pancreatic lipase.
  • β- Mono acyl glycerol can be converted to α- Mono acyl glycerol  by isomerase enzyme and then hydrolyzed by Pancreatic lipase.
  • The primary product of hydrolysis are β- Mono acyl glycerol (78%), α- Mono acyl glycerol (6%) with free fatty acids and glycerol (14%).

 Enterohepatic circulation of bile salts

Figure-3-Enterohepatic circulation of bile salts   

Role of bile salts in emulsification

Figure-4- Role of bile salts in emulsification


 pancreatic lipase

Figure-5- Triglyceride digestion by pancreatic lipase.

Summary  of Emulsification and Digestion of Triglycerides (Figure-6)

 Summary of pancreatic digestion

Figure-6- steps of emulsification and digestion of dietary triglycerides.

Significance of Pancreatic lipase

  • The enzyme is present in high concentration in pancreas. Only very severe pancreatic deficiency such as cystic fibrosis  results in malabsorption of fats due to impaired digestion.
  • Orlistat, an antiobesity drug inhibits , gastric and pancreatic lipases, thereby decreasing fat digestion and absorption resulting in weight loss.

Cholesteryl ester degradation

  • Dietary cholesterol is mainly present in the free (Non esterified) form
  • Only 10-15% is present in the esterified form
  • Cholesteryl esters are hydrolyzed by pancreatic Cholesteryl esterase (Cholesterol ester hydrolase) to produce cholesterol and free fatty acid (figure-7)
  • The enzymatic activity is greatly increased in the presence of bile salts.


 Cholesterol esterase

Figure-7- degradation of cholesteryl esters by cholestrol esterase (cholesterol ester hydrolase)

Phospholipid degradation

  • The enzyme – Phospholipase A 2requires bile salts for optimum activity.
  • Removes one fatty acid from carbon 2 of Phospholipid to form lysophospholipid (figure-8)
  • The remaining fatty acid at position 1  can be removed by lysophospholipase , leaving a glycerylphosphoryl base that may be excreted in the feces, further degraded or absorbed.

 Reaction catalyzed by phospholipase A2

Figure-8- Reaction catalyzed by phospholipase A2.

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