Digestion and absorption of lipids

1. Digestion and Absorption of Lipids

2.  SOURCES OF LIPIDS
 Exogenous Sources: Lipids obtained through diet,
• TAG
• Phospholipids
• Cholesterol
• Free fatty acids
• Fat soluble vitamins – 10%
 Endogenous Source : Lipids which are synthesized in body,
• Fatty acids, TAG, Cholesterol,
• Cholesterol esters, Phospholipids

3.  Digestion of Lipids
 Lipids are insoluble in aqueous solution.
 The digestive enzymes are present in aq. medium
 This leads certain problem in digestion & absorption of lipids overcomes
by……
• Increasing the surface area of lipids for digestion.
• Emulsification of lipids by bile salts.
• Solubilizing the digested product for absorption.

4.  Lipid 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
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.

5.  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 “short chain TG ” 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

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

7.  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 the 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

8.  Contents of Pancreatic Juice
 Pancreatic Lipase-
For the digestion of triglycerides
 Phospholipase A2-
for the digestion of Phospholipids
 Cholesterol esterase-
For the digestion of Cholesteryl esters

9.  Overview of lipid digestion

10.  Absorption of lipid
 • 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.

11.  Re-synthesis of lipids in enterocytes
 Most fat absorption takes place in the duodenum or jejunum –
micelles carry monoglycerides and free fatty acids to the brush border
where they diffuse into enterocytes
 Bile salts are absorbed in the ileum (enterohepatic circulation)
 Once in the enterocytes, monoglycerides and free fatty acids are
reformed into triglycerides
 The triglycerides, cholesterol, phospholipids, and protein carriers form
LIPOPROTEIN

12.  Transport of dietary lipids via chylomicrons
 Once these lipoproteins leave the cell, they become CHYLOMICRONS and
enter the lymph system
 MCTs, short-chain fatty acids and glycerol are absorbed directly into
bloodstream. They do not enter the lymph system.
 Cholesterol and other sterols are poorly absorbed. Overall, about 50% of
dietary cholesterol is absorbed.
 Dietary fat increases cholesterol absorption
 Fiber (especially soluble fiber) and phytosterols decrease cholesterol
absorption

13.  Overview of lipid absorption of lipids

14.  Summary of digestion and absorption of lipids

16.  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.
 • 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