Lipid digestion and absorption

Digestion and absorption of lipid(fat), National medical college bigunj,Nepal

1. LIPID DIGESTION AND ABSORPTION Anup Shamshsher 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 M edium 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