Digestion and absorption of lipid

Digestion of fat:

1)     Limited processing of dietary lipids in mouth and stomach

a)      Lipid digestion begins in stomach by acid stable lingual lipase. Although it is a component of saliva, lingual lipase cannot start lipid digestion in mouth because food stays in mouth for a brief period. TAG with short chain fatty acid is the best substrate for lingual lipase, however the rate of digestion is slow because fat is not yet emulsified.

b)     TAG with short chain fatty acid also get hydrolyzed by gastric lipase.

c)      In spite of limitations, lingual lipase and gastric lipase account for about 30% fat digestion.

2)     Emulsification of dietary lipids in duodenum.

a)      Emulsification is the process by which large fat particles is broken down into smaller fine particle, increasing the surface area of fat, so that digestive enzymes which work at the interface of the fat droplet and the surrounding aqueous media can perform effectively.

3)     Enzymatic degradation of lipids by pancreatic enzymes.

a)      Pancreatic lipase with the help of colipase digest TAG removing fatty acid from 1^st carbon and 3^rd carbon of glycerol and produces 2-monoacylglycerol.

b)     Some of the 2MAG isomerizes to 1MAG which is then digested by pancreatic lipase to fatty acid and glycerol.

c)      Therefore primary end product of TAG digestion are fatty acid, 2MAG and glycerol.

d)       Most dietary cholesterol is present in free form with only 10-15% in ester. Dietary cholesterol ester is digested by pancreatic cholesterol esterase to fatty acid and free cholesterol.

e)      Dietary phospholipid is digested by pancreatic phospholipase A2 removing fatty acid from 2^nd carbon of glycerol moiety and produces phospholipid.

f)       Theredore final end product of lipid digestion is fatty acid, glycerol, 2-MAG, cholesterol and phospholipid.

Absorption of lipid:

a)      Short chain fatty acid and glycerol are directly absorbed to portal blood.

b)     Long chain fatty acid, cholesterol, 2-MAG and lysophospholipid together interact with bile salts to form globular water soluble molecular aggregates called michelle with their hydrophobic parts interior and hydrophilic parts exterior. These fat michelle absorbed to enterocytes by simple diffusion. After absorption of lipids, bile salt come back to lumen to form michelle again and repeat the same process.

Within the enterocyte 2-MAG is again acylated to TAG lysophospholipid and most of the cholesterol are reacylated to PL and cholesterol ester. TAG, cholesterol ester, free cholesterol and phospholipid are packaged into globular chylomicron. From the enterocyte chylomicron is finally secreted to lymphocytes by the process of exocytosis. From lymphatics through thoracic duct chylomicron is eventually absorbed to blood.