Food Chemistry and Fish in Nutrition

4.1.3.Absorption

The fatty acids, glycerol and monogycerides, in these droplets are absorbed by intestinal cells, where they are largely reassembled into triglycerides. The free fatty acids are activated by thiokinase in the presence of coenzyme A and ATP for the resynthesis of triglyceride. Some free glycerol passes directly to the lymp vessel. The others will be activated by glycerokinase in the presence of ATP to form glycerol 3 phosphate and combine with acyl CoA to form triglycerides. All the long chain fatty acids present are reincorporated into the triglycerides. The triglycerides do not pass into the blood capillaries but into the small lymph vessels in the villi. The choline from phospholipids may be absorbed and send to liver via lymph vessels. Cholesterol is absorbed into the lymphatic vessels and converted into cholesterol esters and transported.

Chylomicrons: The lymph draining the small intestine, called chyle, has a milky appearance after a fat-rich meal, due to the suspended chylomicrons, droplets of highly emulsified triglycerides, about 1µm in diameter. Chylomicrons contain triglycerides, free and esterified cholesterol; have a hydrophilic coat of phospholipids and a special protein, which function to keep the chylomicrons suspended. The chylomicrons pass from the thoracic duct into the subclavian vein and then to liver.

Emulsification: The emulsification and digestion of lipids in the small intestine is facilitated by the bile salts. The major human bile salts are sodium glycocholate and sodium taurocholate, derivative of cholic acid, the most abundant of four major human bile acids. The bile salts are powerful emulsifying agents secreted by the liver into the bile, which empties into the upper portion of the small intestine. After the fatty acids and monoglycerides of the emulsified fat droplets have been absorbed in the lower small intestine, the bile salts aiding this process are also reabsorbed. They return to the liver, to be used over again.