Explain the Absorption and Transport of Vitamin E?
Absorption of vitamin E from the intestine depends on adequate pancreatic function, biliary secretion and micelle formation. Conditions for absorption are like those for dietary lipid, that is, efficient emulsification, solubilisation within mixed bile salt micelles, uptake by enterocytes, and secretion into the circulation via the lymphatic system. Emulsification takes place initially in the stomach and then in the small intestine in the presence of pancreatic and biliary secretions. The resulting mixed micelle aggregates the vitamin E molecules, solubilises the vitamin E, and then transports it to the brush border membrane of the enterocyte, probably by passive diffusion. Within the enterocyte, tocopherol is incorporated into chyloinicrons and secreted into the intracellular space and lymphatic system and subsequently into the blood stream. Tocopherol esters, present in processed foods and vitamin supplements, must be hydrolyzed in the small intestine before absorption.
Vitamin E is transported in the blood by the plasma lipoproteins and erythrocytes. Chylomicrons carry tocopherol from the enterocyte to the liver, where they are incorporated into parenchymal cells as Chylomicrons remnants. The catabolism of Chylomicrons takes place in the systemic circulation through the action of cellular lipoprotein, lipase. During this process, tocopherol can be transferred to high-density lipoproteins (HDLs). The tocopherol in HDLs call transfer to other circulating lipoproteins, such as low-density lipoprotein (LDL) and very low- density lipoproteins (VLDLs). During the conversion of VLDL to LDL in the circulation, some a-tocopherol remains within the core lipids and is thus incorporated in LDL. Most a-tocopherol then enters the cells of peripheral tissues within the intact lipoprotein through the LDL receptor pathway, although some may be taken up by membrane binding sites recognizing apolipoprotein ( A-I and A-II) present on HDL.
Although the process of absorption of all the tocopherol homologues in the diet is similar, the a-form predominates in blood and tissue. From a nutritional perspective, the most important form of vitamin E is a-tocopherol. It is absorbed faster and retained better than other forms. This is due to the action of binding proteins that preferentially select the form over other forms.