Vitamin E and Its Metabolic Pathways in Mammals and Birds

Abstract

Vitamin E is a general term for tocopherols and tocotrienols. Originally, vitamin E was considered a dietary factor of animal nutrition, especially important for normal reproduction. Later, since vitamin E was also found to be required for preventing encephalomalacia in chicks, it was recognized as an essential vitamin for maintaining the integrity of biological cell membranes. Currently, vitamin E is primarily known as an antioxidant in scavenging free radicals and thereby reducing cellular oxidative damage. Past studies regarding the functions of vitamin E were mainly mammal studies. Transport of vitamin E relies on the lipoproteins system through the vascular and lympathetic circulation. In birds, however, due to the lack of a functional intestinal lymphatic system, absorbed lipids enter the liver through the portal system where the dietary lipids are assembled and released as very low density lipoproteins (VLDL). The mode of entry of these dietary lipids into the circulation and liver was released as portomicrons for discerning chylomicrons in mammals. Tocopherols and tocotrienols are metabolized by side chain degradation through ω-hydorxylation and β-oxidation. Because of its physiochemical nature as a lipid, vitamin E tends to accumulate in the adipose tissue. Different forms of vitamin E with a lower affinity for the carrier α-tocopherol transfer protein are metabolized and finally excreted in the bile or urine. Many of the metabolic pathways are still unclear. Further studies are needed to clarify these details.