The compounds are comprised of a 6-chromanol ring and an isoprenoid side chain. Compounds having saturated side chains are classified as tocols. The second class of compounds, known as tocotrienols (trienols), have unsaturated side chains (9). The groups attached to the R1, R2 and R3 positions on the 6-chromanol ring determine whether the vitamer is identified as alpha, beta, gamma, or delta. A large body of the research currently focuses on the alpha tocopherol form of vitamin E, which is the most biologically active (31,32). Recently gamma tocopherol has been a topic of interest by many researchers.
However, those studies fall outside of the scope of this review and will not be addressed. Discussion of vitamin E within the current literature review is limited to the alpha tocopherol form of the vitamer, unless otherwise noted.
Vitamin E is integral part of cellular membranes whose main role is to defend the cell against oxidation. Within cells and organelles (e.g. mitochondria) vitamin E is the first line of defense against lipid peroxidation. The vitamin also plays a very important function in lending red blood cells (RBC) flexibility as they make their way through the arterial network. Vitamin E has not yet been shown to have any significant functions outside of these two roles.
Synthetic vitamin E, designated dl-alpha-tocopherol, is the less expensive cousin of the naturally occurring form, d-alpha tocopherol. The natural form of the vitamin is synthesized only by plants and is found predominantly in plant
oils. Vitamin E (tocopherol) is also present in high amounts within the chloroplast and therefore the leaves of most plants. In contrast, the tocotrienols are synthesized and found in the germ and bran sections of the plant (9). The fat-soluble property of vitamin E allows it to be stored within fatty tissue of animals and humans. Therefore a diet that includes meat supplies additional vitamin E. However, the amount of vitamin E obtained in a meat inclusive diet is less than the amount supplied by plant sources.