It seems that d -tocotrienol is the earliest member of the group to be formed in plants, methylation leading to the other tocotrienols and hydrogenation producing the respective tocopherols. Humans and animals are unable to synthesize vitamin E, they must obtain it from plant sources. Now, owing to the knowledge of the molecular purity and stereochemical composition of preparations of a-tocopherol, an international reference is no longer used.
Vitamin E is mainly present in oils of seeds.
Alfalfa meal, corn and soybean represent the major sources for animals. In human nutrition, the major sources are salad oil, dressings, shortenings and margarines. The vitamin E requirement for human adults is about 15 mg per day a -tocopherol equivalents. It seems possible that these molecules can be used as a measure of the body vitamin E status. The oxidation of a -tocopherol to quinones and related intermediates is of considerable interest in connection with the mode of action of vitamin E. It has been realized, almost since the vitamin was discovered, that its activity is related to its antioxidant properties.