Two main groups of compounds have vitamin E activity – tocopherols and tocotrienol. There are 4 isomers of each: α-, β-, γ- and į- tocopherols; and α-, β-, γ- and į-tocotrienols. Each has differing vitamin potency. Vitamin E functions primarily as an antioxidant in cell membranes, protecting unsaturated fatty acids from oxidative damage. Vitamin E contents of foods are expressed as mg α- tocopherol equivalent; leafy vegetables, seeds and most vegetable oils are good sources.