Vitamin E is part of a family of eight compounds, tocopherols and tocotreinols with their variants, alpha, beta, gamma and delta. These compounds have a basic chromanol ring (chroman ring with an alcoholic hydroxyl group) and a 12-carbon aliphatic side chain containing methyl groups. These structures can be modified spatially, and this property of organic compounds to have different structures but same composition is called isomerism.

Alpha-tocopherol has two isomers, d-alpha and l-alpha, the structures like that of the two hands, hence the letters d and l, meaning dextro (right in Greek) and levo (left in Greek). Natural vitamin E is made of d- forms, and synthetic ones are modified spatially to form l-forms, and they contain both d and l, often referred as dl-forms.

The difference between the natural and synthetic forms is underlined by the fact that the body recognizes only the d form. Also the l form inhibits the d form from entering the cell membranes. Hence, the synthetic form should be avoided.

Bioavailability is the amount of a compound or substance available for the biological functions of the body; the less the number, the more the wastage of the substance. Since the d form is the isomer preferred by the body, its bioavailability is higher than that of l form. Therefore, natural vitamin E supplements, which have the d-form of tocopherols, have greater bioavailability than the synthetic forms, which have all the isomers, i.e. the d and l forms of alpha, beta, gamma and delta. Natural vitamin E is twice as potent as the synthetic form, for instance 67 milligram (mg) of alpha-tocopherol is equivalent to 45 mg of dl alpha-tocopherol.

According to the research done in the National Research Council of Canada, the preference of d form over l form is shown by higher presence of vitamin E in brain, lungs, heart and muscles. Hence, it is advised to take natural vitamin E, even though it costs twice as much as synthetic vitamin E.