The names of all types of vitamin E begin with either "d" or "dl," which refer to differences in chemical structure. The "d" form is natural (also known as RRR-alpha tocopherol) and "dl" is synthetic (more correctly known as all-rac-alpha tocopherol). The natural form is more active and better absorbed. Little is known about how the "unnatural" "l" portion of the synthetic "dl" form affects the body, though no clear toxicity has been discovered.

In theory, when a vitamin E supplement is labeled "400 IU" it should have the same level of activity regardless of its source. This is purportedly achieved by using more synthetic vitamin E to reach the same potency as a lesser amount of natural vitamin E. For example, 100 IU of vitamin E requires about 67 mg of the natural form but closer to 100 mg of the synthetic. However, a recent review of the scientific evidence suggests that natural vitamin E probably has greater activity in the body than indicated on the label.⁴⁴³ Natural vitamin E may be as much as twice as bioavailable as synthetic vitamin E, not 1.36 times as is generally accepted.⁴⁴⁴ Many doctors advise people to use only the natural, the "d" form, of vitamin E.

After the "d" or "dl" designation, often the Greek letter "alpha" appears, which also describes the structure. Synthetic "dl" vitamin E is found only in the alpha form--as in "dl-alpha tocopherol." Natural vitamin E may be found either as alpha--as in "d-alpha tocopherol"--or in combination with beta, gamma, and delta, labeled "mixed"--as in mixed natural tocopherols.

Little is known about the importance of the beta and delta forms of vitamin E, but a debate has arisen concerning gamma tocopherol. In a test tube study, gamma tocopherol was found to be more effective than alpha tocopherol in protecting against certain specific types of oxidative damage.⁴⁴⁵ In addition, some research has shown that supplementation with large amounts of alpha tocopherol (such as 1,200 IU per day) increases the breakdown, and decreases blood levels, of gamma tocopherol.⁴⁴⁶

Human trials with vitamin E have almost always been done with the alpha (not gamma) form. Historically the synthetic "dl" form was used in most trials, but some trials are now using the natural form. The issue of alpha vs. gamma form requires more research before it can be fully understood.

Almost all vitamin E research shows that, when positive results are obtained, hundreds of units per day are required--an amount easily obtained with supplements but impossible with food. Therefore, switching to food sources, as suggested by some researchers, is impractical. On the other hand, the vitamin E occurring naturally in food contains gamma tocopherol and other tocopherols. Thus, it possibly may turn out to be more effective than the vitamin E taken in supplement form. Additional research is needed in this area.

Vitamin E forms are listed as either plain "tocopherol" or tocopheryl followed by the name of what is attached to it, as in "tocopheryl acetate." The two forms are not greatly different. However, plain tocopherol may be absorbed a little better, while tocopheryl attached forms have a slightly better shelf life. Both forms are active when taken by mouth. However, the skin utilizes the tocopheryl forms very slowly,^{447, 448} so those planning to apply vitamin E to the skin should buy plain tocopherol. In health food stores, the most common forms of vitamin E are d-alpha tocopherol and d-alpha tocopheryl acetate or succinate. Both of these d (natural) alpha forms are frequently recommended by doctors. Although the succinate form is slightly weaker than the acetate form, more milligrams of the succinate form are added to supplements to compensate for this small difference in potency. Therefore, 400 IU of either form should have equivalent potency.