Major sources of vitamin E in diet (‘RRR-alpha-tocopherol’, also referred to as ’natural‘ or ‘d-alpha-tocopherol’) include vegetable oils (olive, sunflower, and safflower oils), nuts, whole grains, and green leafy vegetables. All eight forms of vitamin E (alpha-, beta-, gamma-, and delta-tocopherols and tocotrienols) occur naturally in foods but in varying amounts.
Many scientists believe it is difficult for an individual to consume more than 15 mg/day alpha-tocopherol from food (RRR-alpha-tocopherol) alone without increasing fat intake above recommended levels (61).
Vitamin E supplements generally contain 100 IU to 1,000 IU alpha-tocopherol. While supplements made from entirely natural sources contain only RRR-alpha-tocopherol, synthetic alpha-tocopherol (‘all-rac-alpha-tocopherol’ or ‘dl-alpha-tocopherol’) is often found in fortified foods and nutritional supplements. All-rac-alpha-tocopherol is a mixture of eight forms (‘isomers’) of alpha-tocopherol; some of these isomers are less available to the body than the natural alpha-tocopherol. Therefore, all-rac-alpha-tocopherol is defined to be slightly less biologically active than RRR-alpha-tocopherol (~26% less, see below); a revision of this definition is currently being discussed.
Alpha-tocopherol supplements are available in two ester forms, ‘alpha-tocopheryl succinate’ and ‘alpha-tocopheryl acetate’, which are more resistant to oxidation during storage than unesterified tocopherols. The bioavailability of alpha-tocopherol from alpha-tocopheryl succinate and alpha-tocopheryl acetate is equivalent to that of free alpha-tocopherol.
There is currently no published evidence that supplements containing ‘alpha-tocopheryl phosphates’ are more efficiently absorbed or have greater bioavailability in humans than supplements containing alpha-tocopherol (71).
The relative biological activity of different vitamin E forms has been defined in International Units (IU). The United States Pharmacopeia (USP) currently defines 1 IU = 1mg of all-rac-alpha-tocopheryl acetate. There are currently efforts to replace IU and express the vitamin E content in foods, food supplements and fortified foods in ‘mg alpha-Tocopherol Equivalents’ (alpha-TE), where 1 mg RRR-alpha-tocopherol = 1 alpha-TE. The conversions are summarized in the table below as defined by European and US health authorities.
| IU/mg | 1 alpha-TE in | |
| RRR-alpha-tocopherol | 1.49 | 1.00 mg |
| All-rac-alpha-tocopherol | 1.10 | 1.36 mg |
| RRR-alpha-tocopheryl acetate | 1.36 | 1.10 mg |
| RRR-alpha-tocopheryl succinate | 1.21 | 1.23 mg |
| All-rac-alpha-tocopheryl acetate | 1.00 | 1.49 mg |
| All-rac-alpha-tocopheryl succinate | 0.89 | 1.67 mg |