the esters (e.g., α-tocopheryl acetate or α-tocopheryl succinate) are hydrolyzed and absorbed as efficiently as α-tocopherol (Cheeseman et al., 1995).
Before 1980, for pharmacological uses, one international unit (IU) of vitamin E activity was defined as 1 mg of all rac-α-tocopheryl acetate by the United States Pharmacopeia (USP) (USP, 1979). Using the rat fetal resorption assay, 1 mg of RRR-α-tocopherol was calculated to be equivalent to 1.49 IU of vitamin E (Weiser and Vecchi, 1981).
After 1980, the IU was changed to the USP unit where one USP unit of vitamin E was still defined as having the activity of 1 mg of all rac-α-tocopheryl acetate, 0.67 mg RRR-α-tocopherol, or 0.74 mg RRR-α-tocopheryl acetate (USP, 1980). Although IUs are no longer recognized, many fortified foods and supplements still retain this terminology while USP units are now generally used by the pharmaceutical industry in labeling vitamin E supplements. Both systems are based on the same equivalency.
Since the USP unit was defined before studies were published indicating that the 2S-stereoisomers of all rac-α-tocopherol were not maintained in human plasma (Acuff et al., 1994; Kiyose et al., 1997: Traber, 1999) or in tissues (Burton et al., 1998), it is recommended that the present equivalency used in the USP system be redefined based on the definition presented in this report of what contributes to the active form of vitamin E in humans. Vitamin E is defined here as limited to the 2R-stereoisomeric forms of α-tocopherol (RRR-, RSR-, RRS-, and RSS-α-tocopherol) to establish recommended intakes. Based on this definition, all rac-α-tocopherol has one-half the activity of RRR-α-tocopherol found in foods or present with the other 2R stereoisomeric forms (RSR-, RRS- and RSS-) of α-tocopherol in fortified foods and supplements. Thus to achieve the RDA recommended in this report of 15 mg/day of α-tocopherol, a person can consume 15 mg/day of RRR-α-tocopherol or 15 mg/day of the 2R-stereoisomeric forms of α-tocopherol (e.g., 30 mg/day of all rac-α-tocopherol) or a combination of the two. The factors necessary to convert RRR- and all rac-α-tocopherol and their esters based on this new definition of vitamin E to USP units (IUs) are shown in Table 6-1.