from most recent recommendations. A large and growing body of experimental evidence suggests that high intakes of vitamin E may lower the risk of some chronic diseases, especially heart disease. However, the limited and discordant clinical trial evidence available precludes recommendations at this time of higher vitamin E intakes to reduce chronic disease risk. The Tolerable Upper Intake Level (UL) for adults is set at 1,000 mg (2,325 µmol)/day of any form of supplemental α-tocopherol based on the adverse effect of increased tendency to hemorrhage.
Of the eight naturally occurring forms of vitamin E (see section on “Naturally Occurring Forms” and Figure 6-1) only the α-tocopherol form of the vitamin is maintained in human plasma (Traber, 1999). Furthermore, the only forms of α-tocopherol that are maintained in plasma are RRR-α-tocopherol [2,5,7,8-tetramethyl-2R-(4′R, 8′R, 12′ trimethyltridecyl)-6-chromanol], the form of α-tocopherol that occurs naturally in foods, and the 2R-stereoisomeric forms of α-tocopherol (RRR-, RSR-, RRS-, and RSS-α-tocopherol) present in synthetic all racemic- (all rac-) α-tocopherol [2,5,7,8-tetramethyl-2RS-(4′RS, 8′RS, 12′ trimethyltridecyl)-6-chromanol (Traber, 1999) (Figure 6-2). Since the 2S-stereoisomers of α-tocopherol (SRR-, SSR-, SRS-, and SSS-α-tocopherol), part of the synthetic all rac-α-tocopherol, are not maintained in human plasma (Acuff et al., 1994; Kiyose et al., 1997; Traber, 1999) or tissues (Burton et al., 1998), they are not included in the definition of active components of vitamin E for humans. Therefore, vitamin E is defined in this report as limited to the 2R-stereoisomeric forms of α-tocopherol to establish recommended intakes. All forms of supplemental α-tocopherol are used as the basis of establishing the Tolerable Upper Intake Level (UL) for vitamin E. These recommended intakes and ULs are at variance with past definitions and recommendations for vitamin E (NRC, 1989).
Naturally occurring structures (Figure 6-1) classified in the past as having vitamin E antioxidant activity include 4 tocopherols (α-tocopherol, trimethyl [3 methyl groups on the chromanol ring]; β-