convertible in the human and thus do not behave the same metabolically.

Synthetic forms of α-tocopherol are present in fortified foods and in vitamin supplements. Vitamin E supplements are sold as esters of either the natural RRR- or the synthetic mixture (all rac-) forms of α-tocopherol. Because α-tocopherol has three asymmetric carbon atoms, it has eight possible stereoisomers, seven of which are only found in synthetic preparations. Synthetic vitamin E, all rac-α-tocopherol (historically and incorrectly labeled dl-α-tocopherol) (Horwitt, 1976),¹ is produced by coupling trimethylhydroquinone with isophytol; it contains all eight stereoisomers in equal amounts (Figure 6-2). Four of the stereoisomers are in the 2R-stereoisomeric form (RRR-, RSR-, RRS-, and RSS-α-tocopherol) and four are in the 2S-stereoisomeric form (SRR- SSR-, SRS-, and SSS-α-tocopherol). Although RRR-α-tocopherol is the most biologically active of the eight stereoisomers in rats, the other 2R-stereoisomers generally have a higher activity than the 2S stereoisomers (Weiser and Vecchi, 1982; Weiser et al., 1986).

The naturally occurring stereoisomer is RRR-α-tocopherol (historically and incorrectly labeled d-α-tocopherol) (Horwitt, 1976). RRR-α-Tocopherol can be derived by methylating γ-tocopherol isolated from vegetable oil. This is labeled “natural source” vitamin E when marketed.

Esterification of the labile hydroxyl (OH) group on the chromanol ring of vitamin E prevents its oxidation and extends its shelf life. This is why esters of α-tocopherol are often used in vitamin E supplements and in fortified foods. In apparently healthy humans,