It is now known that vitamin E forms are not interconvertible in the human and that their plasma concentrations are dependent on the affinity of hepatic α-tocopherol transfer protein (α-TTP) for them (see section on “Hepatic α-Tocopherol Transfer Protein”). Kinetic studies have shown that while RRR-α-tocopherol concentrations are maintained in human plasma, the same is not true for either synthetic SRR-α-tocopherol or natural γ-tocopherol (Traber et al., 1990a, 1992). These compounds are efficiently absorbed and delivered to the liver in chylomicrons but are packaged poorly into newly secreted lipoproteins for delivery to peripheral tissues (see section on “Preferential Secretion of α-Tocopherol from the Liver”). In light of these new findings in humans, it becomes necessary to reevaluate the relative biological potencies of different forms of vitamin E. Therefore, it is best to measure and report the actual concentrations of each of the various vitamin E forms in food and biological samples.
Current information suggests that the number of methyl groups and the stereochemistry of the phytyl tail at the point where it meets the chromanol ring (2 position) determine the affinity of the α-TTP for the vitamin E form and that this protein in turn determines the effective vitamin E biological activity (Hosomi et al., 1997). Since the 2S-stereoisomers (Figure 6-2) are not maintained in human plasma or in tissues, the difference in relative activity of all rac-α-tocopherol compared to RRR-α-tocopherol is 50 percent as demonstrated in Figure 6-3.
Vitamin E activity in food is often reported as α-tocopherol equivalents (α-TE) (Bieri and Evarts, 1973, 1974; Eitenmiller and Landen, 1995) as have been dietary recommendations (NRC, 1989). Previously, factors for the conversion of the tocopherols and tocotrienols to α-TE units were based on the biological activity of the various forms as determined using the rat fetal resorption assay (Bieri and McKenna, 1981). α-TEs were defined as α-tocopherol, mg × 1.0; β-tocopherol, mg × 0.5; γ-tocopherol, mg × 0.1; δ-tocopherol, mg × 0.03; α-tocotrienol, mg × 0.3; and β-tocotrienol, mg × 0.05 (NRC, 1989). The biological activities of γ- and δ-tocotrienol were below detection.
Based on a review of the data, the 2R-stereoisomeric forms of α-tocopherol (RRR-, RSR-, RRS-, and RSS-α-tocopherol) are now used to estimate the vitamin E requirement. The 2S-stereoisomeric forms of α-tocopherol and the other tocopherols (β-, γ-,and δ-tocopherol) and the tocotrienols are not used to estimate the vitamin E require-