Vitamin E is a name given to eight isomer compounds that have the same activity as α-tocopherol. These are α, β, γ, and δ-tocopherol and α , β, γ, and δ-tocotrienol. The vitamin E isomers are known to be in vivo antioxidants that reside mainly in the cell membranes where they protect fatty acids from free radical damage by reducing carbon centred lipid radicals and peroxyl radicals. While the vitamin E isomers share the antioxidant activity of α-tocopherol, there is mounting evidence that the different isomers have distinct but overlapping functions within humans. For example some evidence suggests that subjects with cardiovascular disease have similar α-tocopherol levels to healthy subjects, but lower levels of γ-tocopherol. Recent dietary changes in the predominant oils consumed in Western nations is altering tissue and plasma profiles of the tocopherols.
In a cross sectional survey of 88 elderly people, researchers1 measured the α-tocopherol and γ-tocopherol levels of individuals. The results showed that those individuals with the highest levels of α-tocopherol also had the lowest levels of γ-tocopherol. The researchers then took 8 health volunteers and supplemented their diet with 1200 IU per day of α-tocopherol (synthetic all-rac-α-tocopherol) for 8 weeks. After 8 weeks the β-tocopherol and γ-tocopherol levels decreased in all subjects. Typically, α-tocopherol plasma levels increased by between 200 to 400 % while γ-tocopherol plasma levels decreased 30 to 50 %. The authors concluded that competition for intestinal uptake or transport on the tocopherol binding protein may have accounted for the ability of α-tocopherol to displace γ-tocopherol. Further, the α-tocopherol to γ-tocopherol ratio is a more useful measure of vitamin E status than α-tocopherol alone.
These results are interesting because they suggest that the vitamin E isomers compete for absorption or transport. Western diets have increasing levels of soya oil in recent years which is a good source of α-tocopherol. Supplementation of Swedish women with corn and sesame oil raised plasma levels of γ-tocopherol. Most dietary vitamin E supplements contain mainly or exclusively α-tocopherol. The use of pure α-tocopherol in some studies investigating the effects of vitamin E on cardiovascular health may explain the lack of positive findings. Evidence suggests that higher levels of γ-tocopherol are required for beneficial effects on the cardiovascular system. It is therefore recommended that vitamin E supplements contain a mixture of all the vitamin E isomers (including the tocotrienols) in order to maintain a more natural ratio of mixed isomers in cell membranes and plasma.
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