Vitamin E is not a single vitamin, but a group of vitamers. In fact vitamin E is composed of the structurally related isomers α-, β-, γ- and δ-tocopherol amd α-, β-, γ-, and δ-tocotrienol. All of the vitamin E vitamers share the same biological activity as α-tocopherol, the main biologically active molecule. The complexity of the vitamin E compounds means that plasma levels are often difficult to judge without a detailed analysis of each individual isomer. In addition, plasma levels of vitamin E can be affected by other factors such as the level of lipoproteins, because it is in these that vitamin E is transported. Measuring plasma levels of vitamin is therefore problematic. Because vitamin E and its quinone oxidation product may inhibit the aggregation of platelets, an understanding of the platelet levels of various fractions of the vitamer may be nutritionally more relevant than plasma levels. Pharmacological doses of vitamin E have been used in this regard to reduce the risk of myocardial infarction, with some degree of success.
In one study1, researchers assessed the platelet concentration of various vitamin E vitamers. As with plasma, the α-tocopherol and γ-tocopherol isomers accounted for nearly all the tocopherol content of platelets. As expected the α-tocopherol content of platelets was 4 fold higher than the γ-tocopherol content (similar to plasma). As has been shown previously the α-tocopherol and γ-tocopherol concentration in plasma correlated well with the total lipid, cholesterol and triglyceride concentrations in plasma. However, the platelet concentrations of vitamin E were independent from the plasma lipid concentrations. This suggests that the platelet content of vitamin E might be a better biomarker for vitamin E nutriture than plasma levels. This may to some extent explain discrepancies in results from studies investigating the effects of vitamin E on myocardial infarction. This is because studies that measure plasma levels may not be reliable in their estimate of the vitamin E status of the subjects.
Dr Robert Barrington’s Nutritional Comments: The chemistry of vitamin E is highly complex. The main biologically active isomer of vitamin E is α-tocopherol. This is because the tocopherol transfer protein that is required to package the vitamin E into lipoproteins preferentially recognises this isomer. However, this does not mean that other isomers are not important. In fact the other isomers have been shown in some cases to have unique effects. Because food contains a range of vitamin E isomers, any supplements should reflect this pattern. Measuring the vitamin E status of an individual is more complex than a simple measurement of the plasma levels, and such an oversimplification may explain some of the discrepancies seen in vitamin E studies.
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