Vitamin E is a collection of phenolic isomers that share the same biological activity, with alpha tocopherol having the most biologically activity of all the isomers. Once vitamin E is absorbed from the gut it is passed to the liver and packaged into lipoproteins via the tocopherol transfer protein (TTP). Alpha tocopherol shows the highest affinity for the tocopherol transfer protein and this explains is superior biological activity in humans. The main transport route for alpha tocopherol in human plasma are lipoproteins, which are packaged with vitamin E in the liver. The distribution of alpha tocopherol within various lipoproteins has been studied and differences in the concentrations found. For example, in one study1 researchers assessed the alpha tocopherol concentrations of human plasma including that in low density lipoproteins (LDL), very low density lipoproteins (VLDL) and high density lipoproteins (HDL). As expected almost all of the vitamin E recovered from the plasma was present in the lipoproteins.
Analysis of male and female samples showed no difference between the two sexes in terms of total alpha tocopherol concentrations in the lipoproteins. In addition, in both men and women the LDL and HDL lipoproteins were the main carriers of alpha tocopherol. However, while in the males subjects most of the alpha tocopherol was found in the LDL fraction, in the female subjects most of the alpha tocopherol was found in the HDL fraction. In males 59 % of the alpha tocopherol was in the LDL fraction, 33 % was in the HDL fraction and 8 % was in the VLDL fraction. In women, 56 % of alpha tocopherol was in the HDL fraction, 42 % was in the LDL fraction and 2 % in the VLDL fraction. When the authors analysed the protein content of the lipoproteins they found that the protein content of the lipoproteins was associated with the amount of alpha tocopherol present in the various lipoprotein fractions.
Therefore the differences in the alpha tocopherol content of lipoproteins in males and females may reflect the differing protein levels. Because females have higher levels of protein in their HDL particles, they also have higher levels of alpha tocopherol. In males, while the LDL particle was able to transport twice the amount of alpha tocopherol compared to the HDL particle, the amount of alpha tocopherol in the LDL particle of male subjects was correlated only weakly with the total protein content. This suggests that the binding of alpha tocopherol to the protein in LDL is relatively non-specific compared to that in the HDL particle in men. Some evidence suggests that the HDL alpha tocopherol fraction might be the most important for the transfer of the vitamin to red blood cells. The VLDL particle actually had the highest capacity to transport alpha tocopherol, based on the amount of protein in the particle. However, as the particle is present in small concentrations its overall contribution to transport was low.
Dr Robert Barrington’s Nutritional Recommendation: While alpha tocopherol is the most biologically active form of vitamin E on account of its high affinity of the tocopherol transfer protein, the other isoforms of vitamin E may have important biological roles. High intakes of alpha tocopherol in isolation can result in suppressed levels of other vitamin E isomers, and the long term effects of this may be detrimental to health. Therefore supplements of vitamin E should contain a mixture of vitamin E isomers (including the tocotrienols). Obtaining vitamin E the diet tends to provide a wide range of vitamin E isomers as long as the diet is of high quality and varied.
RdB
