Alpha-tocopherol is one of 8 isomer that makes up the vitamin E group of essential nutrients. All isomers share the same biological activity as α-tocopherol, and α-tocopherol is considered the most biologically active form of the vitamin. The higher biological activity for α-tocopherol is not fully understood but may relate to the higher affinity that the isomer has for the tocopherol binding protein (TBP). The TBP is involved with the transfer of vitamin E absorbed from the diet to lipoproteins in the liver. This ensures that vitamin E can be distributed to the tissues. Levels of vitamin E in the plasma tend to rise more efficiently after consumption of α-tocopherol compared to other isomers because of this higher affinity. Biological activity is also determined by excretion rates and clearance from the plasma is one area of excretion that has an important influence of the bioavailability of vitamin E to the tissues. A number of studies have investigated the plasma clearance of α-tocopherol.
For example one group of researchers1 used radiolabeled collard green as a source of vitamin E for a group of subjects. Following consumption of the greens the α-tocopherol plasma levels were measured over time. The results of the study showed that vitamin E levels in the plasma correlated with plasma lipid levels, as has been shown previously. This is understandable as plasma lipids are the main transport route for vitamin E and as lipid levels rise, α-tocopherol levels would also be expected to rise. Unsurprisingly, the half lives of lipids was also associated with the half lives of α-tocopherol. Another interesting finding was that the radiolabelled vitamin E rose higher in older subjects, and this may reflect the higher lipid concentrations associated with aging. The radiolabelled vitamin E detected in plasma was not predicted by the plasma vitamin E status of the subject suggesting that plasma levels are not a good predictor of the amount of absorption of vitamin E.
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