itamin E comprises of eight isomers; α-, β-, γ- and δ-tocopherol and α-, β-, γ- and δ-tocotrienol. The tocopherols are the more abundant vitamers in the diet with α- and γ-tocopherol contributing the most to intake. It is known that α-tocopherol is present in human plasma at higher concentrations than the other isomers. While this could be assumed to be related to the dietary intake, evidence suggests that α-tocopherol is the preferred form of the vitamin retained in humans. This is demonstrated by the fact that high intakes of α-tocopherol decrease plasma concentrations of γ-tocopherol. This phenomenon has lead some to speculate that the inconsistent findings in studies investigating the use of high concentrations of α-tocopherol in cardiovascular disease, may be caused by the inadvertent reduction of plasma levels of γ-tocopherol. This argues for a balanced intake of all tocopherols, if high intakes of supplements are to be used.
The preference in humans to retain α-tocopherol in the plasma does not appear to be due to the selective absorbance of one isomer over the other. This was demonstrated by researchers1 in a study in 1989, where subjects were fed 1000mg of D-α-tocopherol or 1000 mg D-γ-tocopherol. When the subjects had their lipoprotein vitamin E levels measured 12 hours later, equal concentrations of both tocopherols were present, suggesting that both had been absorbed and incorporated into lipoproteins in equal concentrations. However, when the same measurements were taken at 24 hours following ingestion, the concentrations of γ-tocopherol had decrease significantly in lipoproteins, but that of α-tocopherol had not. Therefore the intestinal barrier cannot be the location of the preferential selection of the α- versus the γ-form of the vitamin. Other mechanisms therefore must explain the differences.
When the same authors gave subjects with a genetic defect of lipoprotein lipase both α- and γ-tocopherol, they found that both isomers at similar concentrations in lipoproteins 24 hours later. When vitamin E is absorbed in chylomicrons, the lipoprotein lipase catabolises the particle and the chylomicron remnants are taken up by the liver along with their vitamin E content. The lack of lipoprotein lipase prevented the uptake of the chylomicron remnants by the liver and allowed the chylomicrons to stay in circulation longer. Therefore by removing the processing by the liver, no difference was seen between the two tocopherol isomers. This suggests that it is the liver that preferentially selects the α-isomer of tocopherol for incorporation into hepatically produced lipoproteins. This probably occurs through preferential selection of D-α-tocopherol by the tocopherol transfer protein (TTP). Studies from gallbladder patients suggest that the γ-tocopherol is preferentially secreted in the bile.
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