Vitamin E is a group of compounds that share the same biological activity as alpha-tocopherol. The biological role of vitamin E is thought to be primarily as an antioxidant, and in this regard vitamin E concentrates in cells membranes where it protects them from the effects of free radicals. However, other roles have not been discounted. The vitamin E group of chemicals is actually made up of eight isomers; alpha-, beta-, gamma- and delta-tocopherol and alpha-, beta-, gamma- and delta-tocopherol. These are the natural forms of vitamin E found in plant material, where they are usually associated with polyunsaturated oils. These natural molecules of vitamin E are in the dextral orientation (which means they rotate plane polarized light to the right (R)). However, the technicalities are not important if it is understood that the dextral form of vitamin E is the form recognised by the body and used biologically. Plants contain a range of vitamin E isomers but alpha and gamma-tocopherol make up most of the human intake.
The chemistry of vitamin E is important to understand because vitamin E can also be synthesised in the laboratory. However, the chemical reactions used to synthesise vitamin E in the laboratory are not as selective as the enzymatic pathways that synthesise vitamin E in plants. As a result some (roughly half) of the molecule of vitamin E that result from chemical reactions are in the sinistral orientation (which means they rotate plane polarised light to the left (L)). This sinistral form of vitamin E is not recognised by the body and so any ingested L forms of vitamin E are not used but metabolised and excreted. A further complication in the chemistry of vitamin E is added by the addition (conjugation) of a chemical compound to the molecule to allow it to be added to tablets as a powder. Normally vitamin E is an oil and is produced into capsules with soybean oil for consumption. However, addition of a succinate or acetate molecule allows the vitamin E to be dried and powdered and this alters its absorption characteristics somewhat.
Studies have assessed the absorption of the different forms of vitamin E in order to ascertain which form might serve best as a supplement to the human diet. For example, in one study1, researchers compared the blood levels of alpha-tocopherol after consumption of different forms of vitamin E preparations. The absorption of the different preparations of vitamin E were tested on healthy human subjects after they had been fed a vitamin E depleting diet for a number of days. The forms of vitamin E analysed were natural alpha tocopherol (RRR-α-tocopherol), powdered synthetic alpha tocopherol in its succinate form (all-rac-α-tocopheryl succinate), powdered synthetic alpha tocopherol in its acetate form (all-rac-α-tocopheryl acetate), powdered natural alpha tocopherol in its acetate form (RRR-α-tocopheryl-acetate) and powdered natural alpha tocopherol acetate with an apple pectin supplement. The intake of the compounds was adjusted so that the vitamin E content of the diet was the same.
The increases in the plasma levels of vitamin E were 71.2 % after natural alpha tocopherol, 63.2 % after natural alpha tocopherol acetate, 60.9 % after natural alpha tocopherol and apple pectin, 31.6 % following synthetic alpha tocopherol acetate and 41.2 % following synthetic alpha tocopherol succinate. These results show that of the compounds tested, natural alpha tocopherol is the most bioavailable form, something confirmed in other studies. This relates to the requirement for transfer of dietary vitamin E to lipoproteins, a process that occurs in the liver. This process involves the tocopherol transfer protein (TTP), and this protein has a high affinity for the natural R-alpha tocopherol, but does not recognise the synthetic L-form. As a result compounds containing R-alpha tocopherol more easily raise blood levels of vitamin E. From these results it can also be seen that the succinate form of vitamin E is superior to the acetate form, and that apple pectin inhibits the rise in vitamin E when accompanying tocopheryl acetate.
Dr Robert Barrington’s Nutritional Recommendation: The natural RRR-form of vitamin E in its tocopherol form is the most biologically active form of vitamin E. However, in nature vitamin E is present as eight isomers. Evidence suggests that consuming high intakes of only alpha tocopherol may inhibit the absorption of other isomers, particularly the biologically important gamma-tocopherol. As other isomers may have unique biological roles efforts should be made to obtain vitamin E as close to its natural form as possible. This means that if supplements are to be taken, they should contain mixed tocopherols, and if possible, mixed tocotrienols, all in their R- or natural form.
RdB
