Vitamin E is a group of structurally related isomers that are required for human health. The need for vitamin E stems from its role as the primary lipid soluble antioxidant in cell membranes. Here it protects the delicate fatty acids in the membranes of cells from free radical damage and thus helps maintain correct cell function. In its role as a cell membrane antioxidant, vitamin E may protect from cardiovascular disease.
Oxidation of lipids is thought to play a role in cardiovascular disease. In particular, free radical stress may occur in the walls of arteries and this may lead to endothelial dysfunction. In addition, lipoprotein particles are surrounded by cell membranes and these may become oxidises and thus disease forming. By protecting these cells membranes vitamin E may confer protection from cardiovascular disease.
Not All Created Equally
Vitamin E therefore provides antioxidant protection to cells and may decrease the risk of cardiovascular disease. However, not all vitamin E is created equally. Vitamin E is actually a group of four isomers, alpha, beta, gamma and delta tocopherol and alpha, beta, gamma and delta tocotrienols, all of which may have different physiological effects. For example, the tocotrienols are more portent antioxidants and may confer better protection than the tocopherols.
Alpha tocopherol is the preferential form of vitamin E transported and stored in the body. This is because of the way the liver processes vitamin E. Following absorption, vitamin E is transferred to very low density lipoproteins for transport to the peripheral tissues. This transfer of vitamin E to the lipoproteins is performed by the tocopherol transfer protein. Because the tocopherol transfer protein preferentially recognises alpha tocopherol, this is the main form of the vitamin in the body.
Research On Vitamin E
Because alpha tocopherol is found in most supplements, and it the predominate form in the plasma, most research has focussed on alpha tocopherols. The results from some studies do show beneficial effects of vitamin E at preventing cardiovascular disease, but the results are inconsistent and therefore controversial. The reason for the inconsistent results have been analysed, and some conclusions have been drawn from this analysis.
Synthetic Vitamin E
Vitamin E can be extracted from plant oils and added to supplements, or synthesised artificially. Metabolic studies show that due to a lower affinity for the tocopherol transfer protein, synthetic vitamin E is only absorbed at around half the efficiency as natural vitamin E. This means that 400 IU of natural vitamin E is around twice as potent a an equivalent amount of synthetic vitamin E. Studies using synthetic vitamin E may therefore by using de facto lower dose.
Another problem with some studies is that they use a single form of vitamin E. However, in nature vitamin E is found as a number of isomers. High intakes of alpha tocopherol, because it is preferentially absorbed, inhibit the absorption of other forms of vitamin E and this lowers their tissue concentrations. Because some of the other forms of vitamin E may have unique beneficial cardioprotective roles, high intakes of just alpha tocopherol may be problematic.