Natural vitamin E includes four tocopherols, called alpha, beta, gamma and delta tocopherol respectively. The form used in the human body is alpha tocopherol, and this form is necessary to treat vitamin E deficiency. Only this form is bound by the liver transport protein called alpha-tocopherol transport protein, which carries it to the site where it is incorporated into lipoproteins and carried to other parts of the body.

Functional activity of alpha-tocopherol

The function of alpha tocopherol is as a fat-soluble antioxidant, which breaks the chain of oxidation reactions which are propagating down a cell membrane or plasma protein by means of newly produced free radicals.

All cell membranes contain fats, and these are easily oxidized by free radical attack through the process of lipid peroxidation. This chain is broken by alpha-tocopherol which is a thousand times more susceptible to reaction with the peroxyl radicals than unsaturated fatty acids.

This causes the inactivation of the peroxyl radicals, while alpha-tocopherol itself becomes oxidized and loses its anti-oxidant activity. Vitamin C can regenerate the activity of alpha-tocopherol after it is oxidized.  

This action of alpha-tocopherol is essential to the protection of the lipid bilayer of all cell membranes, as well as enzymes and membrane proteins.

It also strengthens cell-mediated immunity in several ways. However, this form of vitamin E does not seem to have a protective effect on chronic diseases such as cardiovascular disease, cancer or cataracts. There is some evidence that it slows the progression of age-related macular degeneration and was beneficial in the management of diabetes type 2, as well as non-alcoholic liver disease.

Chemistry of tocopherols

Tocopherols are made up of a chromanol ring and a 15-carbon saturated isoprenoid tail. Natural and synthetic alpha tocopherol are different in their composition. The natural form is made up of a molecule with methyl (-CH3) groups at the 2, 4’ and 8’ positions on the chromanol ring.

The synthetic form is made up of eight stereoisomers, all with identical antioxidant activity in vitro. However, only four of them have the methyl ring at position 2, meaning that chemically synthesized alpha-tocopherol has 50% the biological activity of the natural form. Alpha-tocopherol esters in supplements are chiefly succinate and acetate, and these are converted to alpha-tocopherol in the intestine before absorption.

Sources of tocopherols

Tocopherols are found in almost all plant seeds, the best sources of alpha-tocopherol include plant seeds such as almonds, sunflower seeds and hazelnuts. Olive and canola oil are also good sources, as are tomato, avocado and spinach. Tocopherols are the primary form of vitamin E in the leaves of all plants, and the seeds of dicots.

Gamma-tocopherol is the most common isomer of tocopherol found in the American diet, but its absorption and use appear to be significantly less, and its active excretion and metabolism higher, than for alpha-tocopherol.

Table 1. Sources of natural alpha-tocopherol in the diet.

Toxicity of tocopherols

High doses of alpha tocopherol (2000 IU/day) may cause bleeding manifestations in individuals on anticoagulants by interfering with the vitamin K- mediated coagulation cascade. This may also occur in individuals with vitamin K deficiency, bleeding ulcers, inherited bleeding disorders or a past history of hemorrhagic stroke.

References

• http://lpi.oregonstate.edu/mic/vitamins/vitamin-E
• https://www.nrv.gov.au/nutrients/vitamin-e
• http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3997530/