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The term vitamin E describes a family of eight antioxidants, four tocopherols, alpha-, beta-, gamma- and delta-, and four tocotrienols (also alpha-, beta-, gamma- and delta-). Alpha-tocopherol is the only form of vitamin E that is actively maintained in the human body and is therefore, the form of vitamin E found in the largest quantities in the blood and tissue. Because alpha-tocopherol is the form of vitamin E that appears to have the greatest nutritional significance, it will be the primary topic of the following discussion. It is also the only form that meets the latest Recommended Dietary Allowance (RDA) for vitamin E.

Alpha-tocopherol

The main function of alpha-tocopherol in humans appears to be that of an antioxidant.  Free radicals are formed primarily in the body during normal metabolism and also upon exposure to environmental factors such as cigarette smoke or pollutants. Fats, which are an integral part of all cell membranes, are vulnerable to destruction through oxidation by free radicals.  The fat-soluble vitamin, alpha-tocopherol, is uniquely suited to intercepting free radicals and preventing a chain reaction of lipid destruction.  Aside from maintaining the integrity of cell membranes throughout the body, alpha-tocopherol also protects the fats in low density lipoproteins (LDLs) from oxidation. Lipoproteins are particles composed of lipids and proteins, which are able to transport fats through the blood stream. LDLs transport cholesterol from the liver to the tissues of the body. Oxidized LDLs have been implicated in the development of cardiovascular diseases .  When a molecule of alpha-tocopherol neutralizes a free radical, it is altered in such a way that its antioxidant capacity is lost.  However, other antioxidants, such as vitamin C, are capable of regenerating the antioxidant capacity of alpha-tocopherol.

Several other functions of alpha-tocopherol have been identified, which likely are not related to its antioxidant capacity. Alpha-tocopherol is known to inhibit the actvity of protein kinase C, an important cell signaling molecule, as well as to affect the expression and activity of immune and inflammatory cells. Additionally, alpha-tocopherol has been shown to inhibit platelet aggregation and to enhance vasodilation .

Gamma-tocopherol

The function of gamma-tocopherol in humans is presently unclear. Although the most common form of vitamin E in the American diet is gamma-tocopherol , blood levels of gamma-tocopherol are generally ten times lower than those of alpha-tocopherol.  This phenomenon appears due to the action of the alpha-tocopherol transfer protein (a-TTP) in the liver, which preferentially incorporates alpha-tocopherol into lipoproteins that are circulated in the blood . and ultimately deliver alpha-tocopherol to different tissues in the body. See the Linus Pauling Institute Newsletter for more information about a-TTP and vitamin E adequacy. Because gamma-tocopherol is initially absorbed in the same manner as alpha-tocopherol, small amounts are detectable in blood and tissue.  Products of the metabolism of tocopherols, known as metabolites, can be detected in the urine. More gamma-tocopherol metabolites are excreted in the urine than alpha-tocopherol metabolites, suggesting less gamma-tocopherol is needed for use by the body. Limited research in the test tube and in animals indicates that gamma-tocopherol or its metabolites may play a role in the protection of the body from damage by free radicals , but these effects have not been convincingly demonstrated in humans. Recently, concern has been raised regarding the fact that taking alpha-tocopherol supplements lowers gamma-tocopherol levels in the blood.  However, no adverse effects of moderate alpha-tocopherol supplementation have been demonstrated, while many benefits have been documented (see Disease Prevention and Disease Treatment). In one recent prospective study, increased plasma gamma-tocopherol levels were associated with a significantly reduced risk of developing prostate cancer, while significant protective associations for increased levels of plasma alpha-tocopherol and toenail selenium were found only when gamma-tocopherol levels were also high . These limited findings, in addition to the fact that taking alpha-tocopherol supplements lower gamma-tocopherol levels in the blood, have led some scientists to call for additional research on the effects of dietary and supplemental gamma-tocopherol on health . For more information see the article, Which Form of Vitamin E, Alpha- or Gamma-Tocopherol, is Better? in the Linus Pauling Institute Research Report.