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The α-tocopherol form of vitamin E.


Vitamin E is a generic term for tocopherols and tocotrienols. Vitamin E is a family of α-, β-, γ-, and δ-tocopherols and corresponding four tocotrienols. Vitamin E is a fat-soluble antioxidant that stops the production of reactive oxygen species formed when fat undergoes oxidation. Of these, α-tocopherol (also written as alpha-tocopherol) has been most studied as it has the highest bioavailability.

It has been claimed that α-tocopherol is the most important lipid-soluble antioxidant, and that it protects cell membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction. This would remove the free radical intermediates and prevent the oxidation reaction from continuing. The oxidised α-tocopheroxyl radicals produced in this process may be recycled back to the active reduced form through reduction by other antioxidants, such as ascorbate, retinol or ubiquinol. However, the importance of the antioxidant properties of this molecule at the concentrations present in the body are not clear and it is possible that the reason why vitamin E is required in the diet is unrelated to its ability to act as an antioxidant.. Other forms of vitamin E have their own unique properties. For example, γ-tocopherol (also written as gamma-tocopherol) is a nucleophile that can react with electrophilic mutagens.

However, the roles and importance of all of the various forms of vitamin E are presently unclear, and it has even been suggested that the most important function of vitamin E is as a signaling molecule, and that it has no significant role in antioxidant metabolism.

So far, most studies about vitamin E have supplemented using only the synthetic alpha-tocopherol, but doing so leads to reduced serum gamma- and delta-tocopherol concentrations. Moreover, a 2007 clinical study involving synthetic alpha-tocopherol concluded that supplementation did not reduce the risk of major cardiovascular events in middle aged and older men. For more info, read article tocopherol.

Compared with tocopherols, tocotrienols are poorly studied. Less than 1% of PubMed papers on vitamin E relate to tocotrienols. Current research direction are starting to give more prominence to the tocotrienols, the lesser known but more potent antioxidants in the vitamin E family. Tocotrienols have specialized roles in protecting neurons from damage, cancer prevention and cholesterol reduction by inhibiting the activity of HMG-CoA reductase[16-1];δ-tocotrienol blocks processing of sterol regulatory element‐binding proteins (SREBPs)[16-1].

Oral consumption of tocotrienols is also proven to protect against stroke-associated brain damage in vivo. Disappointments with outcomes-based clinical studies testing the efficacy of α-tocopherol need to be handled with caution and prudence recognizing the untapped opportunities offered by the other forms of natural vitamin E. Toxicity studies of a specific form of tocopherol in excess should not be used to conclude that high-dosage “vitamin E” supplementation may increase all-cause mortality. Such conclusion incorrectly implies that tocotrienols are toxic as well under conditions where tocotrienols were not even considered. For more info, read article tocotrienol.

Food sources of Vitamin E

Particularly high levels of vitamin E can be found in the following foods:

Vitamin E to prevent prostate cancer study discontinued

There have been some theories that Vitamin E, especially when coupled with selenium, may reduce the risk of prostate cancerAmerican Cancer Society, Vitamin E, updated Oct. 27, 2008 by 30 percent. However, the Selenium and Vitamin E Cancer Prevention Trial, ("SELECT"), run from 2004 to 2008, found that vitamin E, whether taken alone or in combination with selenium, did not prevent prostate cancer. The SELECT study was discontinued after independent reviewers determined that there was no benefit to the 35,000 men who were the subject of the study.

Congenital heart defects

A case control study done in the Netherlandsmarker using food frequency questionnaires found that high maternal Vitamin E by diet and supplements is associated with an increased risk of CHD (congenital heart defects) offspring, especially when the supplements are taken in the periconception period. (Note: case control studies are rated as low quality, grade 3 or 4, on a standard scale of medical evidence.) The National Health Service in the United Kingdommarker concludes that pregnant women should: "consider avoiding taking supplemental Vitamin E tablets."

References

  1. USDA National Nutrient Database
  2. National Cancer Institute, The SELECT Prostate Cancer Prevention Trial, Oct. 27, 2008
  3. National Cancer Institute, Selenium and Vitamin E Cancer Prevention Trial (SELECT), Oct. 31, 2008
  4. Bob Phillips; Chris Ball, Dave Sackett, Doug Badenoch, Sharon Straus, Brian Haynes, Martin Dawes (May 2001). "Levels of Evidence". Oxford Centre for Evidence-based Medicine. http://www.cebm.net/index.aspx?o=1047.
  5. http://www.nhs.uk/news/2009/04April/Pages/VitaminEPregnancyRisk.aspx


Further reading



External links

  • Vitamin E Medline Plus, Medical Encyclopedia, U.S. National Library of Medicine
  • Vitamin E Office of Dietary Supplements, National Institutes of Health



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