vitamin E

vitamin E
Name	Vitamin E
Caption	Chemical structure of α-tocopherol, the most biologically active form
Pronounce	/ˈtoʊkəˌfɛrɒl/
Other names	Tocopherol, tocotrienol
Class	Fat-soluble vitamin
Biosynthesis	In plants and some cyanobacteria
Drug class	Dietary supplement
Legal status	Over-the-counter
Routes of administration	Oral, topical

vitamin E is a group of eight fat-soluble compounds that include four tocopherols and four tocotrienols. It is a vital antioxidant nutrient, primarily protecting cell membranes from damage by free radicals. The compound was first isolated in 1936 from wheat germ oil by Herbert McLean Evans and Katherine Scott Bishop at the University of California, Berkeley. Its discovery was pivotal in reproductive biology, as it was identified as a factor essential for fertility in rat studies.

Chemical forms and structure

The eight chemical forms are divided into two classes: tocopherols and tocotrienols, each with alpha-, beta-, gamma-, and delta- variants. These compounds share a chromanol ring structure, with tocopherols having a saturated phytyl tail and tocotrienols possessing an unsaturated isoprenoid tail. The biological activity varies significantly, with RRR-alpha-tocopherol being the most potent form as assessed by the Food and Nutrition Board of the Institute of Medicine. Synthetic forms, such as all-rac-alpha-tocopherol, are commonly used in fortification and supplements. The structural differences influence their distribution within cell membranes and their metabolic fate, governed by the alpha-tocopherol transfer protein in the liver.

Dietary sources

Primary natural sources include plant oils such as wheat germ oil, sunflower oil, and safflower oil. Nuts and seeds, including almonds, sunflower seeds, and hazelnuts, are also rich sources. Some green leafy vegetables like spinach and broccoli provide smaller amounts. Fortified foods, such as certain breakfast cereals and fruit juices, contribute to intake in some populations. The United States Department of Agriculture maintains databases, like the FoodData Central, that detail the content in various foods. Absorption is enhanced by the presence of dietary fat, as demonstrated in studies by the National Institutes of Health.

Biological functions and mechanism

Its primary recognized function is as a chain-breaking antioxidant, neutralizing lipid peroxyl radicals in cell membranes and preventing lipid peroxidation. This protects polyunsaturated fatty acids and cellular components from oxidative damage. The nutrient also modulates signal transduction and gene expression, influencing pathways such as protein kinase C and nuclear factor kappa B. It is incorporated into lipoproteins, like LDL cholesterol, and helps protect them from oxidation during transport. The alpha-tocopherol transfer protein is crucial for maintaining plasma concentrations and delivering the vitamin to tissues like the adrenal glands and pituitary gland.

Deficiency and symptoms

Deficiency is rare in humans but can occur in individuals with fat malabsorption disorders such as cystic fibrosis, Crohn's disease, or abetalipoproteinemia. It is also seen in those with specific genetic mutations affecting the alpha-tocopherol transfer protein. Symptoms primarily involve neurological disorders due to impaired nerve conduction, including peripheral neuropathy, ataxia, and skeletal myopathy. In severe cases, it can lead to retinopathy and immune system impairment. The condition was historically observed in premature infants before the advent of parenteral nutrition protocols.

Supplementation and health effects

Supplementation is widespread, often with the aim of preventing chronic diseases associated with oxidative stress. Large-scale clinical trials, such as the Heart Outcomes Prevention Evaluation and the SELECT trial, have investigated its effects on cardiovascular disease and prostate cancer with largely null or mixed results. Some meta-analyses, including those by the Cochrane Collaboration, have not found clear mortality benefits in the general population. However, specific high-dose supplementation is a standard treatment for deficiency in conditions like ataxia with vitamin E deficiency. Research into its role in age-related macular degeneration and cognitive decline continues.

Safety and interactions

The Tolerable Upper Intake Level for adults is set at 1000 mg per day of any form of supplemental tocopherol by the Institute of Medicine. High-dose supplementation can act as an anticoagulant, potentially increasing the risk of hemorrhagic stroke, as suggested by findings in the Women's Health Study. It may also antagonize the effects of vitamin K and interact with medications like warfarin and chemotherapy agents such as simvastatin. Topical application, common in cosmeceuticals, is generally considered safe. Regulatory bodies like the European Food Safety Authority and the Food and Drug Administration monitor its safety in supplements and fortified foods.

Category:Vitamins Category:Antioxidants Category:Nutrients