From Wikipedia, the free encyclopedia
| Vitamin E | |
|---|---|
| Drug class | |
The α-tocopherol form of vitamin E
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| Use | Vitamin E deficiency, antioxidant |
| Biological target | Reactive oxygen species |
| ATC code | A11H |
| External links | |
| MeSH | D014810 |
| AHFS/Drugs.com | MedFacts Natural Products |
Vitamin E refers to a group of compounds that include both tocopherols and tocotrienols.[1] Of the many different forms of vitamin E, γ-tocopherol is the most common in the North American diet.[2] γ-Tocopherol can be found in corn oil, soybean oil, margarine, and dressings.[3][4] α-tocopherol, the most biologically active form of vitamin E, is the second-most common form of vitamin E in the diet. This variant can be found most abundantly in wheat germ oil, sunflower, and safflower oils.[4][5] As a fat-soluble antioxidant, it stops the production of reactive oxygen species formed when fat undergoes oxidation.[6][7][8] Regular consumption of more than 1,000 mg (1,500 IU) of tocopherols per day[9] may be expected to cause hypervitaminosis E, with an associated risk of vitamin K deficiency and consequently of bleeding problems.
Forms
The ten forms of vitamin E are divided into two groups; five are tocopherols and five are tocotrienols. They are identified by prefixes alpha- (α-), beta- (β-), gamma- (γ-), delta- (δ-), and epsilon (ε-). Natural tocopherols occur in the RRR-configuration only. The synthetic form contains eight different stereoisomers and is called 'all-rac'-α-tocopherol.[10] Water soluble forms such as d-alpha-tocopheryl succinate are used as food additive.α-Tocopherol
α-Tocopherol is an important lipid-soluble antioxidant. It performs its functions as antioxidant in the glutathione peroxidase pathway,[11] and it protects cell membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction.[7][12] This would remove the free radical intermediates and prevent the oxidation reaction from continuing. The oxidized 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.[13] However, the importance of the antioxidant properties of this molecule at the concentrations present in the body are not clear and the reason vitamin E is required in the diet is possibly unrelated to its ability to act as an antioxidant.[14] Other forms of vitamin E have their own unique properties; for example, γ-tocopherol is a nucleophile that can react with electrophilic mutagens.[15]
Tocotrienols
Compared with tocopherols, tocotrienols are sparsely studied.[16][17][18] Less than 1% of PubMed papers on vitamin E relate to tocotrienols.[19] The current research direction is starting to give more prominence to the tocotrienols, the lesser known but more potent antioxidants in the vitamin E family. Some studies have suggested that tocotrienols have specialized roles in protecting neurons from damage[19] and cholesterol reduction[20] by inhibiting the activity of HMG-CoA reductase; δ-tocotrienol blocks processing of sterol regulatory element‐binding proteins (SREBPs).Oral consumption of tocotrienols is also thought to protect against stroke-associated brain damage in vivo.[21] Until further research has been carried out on the other forms of vitamin E, conclusions relating to the other forms of vitamin E, based on trials studying only the efficacy of α-tocopherol, may be premature.[22]
Functions
Vitamin E has many biological functions, the antioxidant function being the most important and best known.[23] Other functions include enzymatic activities, gene expression, and neurological function(s). The most important function of vitamin E has been suggested to be in cell signaling (and it may not have a significant role in antioxidant metabolism).[24][25]- As an antioxidant, vitamin E acts as a peroxyl radical scavenger, preventing the propagation of free radicals in tissues, by reacting with them to form a tocopheryl radical, which will then be reduced by a hydrogen donor (such as vitamin C) and thus return to its reduced state.[26] As it is fat-soluble, it is incorporated into cell membranes, which protects them from oxidative damage. Vitamin E has also found use as a commercial antioxidant in ultra high molecular weight polyethylene (UHMWPE) used in hip and knee replacements, to help resist oxidation.[27]
- As an enzymatic activity regulator, for instance, protein kinase C (PKC), which plays a role in smooth muscle growth, can be inhibited by α-tocopherol. α-Tocopherol has a stimulatory effect on the dephosphorylation enzyme, protein phosphatase 2A, which in turn, cleaves phosphate groups from PKC, leading to its deactivation, bringing the smooth muscle growth to a halt.[28]
- Vitamin E also has an effect on gene expression. Macrophages rich in cholesterol are found in the atherogenetic tissue. Scavenger receptor CD36 is a class B scavenger receptor found to be up-regulated by oxidized low density lipoprotein (LDL) and binds it.[29] Treatment with α-tocopherol was found to downregulate the expression of the CD36 scavenger receptor gene and the scavenger receptor class A (SR-A)[29] and modulates expression of the connective tissue growth factor (CTGF).[30][31] The CTGF gene, when expressed, is responsible for the repair of wounds and regeneration of the extracellular tissue lost or damaged during atherosclerosis.[31]
- Vitamin E also plays a role in neurological functions,[32] and inhibition of platelet aggregation.[33][34][35]
- Vitamin E also protects lipids and prevents the oxidation of polyunsaturated fatty acids.[36]
Deficiency
Vitamin E deficiency can cause:- spinocerebellar ataxia[12]
- myopathies[4]
- peripheral neuropathy[6][38][39]
- ataxia[6][38][39]
- skeletal myopathy[6][38][39]
- retinopathy[6][38][39]
- impairment of the immune response[6][38][39]
- red blood cell destruction[36]
Supplementation
While vitamin E supplementation was initially hoped to have a positive effect on health, research has not supported this hope.[40] Vitamin E does not decrease mortality in adults, even at large doses,[41] and high-dosage supplementation may slightly increase it.[42][43] It does not improve blood sugar control in an unselected group of people with diabetes mellitus[41] or decrease the risk of stroke.[44] Daily supplementation of vitamin E does not decrease the risk of prostate cancer and may increase it.[45] Studies on its role in age-related macular degeneration are ongoing as, though it is of a combination of dietary antioxidants used to treat the condition, it may increase the risk.[46]A 2012 Cochrane Review examined the potential effectiveness of antioxidant vitamin supplementation in preventing and slowing the progression of age-related cataract. The included studies involved supplementation of vitamin E, along with β-carotene and vitamin C, either dosed independently or in combination, and compared to the placebo. The systematic review showed that vitamin E supplementation had no protective effect on reducing the risk of cataract, cataract extraction, progression of cataract, and slowing the loss of visual acuity.[47]
Overdose
Vitamin E can act as an anticoagulant, increasing the risk of bleeding problems. As a result, many agencies have set a tolerable upper intake levels (UL) at 1,000 mg (1,500 IU) per day.[9] In combination with certain other drugs such as aspirin, hypervitaminosis E can be life-threatening. Hypervitaminosis E may also counteract vitamin K, leading to a vitamin K deficiency.Dietary sources
mg/(100 g)
[note 1]Some foods with vitamin E content[6] low high 150 Wheat germ oil 41 Sunflower oil 95 Almond oil 34 Safflower oil 15 26 Nuts and nut oils, such as almonds and hazelnuts[note 2] 15 Palm oil[48] 14 Olive oil 12.2 Common purslane[49] 1.5 3.4 High-value green, leafy vegetables: spinach, turnip, beet greens, collard greens, and dandelion greens[note 3] 2 Avocados[50] 1.4 Sesame oil[51] 1.1 1.5 Asparagus[note 4] 1.5 Kiwifruit (green) 0.78 1.5 Broccoli[note 5] 0.8 1 Pumpkin[note 6] 0.26 0.94 Sweet potato[note 7] 0.9 Mangoes 0.54 0.56 Tomatoes[note 8] 0.36 0.44 Rockfish[note 9] 0.3 Papayas 0.13 0.22 Low-value green, leafy vegetables: lettuce[note 10]
Recommended daily intake
The Food and Nutrition Board at the Institute of Medicine (IOM) of the US National Academy of Sciences reported the following dietary reference intakes for vitamin E:[6][52]mg/day Age Infants 4 0 to 6 months 5 7 to 12 months Children 6 1 to 3 years 7 4 to 8 years 11 9 to 13 years Adolescents and adults 15 14 and older
History
Vitamin E was discovered in 1922 by Herbert McLean Evans and Katharine Scott Bishop[53] and first isolated in a pure form by Gladys Anderson Emerson in 1935 at the University of California, Berkeley.[54] Erhard Fernholz elucidated its structure in 1938 and shortly afterwards the same year, Paul Karrer and his team first synthesized it.[55]The first use for vitamin E as a therapeutic agent was conducted in 1938 by Widenbauer, who used wheat germ oil supplement on 17 premature newborn infants suffering from growth failure. Eleven of the original 17 patients recovered and were able to resume normal growth rates.[23]
In 1945, Drs. Evan V. Shute and Wilfred E. Shute, siblings from Ontario, Canada, published the first monograph arguing that megadoses of vitamin E can slow down and even reverse the development of atherosclerosis.[56] Peer-reviewed publications soon followed.[57][58] The same research team also demonstrated, in 1946, that α-tocopherol improved impaired capillary permeability and low platelet counts in experimental and clinical thrombocytopenic purpura.[59]
Later, in 1948, while conducting experiments on alloxan effects on rats, Gyorge and Rose noted rats receiving tocopherol supplements suffered from less hemolysis than those that did not receive tocopherol.[60] In 1949, Gerloczy administered all-rac-α-tocopheryl acetate to prevent and cure edema.[61][62] Methods of administration used were both oral, that showed positive response, and intramuscular, which did not show a response.[23] This early investigative work on the benefits of vitamin E supplementation was the gateway to curing the vitamin E deficiency-caused hemolytic anemia described during the 1960s. Since then, supplementation of infant formulas with vitamin E has eradicated this vitamin’s deficiency as a cause for hemolytic anemia.[23]
Vitamin E Supplementation and Cardiovascular Disease
What is Cardiovascular Disease
CVD is the general name given to diseases which affect the heart and blood vessels whereas coronary heart disease (CHD) refers to diseases affecting the heart and coronary blood vessels.[63] Common types of CVD include thrombosis, angina, pectoris, myocardial infarction (MI or commonly refer as heart attack) and stroke. The main processes involved in both CVD and CHD are atherosclerosis and hypertension.[63]Vitamin E and Atherosclerosis
Atherosclerosis is a disease condition refer to the build up of plaque, which is a substance containing lipid and cholesterol (mainly the low-density lipoprotein or LDL cholesterol) on the inner layer of the arterial lumen.[63] With the existing plaque, instead of being smooth and elastic, the layers become thickened and irregular and the lumen of the artery become narrower. This vessel-narrowing effect lead to a reduction of blood circulation and can lead to or worsen the condition of hypertension.[64]There are currently multiple theories explaining factors causing and affecting the cholesterol plaque build up within arteries with the most popular theory indicating that the rate of build up is affected by the oxidation of the LDL cholesterol. LDL cholesterol is one of the five major groups of lipoproteins with one of the physiological roles being lipid transportation . A typical LDL particle contain 2,700 fatty acid molecules and half of them are poly-unsaturated fatty acids, which are very oxidation sensitive.[65] Once the oxidation of LDL occur, it will start a series of undesirable effects starting from the increase production of inflammatory cytokines by stimulating the endothelial cells and monocytes, followed by increased production of tissue factors, production of macrophages and monocytes, which eventually lead to the formation of foam cells and accelerated development of atherosclerosis. With the presence of adequate concentration of vitamin E, which is a very potent fat-soluble antioxidant, it can inhibit the oxidation of LDL, and this inhibition contributes protection against the development of atherosclerosis and can stabilize the existing plaque.[65]
Critical Evaluation of Current Related Literature
Many observational and interventional studies have been conducted to clarify the association between vitamin E and CVD and it’s risk factors. The many observational studies supported a protective role for dietary and supplementary vitamin E intake on the risk of CVD. For randomized controlled trials (RCTs), however, the results are more controversial.According to Asplund (2002)’s [66] meta-analysis of nine cohort studies showed that high intake of tocopherol was associated with a lower risk of CVD events compared with lower intake. The odds ratio (OR) was 0.74 (95% confidential interval (CI): 0.66-0.83). In this study, higher dietary, supplementation and combined vitamin E intake was also associated with lower CHD incidents, as presented in Appendix II. A large cohort study conducted by Rimm et al [67] in 1993 included 39,919 mail health professionals aged between 40 to 75 showed that when consuming greater than 60IU of vitamin E (any form) per day was associated with a lowed incidence of CHD compared with less than 7.5 IU/day intake. This study also showed an inverse association between vitamin E supplementation and the incidence of CHD. The relative risk (RR) of at least 100 IU/day for at least two years was 0.63 (95% CI: 0.74-0.84). A European cohort study was conducted by Knekt et al in 1994. This study also found an inverse relationship between higher vitamin E (any form) intake and lower CHD risk in men and women. In addition, Kushi et al (1996) discovered an inverse relationship between food vitamin E intake and CHD mortality among 34,486 postmenopausal women (RR=0.38, 95% CI: 0.18-0.8; trend: P=0.014).
For the result of RCTs, as mentioned previously, it was controversy. A meta-analysis of 6 RCTs showed no significant association between vitamin E supplementation and CVD mortality; the pooled OR (95% CI) was 1.0 (0.94-1.06) (Vivekananthan et al, 2003). Another meta-analysis of 7 RCTs also snowed similar results, with the pooled Ors (95% CI) of cardiovascular events, non-fatal MI, non-fatal stroke, and CVD deaths being 0.98 (0.94-1.03), 1.00 (0.92-1.09), 1.03 (0.93-1.14), and 1.00 (0.94-1.05), respectively [68]
