The term vitamin E describes a family of eight antioxidants: four tocopherols (alpha-, beta-, gamma-, and delta-) and four tocotrienols (alpha-, beta-, gamma-, and delta-). Alpha-tocopherol is the only form of vitamin E that is actively maintained in the human body; therefore, it is the form of vitamin E found in the largest quantities in blood and tissues (1). 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 anantioxidant. 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 intercept free radicals and thus prevent 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 that transport fats through the bloodstream. LDLs specifically transport cholesterol from the liver to the tissues of the body. Oxidized LDLs have been implicated in the development of cardiovascular diseases (See Disease Prevention). 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 (2, 3).
Several other functions of alpha-tocopherol have been identified that are not likely related to its antioxidant capacity. For instance, alpha-tocopherol is known to inhibit the activity of protein kinase C, an important cell-signalingmolecule. Alpha-tocopherol appears to also affect the expression and activities of molecules and enzymes in immune and inflammatory cells. Additionally, alpha-tocopherol has been shown to inhibit platelet aggregation and to enhance vasodilation (4, 5).
The isomeric form of alpha-tocopherol found in foods is RRR-alpha-tocopherol (also referred to as "natural" or d-alpha-tocopherol). Synthetic alpha-tocopherol, which is labeled all-rac- or dl-alpha-tocopherol, has only one-half the biological activity of RRR-alpha-tocopherol (see Supplements). Often vitamin E-fortified foods contain synthetic alpha tocopherol, and the amounts are given as a percentage of the daily value of 30 IU. Throughout this article, amounts of alpha-tocopherol are expressed in both international units (IU) and milligrams (mg).
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 (see Food Sources), blood levels of gamma-tocopherol are generally ten times lower than those of alpha-tocopherol. This phenomenon is apparently due to two mechanisms. 1) Alpha-tocopherol is preferentially retained in the body by 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 (1) and ultimately delivers alpha-tocopherol to different tissues in the body (6). See the Linus Pauling Institute Newsletter for more information about a-TTP and vitamin E adequacy. 2) Forms of vitamin E other than alpha-tocopherol are actively metabolized (6). Because gamma-tocopherol is initially absorbed in the same manner as alpha-tocopherol, small amounts of gamma-tocopherol are detectable in blood and tissue. Breakdown products of tocopherols, known as metabolites, can be detected in urine. More gamma-tocopherol metabolites are excreted in urine than alpha-tocopherol metabolites, suggesting less gamma-tocopherol is needed for use by the body (7). Limited research in the test tube and in animals indicates that gamma-tocopherol or its metabolites may play a role in protecting the body from free radical-induced damage (8, 9), 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 prospective study, increased levels of plasma gamma-tocopherol were associated with a significantly reduced risk of developing prostate cancer. In this study, increased levels of plasma alpha-tocopherol and toenail selenium were protective against prostate cancer development only when gamma-tocopherol levels were also high (10). These limited findings, in addition to the fact that alpha-tocopherol supplementation lowers gamma-tocopherol levels in blood, have led some scientists to call for additional research on the effects of dietary and supplemental gamma-tocopherol on health (11). For more information, see the article, Which Form of Vitamin E, Alpha- or Gamma-Tocopherol, is Better?, in the Linus Pauling Institute Research Report. Importantly, relatively high plasma gamma-tocopherol concentrations may indicate a high level of vegetable and vegetable oil intake.
Vitamin E deficiency has been observed in individuals with severe malnutrition, genetic defects affecting the alpha-tocopherol transfer protein, and fat malabsorption syndromes. For example, children with cystic fibrosisor cholestatic liver disease, who have an impaired capacity to absorb dietary fat and therefore fat-soluble vitamins, may develop symptomatic vitamin E deficiency. Severe vitamin E deficiency results mainly in neurologicalsymptoms, including impaired balance and coordination (ataxia), injury to the sensory nerves (peripheral neuropathy), muscle weakness (myopathy), and damage to the retina of the eye (pigmented retinopathy). For this reason, people who develop peripheral neuropathy, ataxia, or retinitis pigmentosa should be screened for vitamin E deficiency (2). The developing nervous system appears to be especially vulnerable to vitamin E deficiency. For instance, children who have with severe vitamin E deficiency from birth and are not treated with vitamin E rapidly develop neurological symptoms. In contrast, individuals who develop malabsorption of vitamin E in adulthood may not develop neurological symptoms for 10-20 years. It should be noted that symptomatic vitamin E deficiency in healthy individuals who consume diets low in vitamin E has never been reported (2, 12).
Although true vitamin E deficiency is rare, marginal intake of vitamin E is relatively common in the U.S. The National Health and Nutrition Examination Survey III (NHANES III) examined the dietary intake and blood levels of alpha-tocopherol in 16,295 adults (over the age of 18). Twenty-seven % of white participants, 41% of African Americans, 28% of Mexican Americans, and 32% of the other participants were found to have blood levels of alpha-tocopherol less than 20 micromoles/liter. This cutoff value was chosen because the literature suggests an increased risk for cardiovascular disease below this level (13). More recently, data from the NHANES 1999-2000 indicate that mean dietary intake of alpha-tocopherol is 6.3 mg/day and 7.8 mg/day for women and men, respectively (14). These intakes are well below the current intake recommendations of 15 mg/day (see RDA). In fact, it has been estimated that more than 90% of Americans do not meet daily dietary recommendations for vitamin E (15).
The RDA for vitamin E was previously 8 mg/day for women and 10 mg/day for men. The RDA was revised by the Food and Nutrition Board of the Institute of Medicine in 2000 (4). This new recommendation was based largely on the results of studies done in the 1950s in men fed vitamin E deficient diets. In a test-tube analysis, hydrogen peroxide was added to blood samples and the breakdown of red blood cells, known as hemolysis, was used to indicate vitamin E deficiency. Because hemolysis has also been reported in children with severe vitamin E deficiency, this analysis was considered to be a clinically relevant test of vitamin E status. Importantly, this means that the latest RDA for vitamin E continues to be based on the prevention of deficiency symptoms rather than on health promotion and prevention of chronic disease.
| The Recommended Dietary Allowance (RDA) for RRR-alpha-tocopherol (d-alpha-tocopherol) | |||
| Life Stage | Age | Males; mg/day (IU/day) | Females; mg/day (IU/day) |
| Infants (AI) | 0-6 months | 4 mg (6 IU) | 4 mg (6 IU) |
| Infants (AI) | 7-12 months | 5 mg (7.5 IU) | 5 mg (7.5 IU) |
| Children | 1-3 years | 6 mg (9 IU) | 6 mg (9 IU) |
| Children | 4-8 years | 7 mg (10.5 IU) | 7 mg (10.5 IU) |
| Children | 9-13 years | 11 mg (16.5 IU) | 11 mg (16.5 IU) |
| Adolescents | 14-18 years | 15 mg (22.5 IU) | 15 mg (22.5 IU) |
| Adults | 19 years and older | 15 mg (22.5 IU) | 15 mg (22.5 IU) |
| Pregnancy | all ages | - | 15 mg (22.5 IU) |
| Breast-feeding | all ages | - | 19 mg (28.5 IU) |
Cardiovascular disease
Results of at least five large observational studies suggest that increased vitamin E consumption is associated with decreased risk of myocardial infarction (heart attack) or death from heart disease in both men and women. Each study was a prospective study that measured vitamin E consumption in presumably healthy people and followed them for a number of years to determine how many were diagnosed with or died as a result of heart disease. In two of the studies, individuals who consumed more than 7 mg/day of alpha-tocopherol in food were only approximately 35% as likely to die from heart disease as those who consumed less than 3-5 mg/day of alpha-tocopherol (16, 17). Two other large studies found a significant reduction in risk of heart disease only in women and men who consumed at least 100 IU of supplemental RRR-alpha-tocopherol (67 mg of RRR-alpha-tocopherol) daily (18, 19). More recently, several studies have observed plasma or red blood cell levels of alpha-tocopherol to be inversely associated with the presence or severity of carotid atherosclerosis detected using ultrasonography (20-23). A randomized, placebo-controlled, intervention trial in 39,876 women participating in the Women's Health Study found that supplementation with 600 IU of RRR-alpha-tocopherol (400 mg of RRR-alpha-tocopherol) every other day for ten years had no effect on the incidence of various cardiovascular events (myocardial infarction and stroke), but the vitamin E intervention decreased cardiovascular-related deaths by 24% (24). Analysis of data from the Women's Health Study also showed that women receiving the vitamin E intervention experienced a 21% reduction in risk of venous thromboembolism (25). The benefits of vitamin E supplementation in chronic disease prevention are discussed in a recent review (26). Intervention studies in patients with heart or renal disease, however, have not shown vitamin E supplements to be effective in preventing heart attacks or death (see Disease Treatment).
Cataracts appear to be formed by protein oxidation in the lens of the eye; such oxidation may be prevented by antioxidants like alpha-tocopherol. Several observational studies have examined the association between vitamin E consumption and the incidence and severity of cataracts. Results of these studies are mixed: some report increased vitamin E intake protects against cataract development, while others report no association (27). Aplacebo-controlled intervention trial in 4,629 men and women found that a daily antioxidant supplement containing 500 mg of vitamin C, 400 IU of synthetic vitamin E (dl-alpha-tocopherol acetate; equivalent to 180 mg ofRRR-alpha-tocopherol), and 15 mg of beta-carotene did not affect development and progression of age-related cataracts over a 7-year period(28). Similarly, antioxidant supplementation (500 mg of vitamin C, 400 IU [268 mg] of RRR-alpha-tocopherol, and 15 mg of beta-carotene) did not affect progression of cataracts in a 5-year intervention trial (29). A 4-year randomized, placebo-controlled trial reported that supplements containing 500 IU/day of natural vitamin E (335 mg of RRR-alpha-tocopherol) did not reduce the incidence or progression of cataracts in older adults (30). Another intervention trial found that a daily supplement of 50 mg of synthetic alpha-tocopherol daily (equivalent to 25 mg of RRR-alpha-tocopherol) did not alter the incidence of cataract surgery in male smokers (31). Although results from some observational studies suggest that vitamin E may protect against cataract development, results from clinical trials do not support a preventative effect.
Alpha-tocopherol has been shown to enhance specific aspects of the immune response that appear to decline as people age. For example, elderly adults given 200 mg/day of synthetic alpha-tocopherol (equivalent to 100 mg ofRRR-alpha-tocopherol or 150 IU of RRR-tocopherol) for several months displayed increased formation of antibodies in response to hepatitis B vaccine and tetanus vaccine (32). However, it is not known if such alpha-tocopherol associated enhancements in the immune response of older adults actually translate to increased resistance to infections like the flu (influenza virus)(33). A randomized, placebo-controlled trial in elderly nursing home residents reported that daily supplementation with 200 IU of synthetic alpha-tocopherol (equivalent to 90 mg of RRR-alpha-tocopherol) for one year significantly lowered the risk of contracting upper respiratory tract infections, especially the common cold, but had no effect on lower respiratory tract (lung) infections (34). More research is needed to determine whether supplemental vitamin E may protect the elderly against the common cold or other infections.
Many types of cancer are thought to result from oxidative damage to DNA caused by free radicals. The ability of alpha-tocopherol to neutralize free radicals has made it the subject of a number of cancer prevention studies. However, several large prospective studies have failed to find significant associations between alpha-tocopherol intake and the incidence of lung or breast cancer (4). One study in a cohort of 77,126 men and women reported that use of vitamin E supplements over a 10-year period increased risk of lung cancer in current smokers (35). To date, most clinical trials have found that vitamin E supplementation has no effect on the risk of various cancers, except a possible benefit against development of prostate cancer. Arandomized, placebo-controlled trial in 39,876 women participating in the Women's Health Study found that supplementation with 600 IU of RRR-alpha-tocopherol (400 mg of RRR-alpha-tocopherol) every other day for ten years had no effect on overall cancer incidence or cancer-related deaths (24). This vitamin E intervention also did not affect the incidence of tissue-specific cancers, including breast, lung, and colon cancers. Moreover, a recently published meta-analysis of 12 randomized controlled trials concluded that vitamin E supplementation was not associated with overall cancer incidence, cancer mortality, or total mortality (36). However, vitamin E supplementation may possibly reduce the risk of prostate cancer. A placebo-controlled intervention study that was designed to look at the effect of alpha-tocopherol supplementation on lung cancer development noted a 34% reduction in the incidence of prostate cancer in smokers given daily supplements of 50 mg of synthetic alpha-tocopherol (equivalent to 25 mg ofRRR-alpha-tocopherol) daily (37). A meta-analysis that combined the results of this study with three other randomized controlled trials associated vitamin E supplement use with a 15% lower risk of prostate cancer (36). However, a large randomized, placebo-controlled intervention study using alpha-tocopherol and selenium supplementation (the SELECT trial), alone or in combination, was recently halted because there was no evidence of benefit in preventing prostate cancer (38, 39). After an average of 5.5 years of follow-up in SELECT trial, participants taking vitamin E (400 IU/day of all-rac-alpha-tocopherol) alone had a higher risk of prostate cancer, but the increase was not statistically significant (40).
Observational studies have suggested that supplemental alpha-tocopherol might have value in the treatment of cardiovascular disease. For example, a small observational study of men who had previously undergone a coronary artery bypass surgery found those who took at least 100 IU of supplemental alpha-tocopherol (67 mg of RRR-alpha-tocopherol) daily had a reduction in the progression of coronary artery atherosclerosis measured by angiographycompared to those who took less than 100 IU/day of alpha-tocopherol (41). Arandomized, placebo-controlled intervention trial in Great Britain (the CHAOS study) found that supplementing heart disease patients with either 400 IU or 800 IU of synthetic alpha-tocopherol (equivalent to 180 mg or 360 mg of
RRR-alpha-tocopherol) for an average of 18 months dramatically reduced the occurrence of nonfatal heart attacks by 77%. However, alpha-tocopherol supplementation did not significantly reduce total deaths from heart disease(42). Chronic renal dialysis patients are at much greater risk of dying from cardiovascular disease than the general population, and there is evidence that they are also under increased oxidative stress. Supplementation of renal dialysis patients with 800 IU of natural alpha-tocopherol (536 mg ofRRR-alpha-tocopherol) for an average of 1.4 years resulted in a significantly reduced risk of heart attack compared to placebo (43). In contrast, three other intervention trials failed to find significant risk reductions with alpha-tocopherol supplementation. One study, which was designed mainly to examine cancer prevention, found that 50 mg of synthetic alpha-tocopherol (equivalent to 25 mg of RRR-alpha-tocopherol) daily resulted in a non-significant decrease in nonfatal heart attacks in participants who had had previous heart attacks (44). However, two other large trials in individuals with evidence of cardiovascular disease (previous heart attack, stroke, or evidence of vascular disease) found that daily supplements of 400 IU of natural alpha-tocopherol (equivalent to 268 mg RRR-alpha-tocopherol) or 300 mg of synthetic alpha-tocopherol (equivalent to 150 mg of RRR-alpha-tocopherol) did not significantly change the risk of a subsequent heart attack or stroke (45, 46). A trial in patients with either vascular disease or diabetes mellitus found that daily supplementation with 400 IU of natural alpha-tocopherol for an average of seven years had no effect on major cardiovascular events (myocardial infarction or stroke) or deaths; however, this study noted a slightly increased risk of heart failure in subjects taking vitamin E supplements (47). Thus, results of clinical trials using vitamin E for the treatment of heart disease have been inconsistent.
A more thorough discussion of the complex issues involved in analyzing the results of clinical trials of vitamin E in heart disease can be found in the Fall/Winter 1999 issue of the Linus Pauling Institute Newsletter: Fish Oil, Vitamin E, Genes, Diet, and CHAOS. For a discussion of some of the limitations of the HOPE study, see the article, Vitamin E: Hope or Hopeless, in the Spring/Summer 2000 issue of the Linus Pauling Institute Newsletter.
Alpha-tocopherol supplementation of individuals with diabetes has been proposed because diabetes appears to increase oxidative stress and because cardiovascular complications (heart attack and stroke) are among the leading causes of death in diabetics. One study found a biochemical marker of oxidative stress (urinary excretion of F2-isoprostanes) was elevated in type 2 diabetic individuals, and supplementation with 600 mg of synthetic alpha-tocopherol (equivalent to 300 mg of RRR-alpha-tocopherol) for 14 days reduced levels of the biomarker (48). Studies of the effect of alpha-tocopherol supplementation on blood glucose control have been contradictory. Some studies have shown that supplemental vitamin E improves insulin action and glucose disposal in type 2 diabetic (49) and non-diabetic (49, 50) individuals, while other studies have reported minimal to no improvements in glucose metabolism of type 2 diabetics (51, 52). Increased oxidative stress has also been documented in type 1 (insulin-dependent) diabetes (48). One study reported that supplementing type 1 diabetics with only 100 IU/day of synthetic alpha-tocopherol (equivalent to 45 mg RRR-alpha-tocopherol) for one month significantly improved both glycosylated hemoglobin and triglyceride levels (53). This study also noted nonsignificant improvements in blood glucose levels following alpha-tocopherol supplementation (53). Although there is reason to suspect that alpha-tocopherol supplementation may be beneficial in treatment for type 1 or type 2 diabetes, evidence from well-controlled clinical trials is lacking.
The brain is particularly vulnerable to oxidative stress, which is thought to play a role in the pathology of neurodegenerative diseases like Alzheimer's disease (54). Additionally, some studies have documented low levels of vitamin E in cerebrospinal fluid of patients with Alzheimer's disease (55). A large placebo-controlled intervention trial in individuals with moderate neurological impairment found that supplementation with 2,000 IU of synthetic alpha-tocopherol daily for two years (equivalent to 900 mg/day ofRRR-alpha-tocopherol) significantly slowed progression of Alzheimer'sdementia (56). In contrast, a placebo-controlled trial in patients with mild cognitive impairment reported that the same dosage of vitamin E did not slow progression to Alzheimer's disease over a 3-year period (57). After Alzheimer's disease, vascular dementia (dementia resulting from strokes) is the most common type of dementia in the U.S. A case-control studyexamining risk factors for vascular dementia in elderly Japanese-American men found that supplemental vitamin E and vitamin C intake was associated with a significantly decreased risk of vascular and other types of dementia but not Alzheimer's dementia (58). Among those without dementia, vitamin E supplement use was associated with better scores on cognitive tests. Although these findings are promising, further studies are required to determine the role of alpha-tocopherol supplementation in the treatment of Alzheimer's disease and other types of dementia.
Cancer cells proliferate rapidly and are resistant to death by apoptosis(programmed cell death). Cell culture studies indicate that the vitamin E ester, alpha-tocopheryl succinate, can inhibit proliferation and induce apoptosis in a number of cancer cell lines (59, 60). The ester form, alpha-tocopheryl succinate, not alpha-tocopherol, is required to effectively inhibit proliferation or induce cancer cell death (61). Although the mechanisms for the effects of alpha-tocohpheryl succinate on cancer cells are not yet clear, the fact that the ester form has no antioxidant activity argues against an antioxidant mechanism (62). Limited data from animal models of cancer indicate that alpha-tocopheryl succinate administered by injection may inhibit tumor growth (63-66), but much more research is required to determine whether alpha-tocopheryl succinate will be a useful adjunct to cancer therapy in humans. Certainly, administration by injection would be necessary for any benefit, because alpha-tocopheryl succinate taken orally is cleaved to form alpha-tocopherol in the intestine (67). There is currently no evidence in humans that taking oral alpha-tocopheryl succinate supplements delivers alpha-tocopheryl succinate to tissues.
Food sources
Major sources of alpha-tocopherol in the American diet include vegetable oils (olive, sunflower, and safflower oils), nuts, whole grains, and green leafy vegetables. All eight forms of vitamin E (alpha-, beta-, gamma-, and delta-tocopherols and tocotrienols) occur naturally in foods but in varying amounts. For more information on the nutrient content of foods, search the USDA food composition database.
| Food | Serving | Alpha-tocopherol (mg) | Gamma-tocopherol (mg) |
| Olive oil | 1 tablespoon | 1.9 | 0.1 |
| Soybean oil | 1 tablespoon | 1.1 | 8.7 |
| Corn oil | 1 tablespoon | 1.9 | 8.2 |
| Canola oil | 1 tablespoon | 2.4 | 3.8 |
| Safflower oil | 1 tablespoon | 4.6 | 0.1 |
| Sunflower oil | 1 tablespoon | 5.6 | 0.7 |
| Almonds | 1 ounce | 7.4 | 0.2 |
| Hazelnuts | 1 ounce | 4.3 | 0 |
| Peanuts | 1 ounce | 2.4 | 2.4 |
| Spinach | ½ cup, raw | 0.3 | 0 |
| Carrots | ½ cup, raw chopped | 0.4 | 0 |
| Avocado (California) | 1 fruit | 2.7 | 0.4 |
Alpha-tocopherol
In the U.S., the average intake of alpha-tocopherol from food is approximately 8 mg daily for men and 6 mg daily for women (14); these levels are well below the RDA of 15 mg/day of RRR-alpha-tocopherol (4). Many scientists believe it is difficult for an individual to consume more than 15 mg/day of alpha-tocopherol from food alone without increasing fat intake above recommended levels. All alpha-tocopherol in food is the form of theisomer RRR-alpha-tocopherol. The same is not always true for supplements. Vitamin E supplements generally contain 100 IU to 1,000 IU of alpha-tocopherol. Supplements made from entirely natural sources contain onlyRRR-alpha-tocopherol (also labeled d-alpha-tocopherol). RRR-alpha-tocopherol is the isomer preferred for use by the body, making it the mostbioavailable form of alpha-tocopherol. Synthetic alpha-tocopherol, which is often found in fortified foods and nutritional supplements, is usually labeledall-rac-alpha-tocopherol or dl-alpha-tocopherol, meaning that all eight isomers of alpha-tocopherol are present in the mixture. Because half of the isomers of alpha-tocopherol present in all-rac-alpha-tocopherol are not usable by the body, synthetic alpha-tocopherol is less bioavailable and only half as potent. To calculate the number of mg of bioavailable alpha-tocopherol present in a supplement, use the following formulas:
- RRR-alpha-tocopherol (natural or d-alpha-tocopherol):IU x 0.67 = mg RRR-alpha-tocopherol.Example: 100 IU = 67 mg
- all-rac-alpha-tocopherol (synthetic or dl-alpha-tocopherol):IU x 0.45 = mg RRR-alpha-tocopherol.Example: 100 IU = 45 mg
For more information on the Biological Activity of Vitamin E, see the article by Dr. Maret Traber in the Linus Pauling Institute Newsletter.
Alpha-tocopheryl succinate and alpha-tocopheryl acetate (alpha-tocopheryl esters)
Alpha-tocopherol supplements are available in the ester forms: alpha-tocopheryl succinate and alpha-tocopheryl acetate. Tocopherol esters are more resistant to oxidation during storage than unesterified tocopherols. When taken orally, the succinate or acetate moiety is removed from alpha-tocopherol in the intestine. The bioavailability of alpha-tocopherol from alpha-tocopheryl succinate and alpha-tocopheryl acetate is equivalent to that of free alpha-tocopherol. Because international units (IU) for alpha-tocopherol esters are adjusted for molecular weight, the conversion factors for determining the amount of bioavailable alpha-tocopherol provided by alpha-tocopheryl succinate and alpha-tocopheryl acetate are not different from those used for alpha-tocopherol (see formulas) (4). The ester alpha-tocopheryl succinate, not alpha-tocopherol, is required to effectively inhibit growth and induce death in cancer cells grown in culture (see Disease Treatment: Cancer). However, there is currently no evidence in humans that taking oral alpha-tocopheryl succinate supplements delivers alpha-tocopheryl succinate to tissues.
Alpha-tocopheryl phosphates (Ester-E®)
There is currently no published evidence that supplements containing alpha-tocopheryl phosphates are more efficiently absorbed or have greater bioavailability in humans than supplements containing alpha-tocopherol.
Gamma-tocopherol
Gamma-tocopherol supplements and mixed tocopherol supplements are also commercially available (68). The amounts of alpha- and gamma-tocopherol in mixed tocopherol supplements vary, so it is important to read the label to determine the amount of each tocopherol present in supplements.
Toxicity
Few side effects have been noted in adults taking supplements of less than 2,000 mg of alpha-tocopherol daily (RRR- or all-rac-alpha-tocopherol). However, most studies of toxicity or side effects of alpha-tocopherol supplementation have lasted only a few weeks to a few months, and side effects occurring as a result of long-term alpha-tocopherol supplementation have not been adequately studied. The most worrisome possibility is that of impaired blood clotting, which may increase the likelihood of hemorrhage in some individuals. The Food and Nutrition Board of the Institute of Medicine established a tolerable upper intake level (UL) for alpha-tocopherol supplements based on the prevention of hemorrhage (see table below). The Board felt that 1,000 mg/day of alpha-tocopherol in any form (equivalent to 1,500 IU/day of RRR-alpha-tocopherol or 1,100 IU/day of all-rac-alpha-tocopherol) would be the highest dose unlikely to result in hemorrhage in almost all adults (4). Although only certain isomers of alpha-tocopherol are retained in the circulation, all forms are absorbed and metabolized by the liver. The rationale that any form of alpha-tocopherol (natural or synthetic) can be absorbed and thus could be potentially harmful is the basis for a UL that refers to all forms of alpha-tocopherol.
Some physicians recommend discontinuing high-dose vitamin E supplementation one month before elective surgery to decrease the risk of hemorrhage. Premature infants appear to be especially vulnerable to adverse effects of alpha-tocopherol supplementation, which should be used only under controlled supervision by a pediatrician (68). Supplementation with 400 IU/day of vitamin E has been found to accelerate the progression of retinitis pigmentosa that is not associated with vitamin E deficiency (69).
A meta-analysis that combined the results of 19 clinical trials of vitamin E supplementation for various diseases, including heart disease, end-stage renal failure, and Alzheimer's disease, reported that adults who took supplements of 400 IU/day or more were 6% more likely to die from any cause than those who did not take vitamin E supplements (70). However, further breakdown of the risk by vitamin E dose and adjustment for other vitamin and mineral supplements revealed that the increased risk of death was statistically significant only at a dose of 2,000 IU/day, which is higher than the UL for adults. Additionally, three other meta-analyses that combined the results of randomized controlled trials designed to evaluate the efficacy of vitamin E supplementation for the prevention or treatment of cardiovascular disease found no evidence that vitamin E supplementation up to 800 IU/day significantly increased or decreased cardiovascular disease mortality or all-cause mortality (71-73). Additionally, a more recent meta-analysis of 57 randomized controlled trials found that vitamin E supplementation, up to doses of 5,500 IU/day, had no effect on all-cause mortality (74). Furthermore, a meta-analysis of 68 randomized trials found that supplemental vitamin E, singly or in combination with other antioxidant supplements, did not significantly alter risk of all-cause mortality (75). At present, there is no convincing evidence that vitamin E supplementation up to 800 IU/day increases the risk of death from cardiovascular disease or other causes.
Tolerable Upper Intake Level (UL) for Alpha-Tocopherol | |
| Age Group | mg/day (IU/day d-alpha-tocopherol) |
| Infants 0-12 months | Not Possible to Establish* |
| Children 1-3 years | 200 mg (300 IU) |
| Children 4-8 years | 300 mg (450 IU) |
| Children 9-13 years | 600 mg (900 IU) |
| Adolescents 14-18 years | 800 mg (1,200 IU) |
| Adults 19 and older | 1,000 mg (1,500 IU) |
*Source of intake should be from foods or formula only.
Drug interactions
Use of vitamin E supplements may increase the risk of bleeding in individuals taking anticoagulant drugs, such as warfarin (Coumadin); antiplatelet drugs, such as clopidogrel (Plavix) and dipyridamole (Persantine); and non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin, ibuprofen, and others. Also, individuals on anticoagulant therapy (blood thinners) or individuals who are vitamin K deficient should not take alpha-tocopherol supplements without close medical supervision because of the increased risk of hemorrhage (4). A number of medications may decrease the absorption of vitamin E, including cholestyramine, colestipol, isoniazid, mineral oil, orlistat, sucralfate, and the fat substitute, olestra. Anticonvulsant drugs, such as phenobarbital, phenytoin, or carbamazepine, may decrease plasma levels of vitamin E (4,68).
Antioxidants and HMG-CoA reductase inhibitors (statins)
A 3-year randomized controlled trial in 160 patients with documentedcoronary heart disease (CHD) and low HDL levels found that a combination of simvastatin (Zocor) and niacin increased HDL2 levels, inhibited the progression of coronary artery stenosis (narrowing), and decreased the frequency of cardiovascular events, such as myocardial infarction and stroke(76). Surprisingly, when an antioxidant combination (1,000 mg of vitamin C, 800 IU of alpha-tocopherol, 100 mcg of selenium, and 25 mg of beta-carotene daily) was taken with the simvastatin-niacin combination, the protective effects were diminished. However, in a much larger randomized controlled trial of simvastatin and an antioxidant combination (600 mg of vitamin E, 250 mg of vitamin C, and 20 mg of beta-carotene daily) in more than 20,000 men and women with coronary artery disease or diabetes, the antioxidant combination did not adversely affect the cardioprotective effects of simvastatin therapy over a 5-year period (77). These contradictory findings indicate that further research is needed on potential interactions between antioxidant supplementation and cholesterol-lowering agents like HMG-CoA reductase inhibitors (statins).
Scientists at the Linus Pauling Institute feel there exists credible evidence that taking a supplement of 200 IU (134 mg) of natural source d-alpha-tocopherol (RRR-alpha-tocopherol) daily with a meal may help protect adults from chronic diseases, such as heart disease, stroke, neurodegenerative diseases, and some types of cancer. The amount of alpha-tocopherol required for such beneficial effects appears to be much greater than that which could be achieved through diet alone (see Sources). Since supplements containing 200 IU of d-alpha-tocopherol are often as expensive as supplements containing 400 IU of d-alpha-tocopherol, a less expensive alternative may be to take 400 IU (268 mg) of d-alpha-tocopherol every other day. Alpha-tocopherol supplements are unlikely to be absorbed unless taken with food.
Older adults (65 years and older)
The Linus Pauling Institute's recommendation of a supplement providing 200 IU of natural source d-alpha-tocopherol daily (or 400 IU of d-alpha-tocopherol every other day) with a meal is also appropriate for generally healthy older adults.
Written in November 2004 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University
Updated in June 2008 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University
Reviewed in June 2008 by:
Maret G. Traber, Ph.D.
Professor of Nutrition and Exercise Sciences
Principal Investigator, Linus Pauling Institute
Oregon State University
0 comments:
Post a Comment