“Tocopherol”的意思、由来-开放百科全书

Tocopherols ({{IPAc-en|t|oʊ|ˈ|k|ɒ|f|ə|ˌ|r|ɒ|l}};^[1] TCP) are a class of organic chemical compounds (more precisely, various methylated phenols), many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a dietary fertility factor in rats, it was given the name "tocopherol" from the Greek words "τόκος" [tókos, birth], and "φέρειν", [phérein, to bear or carry] meaning in sum "to carry a pregnancy," with the ending "-ol" signifying its status as a chemical alcohol.

Tocotrienols, which are related compounds, also have vitamin E activity. All of these various derivatives with vitamin activity may correctly be referred to as "vitamin E". Tocopherols and tocotrienols are fat-soluble antioxidants but also seem to have many other functions in the body.

Forms

The tocotrienols have the same methyl structure at the ring and the same Greek letter-methyl-notation, but differ from the analogous tocopherols by the presence of three double bonds in the hydrophobic side chain. The unsaturation of the tails gives tocotrienols only a single stereoisomeric carbon (and thus two possible isomers per structural formula, one of which occurs naturally), whereas tocopherols have 3 centers (and eight possible stereoisomers per structural formula, again, only one of which occurs naturally).

In general, the unnatural l-isomers of tocotrienols lack almost all vitamin activity, and half of the possible 8 isomers of the tocopherols (those with 2S chirality at the ring-tail junction) also lack vitamin activity. Of the stereoisomers which retain activity, increasing methylation, especially full methylation to the alpha-form, increases vitamin activity. In tocopherols, this is due to the preference of the tocopherol binding protein for the alpha-tocopherol form of the vitamin.

As a food additive, tocopherol is labeled with these E numbers: E306 (tocopherol), E307 (α-tocopherol), E308 (γ-tocopherol), and E309 (δ-tocopherol). These are all approved in the USA,^[5] EU^[6] and Australia and New Zealand^[7] for use as antioxidants.

α-Tocopherol

Alpha-tocopherol is the form of vitamin E that is preferentially absorbed and accumulated in humans.^[8] The measurement of "vitamin E" activity in international units (IU) was based on fertility enhancement by the prevention of miscarriages in pregnant rats relative to alpha-tocopherol.

Although the mono-methylated form ddd-gamma-tocopherol is the most prevalent form of vitamin E in oils, there is evidence that rats can methylate this form to the preferred alpha-tocopherol, since several generations of rats retained alpha-tocopherol tissue levels, even when fed only gamma-tocopherol through their lives.

There are three stereocenters in alpha-tocopherol, so this is a chiral molecule.^[11] The eight stereoisomers of alpha-tocopherol differ in the arrangement of groups around these stereocenters. In the image of RRR-alpha-tocopherol below, all three stereocenters are in the R form. However, if the middle of the three stereocenters were changed (so the hydrogen was now pointing down and the methyl group pointing up), this would become the structure of RSR-alpha-tocopherol. These stereoisomers can also be named in an alternative older nomenclature, where the stereocenters are either in the d or l form.^[12]

1 IU of tocopherol is defined as ⅔ milligrams of RRR-alpha-tocopherol (formerly named d-alpha-tocopherol or sometimes ddd-alpha-tocopherol). 1 IU is also defined as 1 milligram of an equal mix of the eight stereoisomers, which is a racemic mixture called all-rac-alpha-tocopheryl acetate. This mix of stereoisomers is often called dl-alpha-tocopheryl acetate, even though it is more precisely dl,dl,dl-alpha-tocopheryl acetate). However, 1 IU of this racemic mixture is not now considered equivalent to 1 IU of natural (RRR) α-tocopherol, and the Institute of Medicine and the USDA now convert IU's of the racemic mixture to milligrams of equivalent RRR using 1 IU racemic mixture = 0.45 "milligrams α-tocopherol".^[9]

Tocotrienols

Function and dietary recommendations

Mechanism of action

Tocopherols are radical scavengers, delivering an H atom to quench free radicals. At 323 kJ/mol, the O-H bond in tocopherols is about 10% weaker than in most other phenols.^[11] This weak bond allows the vitamin to donate a hydrogen atom to the peroxyl radical and other free radicals, minimizing their damaging effect. The thus generated tocopheryl radical is relatively unreactive but revert to tocopherol by a redox reaction with a hydrogen donor such as vitamin C.^[12] As they are fat-soluble, tocopherols are incorporated into cell membranes, which are protected from oxidative damage.

Dietary considerations

The U.S. Recommended Dietary Allowance (RDA) for adults is 15 mg/day.^[17] The RDA is based on the alpha-tocopherol form because it is the most active form as originally tested. Vitamin E supplements are absorbed best when taken with meals.^[13] The U.S. Institute of Medicine has set an upper tolerable intake level (UL) for vitamin E at 1,000 mg (1,500 IU) per day.^[14] The European Food Safety Authority sets UL at 300 mg alpha-tocopherol equivalents /day.^[15]

α-Tocopherol equivalents

For dietary purposes, vitamin E activity of vitamin E isomers is expressed as α-tocopherol equivalents (a-TEs). One a-TE is defined by the biological activity of 1 mg (natural) d-alpha-Tocopherol in the resorption-gestation test. According to listings by FAO and others beta-tocopherol should be multiplied by 0.5, gamma-tocopherol by 0.1, and a-tocotrienol by 0.3.^[3] The IU is converted to aTE by multiplying it with 0.67.^[16] These factors do not correlate with the antioxidant activity of vitamin E isomers, where tocotrienols show even much higher activity in vivo.^[17]

Sources

The U.S. Department of Agriculture (USDA), Agricultural Research Services, maintains a food composition database. The last major revision was Release 28, September 2015.^[18] In general, food sources with the highest concentrations of vitamin E are vegetable oils, followed by nuts and seeds. Adjusting for typical portion sizes, however, for many people in the United States the most important sources of vitamin E include fortified breakfast cereals.^[18]

Deficiency

Vitamin E deficiency is rare, and in almost all instances caused by an underlying disease rather than a diet low in vitamin E.^[14] Vitamin E deficiency causes neurological problems due to poor nerve conduction. These include neuromuscular problems such as spinocerebellar ataxia and myopathies.^[19] Deficiency can also cause anemia, due to oxidative damage to red blood cells.

Supplements

Commercial vitamin E supplements can be classified into several distinct categories:

Synthetic all-racemic

Synthetic vitamin E derived from petroleum products is manufactured as all-racemic alpha tocopheryl acetate with a mixture of eight stereoisomers. In this mixture, one alpha-tocopherol molecule in eight molecules are in the form of RRR-alpha-tocopherol (12.5% of the total).^[20]

The 8-isomer all-rac vitamin E is always marked on labels simply as dl-tocopherol or dl-tocopheryl acetate, even though it is (if fully written out) actually dl,dl,dl-tocopherol. The present largest manufacturers of this type are DSM and BASF.

Natural alpha-tocopherol is the RRR-alpha (or ddd-alpha) form. The synthetic dl,dl,dl-alpha ("dl-alpha") form is not as active as the natural ddd-alpha ("d-alpha") tocopherol form. This is mainly due to reduced vitamin activity of the 4 possible stereoisomers which are represented by the l or S enantiomer at the first stereocenter (an S or l configuration between the chromanol ring and the tail, i.e., the SRR, SRS, SSR, and SSS stereoisomers).^[21] The 3 unnatural "2R" stereoisomers with natural R configuration at this 2' stereocenter, but S at one of the other centers in the tail (i.e., RSR, RRS, RSS), appear to retain substantial RRR vitamin activity, because they are recognized by the alpha-tocopherol transport protein, and thus maintained in the plasma, where the other four stereoisomers (SRR, SRS, SSR, and SSS) are not. Thus, the synthetic all-rac-α-tocopherol in theory would have approximately half the vitamin activity of RRR-alpha-tocopherol in humans. Experimentally, the ratio of activities of the 8 stereoisomer racemic mixture to the natural vitamin, is 1 to 1.36 in the rat pregnancy model (suggesting a measured activity ratio of 1/1.36 = 74% of natural, for the 8-isomer racemic mix).^[22]

Although it is clear that mixtures of stereoisomers are not as active as the natural RRR-alpha-tocopherol form, in the ratios discussed above, specific information on any side effects of the seven synthetic vitamin E stereoisomers is not readily available.

Esters

Manufacturers also commonly convert the phenol form of the vitamins (with a free hydroxyl group) to esters, using acetic or succinic acid. These tocopheryl esters are more stable and are easy to use in vitamin supplements. Alpha tocopheryl esters are de-esterified in the gut and then absorbed as the free tocopherol.^[23]^[24] Tocopheryl nicotinate and tocopheryl linolate esters are also used in cosmetics and some pharmaceuticals.

Mixed tocopherols

"Mixed tocopherols" in the US contain at least 20% w/w other natural R, R,R- tocopherols, i.e. R, R,R-alpha-tocopherol content plus at least 25% R, R,R-beta-, R, R,R-gamma-, R, R,R-delta-tocopherols.{{Citation needed|date=October 2012}}

Some brands may contain 200% w/w or more of the other tocopherols and measurable tocotrienols. Some mixed tocopherols with higher gamma-tocopherol content are marketed as "High Gamma-Tocopherol." The label should report each component in milligrams, except R, R,R-alpha-tocopherol may still be reported in IU. Mixed tocopherols can also be found in other nutritional supplements.{{Citation needed|date=October 2012}}

Uses

Observational studies that measure dietary intake and/or serum concentration, and experimental studies that ideally are randomized clinical trials (RCTs), are two means of examining the effects or lack thereof of a proposed intervention on human health.^[25] Healthcare outcomes can be expected to be in accord between reviews of observational and experimental studies. If there is a lack of agreement then factors other than design need to be considered.^[26] In observational studies on vitamin E, an inverse correlation between dietary intake and risk of a disease, or serum concentration and risk of a disease, can be considered suggestive, but any conclusions should also rest on randomized clinical trials of sufficient size and duration to measure clinically significant results. One concern with correlations is that other nutrients and non-nutrient compounds (such as polyphenols) may be higher in the same diets that are higher in vitamin E. Another concern for the relevance of RCTs described below is that while observational studies are comparing disease risk between low and high dietary intake of naturally occurring vitamin E from food (when worldwide, the adult median dietary intake is 6.2 mg/d for d-alpha-tocopherol; 10.2 mg/day when all of the tocopherol and tocotrienol isomers are included),^[27] the prospective RCTs often used 400 IU/day of synthetic dl-alpha-tocopherol as the test product, equivalent to 268 mg of α-tocopherol equivalents.^[14]

Supplement popularity over time

In the US, the popularity for vitamin E as a dietary supplement may have peaked around 2000. The Nurses' Health Study (NHS) and the Health Professionals Follow-up Study (HPFS) tracked dietary supplement use by people over the age of 40 during years 1986-2006. For women, user prevalence was 16.1% in 1986, 46.2% in 1998, 44.3% in 2002, but had decreased to 19.8% in 2006. Similarly, for men, prevalence for same years was 18.9%, 52.0%, 49.4% and 24.5%. The authors theorized that declining use in these health science aware populations may have due to publications of studies that showed either no benefits or negative consequences from vitamin E supplements.^[28] There is other evidence for declining use of vitamin E. Within the US military services, vitamin prescriptions written for active, reserve and retired military, and their dependents, were tracked over years 2007-2011. Vitamin E prescriptions decreased by 53% while vitamin C remained constant and vitamin D increased by 454%.^[29] A report on vitamin E sales volume in the US documented a 50% decrease between 2000 and 2006,^[30] with a significant cause attributed to a well-publicized meta-analysis that had concluded that high-dosage vitamin E increased all-cause mortality.^[31]

Age-related macular degeneration

A Cochrane review published in 2017 on antioxidant vitamin and mineral supplements for slowing the progression of age-related macular degeneration (AMD) identified only one vitamin E clinical trial. That trial compared 500 IU/day of alpha-tocopherol to placebo for four years and reported no effect on the progression of AMD in people already diagnosed with the condition.^[32] Another Cochrane review, same year, same authors, reviewed the literature on alpha-tocopherol preventing the development of AMD. This review identified four trials, duration 4–10 years, and reported no change to risk of developing AMD.^[33] A large clinical trial known as AREDS compared beta-carotene (15 mg), vitamin C (500 mg) and alpha-tocopherol (400 IU) to placebo for up to 10 years, with a conclusion that the anti-oxidant combination significantly slowed progression. However, because there was no group in the trial receiving only vitamin E, no conclusions could be drawn as to the contribution of the vitamin to the effect.^[34]

Complementary and alternative medicine

Proponents of megavitamin therapy and orthomolecular medicine advocate natural tocopherols.^[35] Meanwhile, clinical trials have largely concentrated on use of either a synthetic, all-racemic d-alpha tocopheryl or synthetic dl-alpha tocopheryl.{{cn|date=April 2018}}

Antioxidant theory

Tocopherol is described as functioning as an antioxidant. A dose-ranging trial was conducted in people with chronic oxidative stress attributed to elevated serum cholesterol. Plasma F2-isoprostane concentration was selected as a biomarker of free radical-mediated lipid peroxidation. Only the two highest doses - 1600 and 3200 IU/day - significantly lowered F2-isoprostane.^[36]

Alzheimer's disease

Cancer

From reviews of observational studies, diets higher in vitamin E content were associated with a lower relative risk of kidney cancer,^[43] bladder cancer,^[44] and lung cancer^[45] When comparisons were made between the lowest and highest groups for dietary vitamin E consumption from food, the average reductions in relative risk were in the range of 16-19%. For all of these reviews, the authors noted that the findings needed to be confirmed by prospective studies.^[43]^[44]^[45] From randomized clinical trials (RCTs) in which alpha-tocopherol was administered as a dietary supplement, results differed from the dietary intake reviews. A RCT of 400 IU/day of alpha-tocopherol did not reduce risk of bladder cancer.^[46] In male tobacco smokers, 50 mg/day had no impact on developing lung cancer.^[47] A review of RCTs for colorectal cancer reported lack of a statistically significant reduction in risk.^[48] In male tobacco smokers, 50 mg/day reduced prostate cancer risk by 32%,^[49] but in a different trial, majority non-smokers, 400 IU/day increased risk by 17%.^[50] In women who consumed either placebo or 600 IU of natural-source vitamin E on alternate days for an average of 10.1 years there were no significant differences for breast cancer, lung cancer or colon cancer.^[51]

The U.S. Food and Drug Administration initiated a process of reviewing and approving food and dietary supplement health claims in 1993. A Qualified Health Claim issued in 2012 allows product label claims that vitamin E may reduce risk of renal, bladder and colorectal cancers, with a stipulation that the label must include a mandatory qualifier sentence: “FDA has concluded that there is very little scientific evidence for this claim.”^[52] The European Food Safety Authority (EFSA) reviews proposed health claims for the European Union countries. As of March 2018, EFSA has not evaluated any vitamin E and cancer prevention claims.

Cataracts

A meta-analysis from 2015 reported that for studies which reported serum tocopherol, higher serum concentration was associated with a 23% reduction in relative risk of age-related cataracts (ARC), with the effect due to differences in nuclear cataract rather than cortical or posterior subcapsular cataract - the three major classifications of age-related cataracts.^[53] However, this article and a second meta-analysis reporting on clinical trials of alpha-tocopherol supplementation reported no statistically significant change to risk of ARC when compared to placebo.^[53]^[54]

Cardiovascular diseases

Research on the effects of vitamin E on cardiovascular disease has produced conflicting results. An inverse relation has been observed between coronary heart disease and the consumption of foods high in vitamin E, and also higher serum concentration of alpha-tocopherol.^[55] In one of the largest observational studies, almost 90,000 healthy nurses were tracked for eight years. Compared to those in the lowest fifth for reported vitamin E consumption (from food and dietary supplements), those in the highest fifth were at a 34% lower risk of major coronary disease.^[56] Diet higher in vitamin E may also be higher in other, unidentified components that promote heart health, or people choosing such diets may be making other healthy lifestyle choices.^[55]^[56] There is some supporting evidence from randomized clinical trials (RCTs). A meta-analysis on the effects of alpha-tocopherol supplementation in RCTs on aspects of cardiovascular health reported that when consumed without any other antioxidant nutrient, the relative risk of heart attack was reduced by 18%.^[57] The results were not consistent for all of the individual trials incorporated into the meta-analysis. For example, the Physicians' Health Study II did not show any benefit after 400 IU every other day for eight years, for heart attack, stroke, coronary mortality or all-cause mortality.^[58] The effects of vitamin E supplementation on incidence of stroke were summarized in 2011. There were no significant benefits for vitamin E versus placebo for risk of stroke, or for subset analysis for ischaemic stroke, haemorrhagic stroke, fatal stroke or non-fatal stroke.^[59]

In 2001 the U.S. Food and Drug Administration rejected proposed health claims for vitamin E and cardiovascular health.^[60] The U.S. National Institutes of Health also reviewed the literature and concluded there was not sufficient evidence to support the idea that routine use of vitamin E supplements prevents cardiovascular disease or reduces its morbidity and mortality.^[14] In 2010 the European Food Safety Authority reviewed and rejected claims that a cause and effect relationship has been established between the dietary intake of vitamin E and maintenance of normal cardiac function or of normal blood circulation.^[61]

Pregnancy

Antioxidant vitamins as dietary supplements have been proposed as having benefits if consumed during pregnancy. For the combination of vitamin E with vitamin C supplemented to pregnant women, a Cochrane review of 21 clinical trials concluded that the data do not support vitamin E supplementation - majority of trials alpha-tocopherol at 400 IU/day plus vitamin C at 1000 mg/day - as being efficacious for reducing risk of stillbirth, neonatal death, preterm birth, preeclampsia or any other maternal or infant outcomes, either in healthy women or those considered at risk for pregnancy complications.^[62] The review identified only three small trials in which vitamin E was supplemented without co-supplementation with vitamin C. None of these trials reported any clinically meaningful information.^[62]

Topical

Although there is widespread use of vitamin E as a topical medication, with claims for improved wound healing and reduced scar tissue, reviews have repeatedly concluded that there is insufficient evidence to support these claims.^[63]^[64] There are reports of vitamin E-induced allergic contact dermatitis from use of vitamin-E derivatives such as tocopheryl linoleate and tocopherol acetate in skin care products. Incidence is low despite widespread use.^[65]

Side effects

The US Food and Nutrition Board set a Tolerable upper intake level (UL) at 1,000 mg (1,500 IU) per day derived from animal models that demonstrated bleeding at high doses.^[66] The European Food Safety Authority reviewed the same safety question and set a UL at 300 mg/day.^[15] A meta-analysis of long-term clinical trials reported a non-significant 2% increase in all-cause mortality when alpha-tocopherol was the only supplement used.^[67] Another meta-analysis reported a non-significant 1% increase in all-cause mortality when alpha-tocopherol was the only supplement. Subset analysis reported no difference between natural (plant extracted) or synthetic alpha-tocopherol, or whether the amount used was less than or more than 400 IU/day.^[68] There are reports of vitamin E-induced allergic contact dermatitis from use of vitamin-E derivatives such as tocopheryl linoleate and tocopherol acetate in skin care products. Incidence is low despite widespread use.^[65]

Drug interactions

The amounts of alpha-tocopherol, other tocopherols and tocotrienols that are components of dietary vitamin E, when consumed from foods, do not appear to cause any interactions with drugs. Consumption of alpha-tocopherol as a dietary supplement in amounts in excess of 300 mg/day may lead to interactions with aspirin, warfarin, tamoxifen and cyclosporine A in ways that alter function. For aspirin and warfarin, high amounts of vitamin E may potentiate anti-blood clotting action.^[14]^[69] One small trial demonstrated that vitamin E at 400 mg/day reduced blood concentration of the anti-breast cancer drug tamoxifen. In multiple clinical trials, vitamin E lowered blood concentration of the immuno-suppressant drug, cyclosporine A.^[69] The US National Institutes of Health, Office of Dietary Supplements, raises a concern that co-administration of vitamin E could counter the mechanisms of anti-cancer radiation therapy and some types of chemotherapy, and so advises against its use in these patient populations. The references it cited reported instances of reduced treatment adverse effects, but also poorer cancer survival, raising the possibility of tumor protection from the oxidative damage intended by the treatments.^[14]

Synthesis

Naturally sourced d-alpha-tocopherol can be extracted and purified from seed oils, or gamma-tocopherol can be extracted, purified, and methylated to create d-alpha-tocopherol. In contrast to alpha-tocopherol extracted from plants, which is also called d-alpha-tocopherol, industrial synthesis creates dl-alpha-tocopherol. "It is synthesized from a mixture of toluene and 2,3,5-trimethyl-hydroquinone that reacts with isophytol to all-rac-alpha-tocopherol, using iron in the presence of hydrogen chloride gas as catalyst. The reaction mixture obtained is filtered and extracted with aqueous caustic soda. Toluene is removed by evaporation and the residue (all rac-alpha-tocopherol) is purified by vacuum distillation." Specification for the ingredient is >97% pure.^[70] This synthetic dl-alpha-tocopherol has approximately 50% of the potency of d-alpha-tocopherol. Manufacturers of dietary supplements and fortified foods for humans or domesticated animals convert the phenol form of the vitamin to an ester using either acetic acid or succinic acid because the esters are more chemically stable, providing for a longer shelf-life. The ester forms are de-esterified in the gut and absorbed as free alpha-tocopherol.

History

During feeding experiments with rats Herbert McLean Evans concluded in 1922 that besides vitamins B and C, an unknown vitamin existed.^[71] Although every other nutrition was present, the rats were not fertile. This condition could be changed by additional feeding with wheat germ. It took several years until 1936 when the substance was isolated from wheat germ and the formula C₂₉H₅₀O₂ was determined. Evans also found that the compound reacted like an alcohol and concluded that one of the oxygen atoms was part of an OH (hydroxyl) group. As noted in the introduction, the vitamin was given its name by Evans from Greek words meaning "to bear young" with the addition of the -ol as an alcohol.^[72]

See also

References

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