June 2006: 295–299Brief Critical Review

How an Increased Intake of Alpha-Tocopherol Can Suppress theBioavailability of Gamma-TocopherolGeorge Wolf, DPhil

�-Tocopherol is the only form of vitamin E in vitaminsupplements, whereas �-tocopherol is the predomi-nant form of vitamin E in the US diet. �-Tocopherolhas beneficial properties as an anti-inflammatory andpossibly anti-atherogenic and anticancer agent. Ex-cess �-tocopherol taken in supplements causes a re-duction of �-tocopherol concentration in plasma. Thebiochemical mechanism of this effect, which is impor-tant to human nutrition, has recently been elucidated.

Key words: �-tocopherol; �-tocopherol transfer pro-tein; antioxidant; anti-inflammatory; �-carboxyethyl-hydroxychroman (�CEHC); �-carboxyethyl-hydroxy-chroman (�CEHC), �-tocopherol; vitamin E© 2006 International Life Sciences Institute

doi: 10.1301/nr.2006.jun.295–299

Vitamin E was discovered by Evans and Bishop in1922 as a vitamin necessary for reproduction in rats.1 Itsfunction as an antioxidant was first described by Cum-mings and Mattill in 1931.2 Since then, its antioxidantfunction as a free radical scavenger protecting the organ-ism against oxidative damage has been firmly estab-lished.3 Vitamin E occurs in plant sources in eightdifferent analogs. �-Tocopherol (�T) (Figure 1) is thepredominant form of the vitamin found in mammalianplasma and tissues. It is the only form present in vitaminsupplements. Another form of vitamin E, �-tocopherol(�T), is the principal vitamin E form in the US diet,being about 2.5 times as abundant in food as �T.4 Itssource is principally vegetable oils,5 and it is less activeas an antioxidant than �T.6 It has one fewer methylgroup on the benzene ring than the �T form (Figure 1).

�T and �T are absorbed equally well from theintestine,7 and are carried to the liver by chylomicrons.

�T is released into the circulation from the liver incombination with a carrier protein, �-tocopherol transferprotein (�TTP), where it is incorporated into very lowdensity lipoprotein (VLDL) for delivery to tissues.7 Inanimals lacking �TTP, such as in transgenic mice(Ttpa–/– mice),8 high dietary doses of �T can overcomethe lack of the carrier protein, with the vitamin beingdelivered directly into the bloodstream.8 In humans suf-fering from ataxia caused by a genetic defect resulting ina lack of �TTP, high dietary �T can overcome thedisease.9 �T can also combine with �TTP, but only to asmall extent (9%).10 Therefore, �TTP is the regulator ofthe supply of �T but not �T to the tissues.

That portion of vitamin E not transported out of theliver is metabolized by �- and �-oxidation through theaction of cytochrome P450. The oxidation products are thenexcreted in urine as glucuronides. �T is metabolized to2,5,7,8-tetramethyl-2-(2�-carboxyethyl)-6-hydroxychroman(�CEHC) and �T to 2,7,8-trimethyl-2-(2�-carboxyethyl)-6-hydroxychroman (�CEHC) (Figure 1). �CEHC has physi-ological activity as a natriuretic agent.11

In view of the much larger amount of �T than �T inthe human diet, the question arose, does �T have afunction in the animal organism other than as a weakantioxidant? Though the level of �T in human plasma islow (�T, 32 �M; �T, 1.9 �M),12 the two forms arepresent in comparable concentrations in human muscle(�T, 155 nmol/g; �T, 107 nmol/g) and human skin (�T,127 nmol/g; �T, 180 nmol/g).12

In 2001, Jiang et al.13 published an important reviewentitled “�-Tocopherol, the major form of vitamin E inthe U.S. diet, deserves more attention.” More attentionwas then paid to �T, and a number of functions of thisform of vitamin E were revealed. �T, in contrast to �T,can act as an anti-inflammatory agent by virtue of adose-dependent reduction of the synthesis of the media-tor of inflammation, prostaglandin E2, through inhibitionof the enzyme cyclooxygenase 2 (COX 2).14

Due to an unsubstituted position on its benzene ring(Figure 1), �T, in contrast to �T, can react with reactivenitrogen species such as peroxynitrite. Peroxynitrite isformed in macrophages from reactive oxygen and the

Dr. Wolf is Professor Emeritus at the Departmentof Nutritional Sciences and Toxicology, University ofCalifornia, Berkeley.

Please address all correspondence to: Dr. GeorgeWolf, c/o Nutrition Reviews Editorial Office, OneThomas Circle NW, Ninth Floor, Washington, DC20005; Phone: 202-659-0074; Fax: 202-659-3859;E-mail: ilsipress@ilsi.org.

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stable nitric oxide free radical during inflammation, andcan damage protein, lipids, and DNA. �T readily mopsup peroxynitrite by forming 5-nitro-�T.15,16 Reactivenitrogen species are especially abundant in cigarettesmoke, and smokers have twice the level of 5-nitro-�T intheir plasma compared with nonsmokers.17

The anti-inflammatory action may also explain theinfluence of �T in lowering the incidence of coronaryheart disease, shown by both animal and clinicaldata.18,19 The most striking result was obtained in a7-year study of 34,486 women20 that revealed a signifi-cant inverse relationship between �T intake (dietaryvitamin E, mostly �T) and the risk of cardiovasculardisease. This was not the case when vitamin E wasconsumed from supplements (mostly �T). Other stud-ies21,22 confirmed this result by showing a stronglysignificant inverse relationship between serum �T andthe risk of coronary heart disease.

In an important prospective, 7-year case-controlstudy of the association of prediagnostic blood levels of�T among 10,458 males, Huang et al.23 found �T con-centrations strongly and inversely associated with sub-sequent risk of prostate cancer. The blood levels of �T

among the 142 men with prostate cancer were lower thanin the 284 matched controls (P � 0.0001).

In light of the beneficial effects of �T and the greatexpansion of the use of dietary supplements, an impor-tant consideration for human nutrition has been theinfluence of �T from supplements on the availability tothe organism of �T from the diet. The early work ofHandelman et al.24 in a survey of 86 elderly adultsshowed an inverse correlation of plasma �T comparedwith �T (P � 0.001) (Figure 2). After supplementationof a group of middle-aged volunteers with 400 IU of �Tthree times daily for 2 months, plasma �T increased to200%–400% and �T decreased to 30%–50% of initiallevels. In later work by Handelman’s group,25 toco-pherols in the adipose tissue of 10 male human subjectswere assayed. After a 1-year supplementation with �T(800 mg/d), followed by another year without supple-mentation, �T had declined by about 50%, whereas �Thad remained unchanged. Similarly, Traber and Kay-den26 found that 24 hours after a single dose of 1000 mg�T given to six human subjects, plasma �T increased,whereas �T declined precipitously. A most convincingrecent study by Huang and Appel27 demonstrated thatsupplementing the diet of 184 adult nonsmokers with296 mg/d of �-tocopheryl acetate for 2 months reducedthe serum �T level by 58%. Clearly, the � form of thevitamin in the organism can somehow suppress the �form.

The questions then arose, is the reduction of �T inplasma and adipose tissue the result of competition in the

Figure 1. Chemical structures of vitamin E and urinary degra-dation products. Chiral centers are in the R configuration intocopherols and in an S configuration in their metabolites. Usedwith permission from Jiang Q, Christen S, Shigenaga MK,Ames B. �-Tocopherol, the major form of vitamin E in the USdiet, deserves more attention. Am J Clin Nutr. 2001;74:714–722.

Figure 2. Relationship between plasma �- and �-tocopherollevels observed in 86 elderly subjects in Boston from 1981 to1982. �-Tocopherol is given as mean � SD. Used with per-mission from Kontush A, Spranger T, Reich A, Baum K,Beisiegel U. Lipophilic antioxidants in blood plasma as mark-ers of atherosclerosis; the role of alpha-carotene and gamma-tocopherol. Atherosclerosis. 1999;144:117–122.

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course of absorption? Does �T suppress the release of�T from the liver into the circulation, either directly orby competition for access to the transfer protein �TTP(even though only a small fraction of �T can bind to�TTP)? Does an increase in �T in liver lead to a greatermetabolic breakdown and excretion of �T, perhaps incompetition for a metabolic enzyme? Do high doses of�T increase an enzyme that preferentially breaks down�T?

Some answers to these questions have been found.As to absorption, Traber et al.7 found that in humansubjects, �T and �T labeled with deuterium were ab-sorbed from the intestine equally well. More recently,however, evidence28 has suggested that, at least in mice,competition between �T and �T may take place duringthe absorption of tocopherols from the intestine. Thescavenger receptor class B type I (SR-BI) has beenshown to be involved in the transfer of �T from theplasma to the tissues.29 This same receptor, acting as atransporter, is expressed abundantly in the intestine.30

Reboul et al.28 found that, although 30% to 50% of �Twas absorbed by passive diffusion, the rest was trans-ported by the SR-BI receptor-transporter from the apicalto the basal side of the enterocyte in mice. SR-BIantibody blocked 80% of uptake. By means of intestinalcells (Caco-2TC-7) in culture, competitive inhibition ofabout 60% by �T of the uptake of �T was observed. Theauthors28 suggest that this competitive effect was prob-ably caused by competitive binding to the SR-BI recep-tor-transporter.

As to the question of the release into the circulationand metabolic breakdown of the tocopherols, two recentreports by Traber’s group31,32 provided important an-swers.

Previous work by the authors8 had established that�T is transferred from the liver to the plasma by �TTP.Nevertheless, in �TTP-knockout mice (Ttpa–/– mice),31

plasma �T levels could be maintained, if high amountsof �T were provided in the diet, by a direct transfer of �Tfrom the liver to the plasma. The authors31 found verylow levels, both of �T and �T, in plasma of Ttpa–/– micefed a normal diet (�T in the diet, 31 mg/kg; �T, 8.8mg/kg). Feeding a diet high in �T, low in �T (�T in thediet, 10.2 mg/kg; �T, 550 mg/kg ) to Ttpa–/– mice wasexpected to result in high plasma levels of �T by directtransfer; however, as was the case with �T, this did notoccur. Even liver �T concentrations in those mice werelow. Indeed, liver �T levels did not increase in propor-tion to increases in diet. Clearly, �T must be rapidlymetabolized to �CEHC. Interestingly, when Ttpa�/�

mice were given a high-�T diet, their plasma �T con-centration was higher than in the corresponding Ttpa–/–

mice, showing that, despite the low fraction of �T trans-

ferred by �TTP, this transfer protein can carry significantamounts of �T into the circulation.

The authors8 identified and determined the P450enzyme in liver metabolizing the tocopherols as Cyp3a.The concentration of Cyp3a was correlated with liver�T, but not �T, levels (Figure 3). The levels of themetabolite of �T, �CEHC, in liver was correlated withthe concentrations of both �T and �T (Figure 4). Theauthors31 suggested that �T influences �T metabolism,in that an increase in dietary �T increases the hepaticenzyme metabolizing not only �T, but also �T, leadingto the excretion of both metabolites in urine. Whereas allremaining �T is taken to the plasma by �TTP, all of the�T is metabolized and excreted. In other words, in-creased �T, by causing an increase in Cyp3a, stimulatesthe metabolic breakdown of �T. This mechanism ex-plains the decline in plasma and adipose �T in humanswhose diets contain high levels of �T.24,25,26,27

The metabolic kinetics of �T and �T in humans wasinvestigated by Traber’s group.32 The authors administereda 1:1 molar mixture of 50 mg deuterium-labeled �- and�-tocopheryl acetate (d-�T; d-�T) to five women and ninemen. Deuterium in metabolites was determined by liquidchromatography/mass spectrometry. Plasma samples werecollected at different times up to 72 hours; urine sampleswere collected throughout. In plasma, d-�T peaked earlier(8.6 � 2.6 h) than d-�T (11.1 � 1.4 h), at one-third theconcentration of d-�T. Areas under the curves were 6 timesgreater for d-�T than for d-�T. Half-lives were calculatedfrom fractional disappearance rates from plasma. Plasma�T had a half-life about four times that of �T (57 � 19 h vs.13 � 4 h). Women had a greater fractional disappearance

Figure 3. Relationship of hepatic �- and �-tocopherol concen-trations with constitutive hepatic Cyp3a concentrations. He-patic �- (A) and �-tocopherol (B) concentrations (nmol/g liver)were correlated with hepatic Cyp3a concentrations (in nano-moles of CYP3A2 cross-reactive protein per milligram of liverhomogenate protein) in male mice (Y) (P � 0.0002) and in allmice (X and Y) (P � 0.0001). Used with permission fromTraber MG, Siddens LK, Leonard SW, et al. �Tocopherolmodulates Cyp3A expression, increases �-carboxyethyl-hy-droxychroman (�CEHC) production, and limits tissue �-to-copherol accumulation in mice fed high �-tocopherol diets.Free Radical Biol Med. 2005;38:773–785.

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rate of �T than men; the rates for �T were the same forwomen and men.

�T was metabolized rapidly to �CEHC, which ap-peared in plasma within a few hours of tocopheroladministration. Women excreted four times as much ofthe �T metabolite than men did. No �CEHC was foundin plasma. The deuterium enrichment in �T and �CEHCwas closely matched at all times, showing rapid equilib-rium between the �T and the �CEHC pools. The muchshorter half-life of �T is clearly caused by the more rapidand greater extent of metabolism of �T than �T, con-firming the conclusions reached from their work withmice.31 The authors32 suggested that “the relativelylesser conversion of �T to �CEHC was not dependentupon competition between �T and �T . . . rather, thedifferences in metabolism seem to be the result of pref-erential metabolism of non-�T forms.”

In conclusion, it appears that the reduction in plasma�T during enhanced intake of �T can be explained by themore rapid metabolism of �T occurring when �T intakeis increased. Recent experiments with mice28 have sug-gested that competition for an intestinal transporter pro-tein may also play a part in the reduction of �T by largedoses of �T.

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Figure 4. Hepatic �-CEHC is related to hepatic �-tocopherolconcentrations. Hepatic �-CEHC concentrations (nmol/g) werecorrelated with �-tocopherol concentrations (A) (nmol/g; P �0.003) and hepatic �-tocopherol concentrations (B) (nmol/g;P � 0.004). These data were from Ttpa�/– mice fed low ormedium amounts of �-tocopherol or deficient diets. Used withpermission from: Traber MG, Siddens LK, Leonard SW, et al.�Tocopherol modulates Cyp3A expression, increases �-CEHCproduction and limits tissue �-tocopherol accumulation in micefed high �-tocopherol diets. Free Radical Biol Med. 2005;38:773–785.

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