Chapter 4. Vitamin E

164 Vitamin E Deficiency Caused by Genetic Defects in the a-Tocopherol Transfer Protein .... Vitamin E deficiency symptomsare varied because the major function of vitamin E is that of a

4 Vitamin E

Maret G Traber

CONTENTS

Introduction 154

History 154

Structures and Antioxidant Chemistry 154

Structure 154

Nomenclature 155

Chemical Properties 155

Antioxidant Network 156

Oxidized Vitamin E 156

Physiologic Relationships 156

Absorption 156

Lipoprotein Transport 157

Tissue Delivery 157

Storage Sites 158

Vitamin E-Specific Proteins 158

a-Tocopherol Transfer Protein 158

Other Tocopherol-Binding Proteins 159

Plasma Vitamin E Kinetics 160

Metabolism and Excretion 160

Cellular and Biochemical Functions 162

Nutritional Requirement 162

Adequacy of Vitamin E Intakes in Normal U.S Populations 163

Food Sources of Vitamin E 163

Deficiency Signs and Methods of Nutritional Assessment 164

Vitamin E Deficiency Caused by Genetic Defects in the a-Tocopherol Transfer Protein 164

Vitamin E Deficiency Caused by Genetic Defects in Lipoprotein Synthesis 165

Vitamin E Deficiency as a Result of Fat Malabsorption Syndromes 165

Pathology of Human Vitamin E Deficiency 166

Assessment of Vitamin E Status 166

Efficacy of Pharmacological Doses of Vitamin E 167

Conclusion 168

Acknowledgment 168

References 168

Vitamin E is unique because it remains a vitamin without a specific function Other vitaminsare cofactors, hormones, or have specific roles in metabolism Vitamin E deficiency symptomsare varied because the major function of vitamin E is that of a lipid-soluble antioxidant.Therefore, vitamin E deficiency symptoms are dependent on a-tocopherol content, uptakeand turnover, as well as susceptibility to and the degree of oxidative stress in a given tissue.Furthermore, vitamin E concentrations depend on the presence of other antioxidants tomaintain a-tocopherol in its unoxidized state (1)

Importantly, vitamin E’s antioxidant function cannot be fulfilled by just any oxidant Specifically, only a-tocopherol meets human vitamin E requirements (2) Plasmaa-tocopherol is controlled by the hepatic a-tocopherol transfer protein (a-TTP) (3,4) and,

anti-in humans, a genetic defect anti-in a-TTP results anti-in severe vitamanti-in E deficiency (5) a-TTP isnecessary for the facilitated transfer of a-tocopherol from the liver to the plasma (6,7).This chapter describes vitamin E structures, antioxidant function, lipoprotein transport,and delivery to tissues Requirements, intake, human deficiency symptoms, and the role ofvitamin E in the prevention of chronic disease are discussed

HISTORY

Vitamin E deficiency in rats fed rancid fat was first described in 1922 by Evans and Bishop(8) In 1936, Evans et al (9) isolated vitamin E from wheat germ and named this factor, ‘‘a-tocopherol;’’ a name derived from the Greek ‘‘tokos’’ (offspring) and ‘‘pherein’’ (to bear) with

an ‘‘ol’’ to indicate that it was an alcohol Subsequently, b-tocopherol and g-tocopherol wereisolated from vegetable oils (10), demonstrating that various forms of vitamin E exist, butthat a-tocopherol is the most effective form in preventing vitamin E deficiency symptoms.Today, it seems likely that the minor a-tocopherol contaminant in these vitamin E prepar-ations provided their vitamin E biologic activity

Specific vitamin E deficiency symptoms (e.g., fetal resorption, muscular dystrophy, andencephalomalacia) were observed in experimental animals fed vitamin E-deficient diets (11).The most popular, though most tedious and time-consuming, assay for the biologic activity ofvitamin E is the fetal resorption assay (11) Here, vitamin E-depleted virgin female rats aremated with normal males After successful mating, various levels of single vitamin E forms arefed in several divided doses to the females, which are killed 20–21 days after mating Thenumber of living, dead, and resorbed fetuses are counted and the percentage of live youngdetermined Thus, the vitamin E biologic activity depends on the amount necessary tomaintain the maximum number of live fetuses The results of this assay remain in use today

to define the international units (IUs) of vitamin E activity

Horwitt (12,13) attempted to induce vitamin E deficiency in men by feeding a diet low invitamin E (2–4 mg a-tocopherol) for six years to volunteers at the Elgin State Hospital inIllinois After about two years, their serum vitamin E levels decreased into the deficient range.Although their erythrocytes were more sensitive to peroxide-induced hemolysis, overt anemiadid not develop The data from the Horwitt study was used in 2000 to set the recommendeddietary allowance (RDA) for vitamin E (2) These latest RDAs are discussed later

STRUCTURES AND ANTIOXIDANT CHEMISTRY

STRUCTURE

Vitamin E is the name for molecules with a-tocopherol antioxidant activity, including alltocol and tocotrienol derivatives (14) These antioxidants include four tocopherols and four

tocotr ienols, which have simila r chroman ol struc tures: trimeth yl ( a-), dimethyl (b - or g-), an dmonomet hyl ( d-) Tocotri enols differ from tocopherol s in that they ha ve an unsatur ated tail.How ever, in 2000, the Food and Nutritio n Boar d (FNB) (2) de fined a-tocophe ro l as the onlyform that meet s hum an vita min E requir ement s, because only a -tocophe rol has been shown

to reverse human vita min E defic iency symptoms

Unlike most other vitamins, chemically synthesized a-tocopherol is not identical to thenaturally occurring form a-Tocopherol synthesized by condensation of trimethyl hydroquinonewith racemic isophytol (15) contains eight stereoisomers, arising from the three chiral centers(2,40 , and 80 , Figure 4.1), and is designated all-rac-a-tocopherol (incorrectly called D,L-a-tocopherol) The FNB (2) defined that only 2R-a-tocopherol forms meet human vitamin Erequirements Thus, only half of the stereoisomers in all-rac-a-tocopherol meet the vitamin

E requirement

Vitam in E supplem ents often co ntain a-tocophe rol esters, includi ng a-tocoph eryl acetate,succina te, or nicoti nate The ester form is not an antioxidan t and thu s has a long shelf life.Vitam in E e sters are readil y hy drolyze d in the gut and are ab sorbed as a-tocoph erol (16) NOMENCLATURE

The FNB (2) de finitio n of vita min E ha s led to con fusion about vita min E uni ts The vitamin Eunit currently used on supplement labels was defined by the U.S Pharmacopoeia (17) The IU

of vitamin E equals 1 mg all-rac-a-tocopheryl acetate, 0.67 mg RRR-a-tocopherol, or 0.74 mgRRR -a-tocop heryl acetat e However, the FNB (Tab le 6.1 in (2)) define d the vita min Erequirement in milligrams of 2R-a-tocopherol and provided conversion factors, such thatall-rac is equal to 1=2 RRR-a-tocopherol To estimate the number of milligrams of2R-a-tocopherol, IU all-rac-a-tocopherol (or its esters) must be multiplied by 0.45; whereas IURRR-a-tocopherol (or its esters) is multiplied by 0.67

CHEMICAL PROPERTIES

All vitamin E forms act as lipid-soluble chain-breaking antioxidants (18) Vitamin E is apotent peroxyl radical scavenger and especially protects PUFA within phospholipids ofbiological membranes and in plasma lipoproteins When lipid hydroperoxides are oxidized

to peroxyl radicals (ROO.), these react 1,000 times faster with vitamin E (Vit E-OH) thanwith PUFA (RH) (19) The chromanol hydroxyl group reacts with a peroxyl radical to form ahydroperoxide and the chromanoxyl radical (Vit E-O.):

In the presence of vitamin E, ROO.þ Vit E-OH ! ROOH þ Vit E-O.

In the absence of vitamin E, ROO.þ RH ! ROOH þ R.

chroma-Vit E-O.þ AH ! Vit E-OH þ A.

Biologically important antioxidants that regenerate chromanols from chromanoxyl radicalsinclude ascorbate (vitamin C) and thiols, especially glutathione Various metabolic processescan then reduce these other antioxidants This phenomenon has led to the idea of vitamin Erecycling, where vitamin E is restored by other antioxidants Since the a-tocopheroxyl radicalcan readily be reduced to a-tocopherol, the amount of vitamin E that is recycled is likelymuch larger than the amount that is further oxidized

OXIDIZED VITAMIN E

The primary oxidation product of a-tocopherol is a-tocopheryl quinone, which can beconjugated to yield the glucuronide after reduction to the hydroquinone The glucuronidecan be excreted into bile or further degraded in the kidneys to a-tocopheronic acid, which isexcreted in the urine (20) Other oxidation products, including dimers and trimers, as well asother adducts have also been described (18)

Specific vitamin E oxidation products have been generated in vitro (21,22) These include4a,5-epoxy- and 7,8-epoxy-8a(hydroperoxy)tocopherones and their respective hydrolysisproducts, 2,3-epoxy-tocopherol quinone and 5,6-epoxy-a-tocopherol quinone However,these products are formed during in vitro oxidation; their importance in vivo is unknown

The bioavailability of vitamin E appears also to be dependent on the fat content of themeal Hayes et al (25) reported that plasma a-tocopherol concentrations doubled whena-tocopheryl acetate (100–200 mg=day) was provided as a microdispersion in milk, comparedwith providing the same dose in orange juice Vitamin E absorption is relatively poor when it

is consumed without fat, as was observed when vitamin E pills were consumed without food (26)

It is well known that increasing dietary fat increases absorption of vitamin E supplements(26,27) Roodenberg et al (28) suggested that a 3% fat intake was sufficient for optimalvitamin E bioavailability However, they measured bioavailability as increased plasmaa-tocopherol concentrations following one week of supplementation with 50 mg a-tocopherol

in either 50 g of a low- or high-fat spread, such that hot meals contained either less than6.5 g fat or less than 45 g fat Thus, dissolving the vitamin E in the spread may have allowednormal vitamin E absorption

During fat absorption, enterocytes synthesize chylomicrons that contain triglycerides, freeand esterified cholesterol, phospholipids, and apolipoproteins (especially apolipoprotein[apo] B48) In addition, fat-soluble vitamins, carotenoids, and other fat-soluble dietarycomponents are incorporated into chylomicrons Chylomicrons are then secreted into thelymph The movement of vitamin E through the absorptive cells is not well understood; nointestinal TTPs have been described Even in healthy individuals, the efficiency of vitamin Eabsorption is low (<50%) Recent findings in the cholesterol field suggest that lipid-solublenutrient absorption may be modulated by adenosine triphosphate-binding cassette (ABC)transporters, nuclear receptors, and various intracellular trafficking proteins (29)

Often it is assumed that differences in plasma concentrations of various forms of vitamin

E result from differences in the degree of intestinal absorption, but this is not the case.Discrimination between forms of vitamin E does not occur during their absorption by theintestine and their secretion in chylomicrons (30,31) Thus, all dietary forms of vitamin E areabsorbed and secreted into chylomicrons

LIPOPROTEINTRANSPORT

During chylomicron catabolism by lipoprotein lipase in the circulation, some of the newlyabsorbed vitamin E is transferred to high-density lipoproteins (HDL) and some remains withthe chylomicron remnants Because HDL readily transfer vitamin E to other lipoproteins, thenewly absorbed vitamin E is distributed to all of the circulating lipoproteins Although theprocess can occur spontaneously, the phospholipid transfer protein (PLTP) may also beinvolved PLTP catalyzes vitamin E exchange between lipoproteins at a rate that representstransfer of approximately 10% of the plasma vitamin E=h (32)

Chylomicron remnants are taken up by the liver, delivering the newly absorbed vitamin E.The liver repackages dietary fats, secreting them into the plasma in very low density lipopro-teins (VLDL) Unlike other fat-soluble vitamins, which have specific plasma transportproteins, vitamin E is transported nonspecifically in lipoproteins in the plasma However,plasma vitamin E concentrations do depend on a-tocopherol secretion from the liver (33).Additionally, the newly absorbed vitamin E appears to be preferentially secreted intothe plasma from the liver (34) Thus, the liver, not the intestine, discriminates betweentocopherols

TISSUE DELIVERY

No plasma-specific vitamin E transport proteins have been described, but rather mechanisms

of lipoprotein metabolism determine the delivery of vitamin E to tissues There are at leastthree major routes by which tissues likely acquire vitamin E: (i) via lipoprotein lipase-mediated lipoprotein catabolism, (ii) via lipoprotein receptors, and (iii) mediated by mem-brane lipids transporters In addition, vitamin E rapidly exchanges between lipoproteins, andbetween lipoproteins and membranes

Delivery of vitamin E from both chylomicrons and VLDL is likely mediated by tein lipase This mechanism may be particularly important for tissues that express lipoproteinlipase, such as adipose tissue, muscle, and brain Sattler et al (35) tested this hypothesis

lipopro-directly by overexpressing lipoprotein lipase in mouse muscle and found increased delivery ofa-tocopherol to the muscle.

Another important mechanism for the delivery of tocopherols to tissues is via the LDLreceptor (24) Other lipoprotein receptors are also involved in tissue uptake of tocopherols.The scavenger receptor-BI (SR-BI) mediates transfer of lipids from HDL, whereas the protein

is released into the circulation (36) Apparently, SR-BI similarly delivers vitamin E fromHDL to cells HDL are major a-tocopherol transporters to lung (37–40), brain (41,42), andliver (43)

Vitamin E delivery by HDL to the liver appears analogous to reverse cholesterol transport(44), in that HDL via SR-BI deliver vitamin E for excretion into bile (45) Vitamin E-deficientrats increase liver SR-BI suggesting that SR-BI is regulated by a-tocopherol (43) IncreasedSR-BI would serve to increase vitamin E delivery to the liver

ABCA1 is an ABC transporter that transfers cholesterol and phospholipids to HDL.ABCA1 is also responsible for the cellular secretion of a-tocopherol (46) Mice lacking ABCA1have severe a-tocopherol deficiency (47) Clearly, ABCA1 is important in a-tocopheroltrafficking, and its physiologic role appears to be involved in cellular tocopherol efflux;but further investigations into its role in regulating tissue vitamin E concentrations arewarranted

STORAGE SITES

No organ functions as an a-tocopherol storage site, releasing it on demand (24) More than90% of the human body a-tocopherol is located in the adipose tissue It has been estimatedthat more than two years are required to reach new steady state levels in response to changes

in dietary intake (48) Thus, the analysis of adipose tissue a-tocopherol content is a usefulestimate of long-term vitamin E intakes El-Sohemy et al (49) reported in nearly 500 CostaRican subjects that adipose tissue g-tocopherol concentrations were related to dietaryg-tocopherol intakes; whereas adipose tissue a-tocopherol concentrations were not related

to intakes Nonetheless, adipose tissue a-tocopherol concentrations were higher thang-tocopherol concentrations These findings suggest that the ability of a-TTP in its role ofdiscriminating between tocopherols and maintaining plasma a-tocopherol concentrations isultimately important for determining tissue, including adipose tissue concentrations

VITAMIN E-SPECIFIC PROTEINS

a-TOCOPHEROLTRANSFERPROTEIN

a-TTP (calculated molecular weight 31,883 Da) has been detected in the liver of rats, humans,mice, dogs, chickens, and so on The gene has been localized to the human 8q13.1–13.3 region

of chromosome 8 (50,51) a-TTP mRNA has been detected in rat brain, spleen, lung, andkidney (52), as well as in human brain (53) a-TTP is also present in pregnant mouse uterusand human placenta (54–56), suggesting that it functions to ensure adequate a-tocopherolconcentrations during pregnancy

a-TTP preferentially transfers a-tocopherol, compared with other dietary vitamin Eforms (57,58) Hypothetically, this ability to transfer tocopherol is necessary for the observed

in vivo a-TTP action because nascent VLDL, secreted from the monkey liver, are tially enriched in RRR-a-tocopherol (59) However, when this hypothesis was tested in ana-TTP-expressing hepatic cell line (McARH7777 cells), a-TTP-mediated a-tocopherol secretionwas not associated with VLDL secretion (60)

preferen-Although the mechanism by which a-TTP facilitates a-tocopherol secretion into plasma isunknown, some progress has been made in this area a-TTP is a cytosolic protein in

hepatocytes; however, following chloroquine treatment, a-TTP was associated with thecytosolic surface of late endosomes (61) Hypothetically, a-TTP translocates from the cytosol

to late endosomes to acquire a-tocopherol and then a-TTP-a-tocopherol moves to theplasma membrane where a-TTP releases a-tocopherol to the membrane (61) Thus, chylo-micron remnant-a-tocopherol could be released from the lipoprotein, enriching the innerleaflet of the endosomal membrane Zha et al (62) have reported that ABCA1 in theendosomal compartment also plays a role in endocytosis by acting as a flippase to translocatephosphatidyl serine to the outer membrane and potentiate membrane budding Since ABCA1can also transfer a-tocopherol (46), ABCA1 could enrich the outer membrane of the endo-cytic vesicles with both RRR- and SRR-a-tocopherols; a-TTP could then preferentiallyremove RRR-a-tocopherol from the outer leaflet of the endosomal membrane for transfer

to the plasma membrane It remains to be clarified as to whether ABCA1 participates ina-tocopherol transfer to a-TTP, as suggested by Horiguchi et al (61), or if some othertransporters or flippases are also involved in a-tocopherol trafficking

a-TTP crystal structure has also been described (63,64) Importantly, the protein has

a pocket that specifically binds a-tocopherol The phytyl tail is bent to fit the pocket(Figure 4.2), thus the 2-position is critical for this conformation Additionally, there arecoordinating sites in the pocket that only allow a-tocopherol, and not other tocopherols, tobind Clearly, the tocotrienols do not fit this binding pocket

OTHER TOCOPHEROL-BINDING PROTEINS

a-TTP belongs to a family of hydrophobic ligand-binding proteins that have a cis-retinalbinding motif sequence (CRAL_TRIO) This motif is also shared with the cellular retinalde-hyde binding protein (CRALBP) and yeast phosphatidylinositol transfer protein (Sec14p).Panagabko et al (65) showed that all of the CRAL_TRIO members bind a-tocopherol tosome extent, but only a-TTP appears to have high enough affinity to serve as a physiologicala-tocopherol mediator The search for tissue a-tocopherol-regulating proteins led to the

identification of the tocopherol-associated protein (TAP), a 46 kDa cytosolic, CRAL_TRIOprotein in bovine liver (66) TAP is controversial because its sequence is identical to super-natant protein factor (SPF) SPF stimulates squalene conversion to lanosterol, and thusenhances cholesterol biosynthesis (67) The actual function of TAP=SPF remains underintense investigation Both the liver and the heart contain an a-tocopherol-binding proteinwith a mass of 14.2 kDa (68).

PLASMA VITAMIN E KINETICS

Plasma vitamin E kinetics have been studied in humans using deuterium-labeled mers of a-tocopherol (RRR- and SRR-) (69) In normal subjects, the fractional disappearancerates of RRR-a-tocopherol (0.4 + 0.1 pools=day) were significantly ( p < 0.01) slower thanthose for SRR- (1.2 + 0.6) In patients with a genetic defect in a-TTP, the fractional disap-pearance rates of both RRR- and SRR-a-tocopherols were fast and the same as for SRR-a-tocopherol in the control subjects

stereoiso-The RRR-a-tocopherol half-life in normal subjects was approximately 48 h, consistentwith the slow disappearance of RRR-a-tocopherol from the plasma (69) Because RRR-a-tocopherol is taken up by the liver and is returned to the plasma, its apparent turnover is slow.This hepatic recirculation of RRR-a-tocopherol results in the daily replacement of nearly all

of the circulating RRR-a-tocopherol

a-Tocopherol and g-tocopherol E kinetics have also been evaluated in humans usingdifferently deuterated a-tocopherol and g-tocopherol (70) Like SRR-a-tocopherol, plasmag-tocopherol fractional disappearance rates (1.4 + 0.4 pools=day) were triple those ofa-tocopherol (0.3 + 0.1) In this study, g-tocopherol half-lives were 13 + 4 h comparedwith 57 + 19 h for a-tocopherol These data suggest that non-RRR-a-tocopherols are quicklyremoved from plasma

Vitamin E kinetics in endurance exercisers suggests that a burst of oxidative stress canincrease a-tocopherol depletion (71) Moreover, studies in cigarette smokers, a model ofchronic oxidative stress and inflammation, not only demonstrate a faster a-tocopheroldisappearance compared with nonsmokers, but faster disappearance in smokers with thelowest plasma ascorbate concentrations (72) These data demonstrate the in vivo antioxidantrole of vitamin E in humans

METABOLISM AND EXCRETION

Vitamin E is not accumulated in the liver (73), suggesting that excretion and metabolismare important The vitamin E metabolites, a-CEHC (2,5,7,8-tetramethyl-2-(20-carboxyethyl)-6-hydroxychroman) and g-CEHC (2,7,8-trimethyl-2-(20-carboxyethyl)-6-hydroxychroman),are derived from a-tocopherols and a-tocotrienols and from g-tocopherols and g-tocotrienols,respectively (74,75) Initially, the tail is hydroxylated, then b-oxidation takes place (75,76)(Figur e 4.3)

Vitamins E appear to be metabolized similarly to xenobiotics, in that they are v-oxidized

by cytochrome P450s (CYPs), then conjugated and excreted in urine (77) or bile (78) HepaticCYP 4F2 is involved in v-oxidation of a-tocopherol and g-tocopherol (76); but CYP 3A mayalso be involved (75,79–81) Similar to other xenobiotics, CEHCs are sulfated or glucuroni-dated (82–84) Xenobiotic transporters are likely candidates for mediating hepatic CEHCexcretion because CEHCs are found in plasma, urine, and bile (85)

High a-tocopherol intakes, for example, most vitamin E supplements, lead to plasmaincreases of a-tocopherol, decreases of g-tocopherol (86), and increases in excretion of botha-CEHC (87) and g-CEHC (74,88) In cultured hepatocytes, g-CEHC production is 100 timesgreater than that of a-CEHC from similar amounts of tocopherols added to the medium (75)

When humans were given equimolar amounts of labeled tocopherols (~50 mg each d6tocopheryl acetate and d2-g-tocopheryl acetate), plasma d6-a-CEHC concentrations werebelow levels of detection, but g-CEHC and g-tocopherol rates of disappearance from plasmawere similar (70) Thus, dietary g-tocopherol is rapidly metabolized (within 9–12 h) by humans.Studies in mice suggest that g-CEHC excretion increases because a-tocopherol upregulateshepatic xenobiotic metabolism (89) Additionally, studies in end-stage renal disease patientsdemonstrated that vitamin E supplementation was accompanied by a concomitant decrease(~1 mM) in circulating g-tocopherol and about 1 mM increase in g-CEHC (88) Thus,metabolism appears to be a regulator of plasma g-tocopherol concentrations.

-a-The mechanisms for the regulation of CEHC production are, however, unknown, buta-tocopherol appears to play an important role in the regulatory process (89) Studies thus far

in isolated hepatocytes or liver cell lines (76) have not provided complete answers to themystery of why a-tocopherol and g-tocopherol, despite their very similar structures andantioxidant activities, are so differently handled by the liver There is speculation, however,that the chemical properties of g-tocopherol and other non-a-tocopherols make them toxic tocells (90)

The major route of excretion of ingested vitamin E is fecal elimination Excretion ofhepatic vitamin E into the bile has been demonstrated to be mediated by the ABC transporter,p-glycoprotein (MDR2) (91), a transporter that also facilitates biliary phospholipid excretioninto bile

O HO

(Carboxylethyl hydroxychroman)

CELLULAR AND BIOCHEMICAL FUNCTIONS

a-To copherol ap pears to modulat e some cell ular functions For exampl e, a-tocophe rolinhibi ts protein kinase C (PKC) and thus inhibi ts smooth muscl e cell pr oliferation (92), aswell as plate let aggrega tion and adhesion (93,94 ) a-Toco pherol supplem entat ion to humans(1200 IU or 900 mg =da y) also decreas es monocyt e superoxi de produ ction via inhibition ofPKC (95,96), decreas es IL-1b release from monocyt es by inhibiting 5-lipoxy genase (97), anddecreas es mono cyte–en dothelial cell ad hesion in vitro, which correlated with dec reases inmess age an d cell surfa ce express ion of E-selecti n in en dothelial c ells (98) Treatm ent ofend othelial cells (HU VEC) with a-tocoph erol significan tly reduce d the express ion of theadh esion molec ules, ICAM- 1 and VCAM -1, on HUVEC induced by oxidiz ed LDL (99).Enr ichment of hum an aorti c endotheli al ce lls with a-tocoph erol signi fican tly inhibi ted LDL-induce d adhesion of monocyt es to end othelial cell s in a dose-depen dent manner wi th acon comitant redu ction in levels of sIC AM-1 (100) This de creased adhesion was media ted

by decreas ed express ion of CD11b and VLA-4, by inhibi ting the acti vation of NF kB (96).Anothe r key fun ction that a-tocoph erol regula tes is vascul ar hom eostasis throu gh its acti on

on PKC in en dothelial cells; a-tocoph erol has been shown to media te NO producti on (101).Singh et al (102) ha ve recent ly reviewed the da ta with respect to inflamm ation.a-To copherol decreas ed the relea se of proinf lammator y cytoki nes an d ch emokines (IL-8 andplasm inogen activator inhibi tor-1 [PAI-1] ) It decreas ed C-react ive pro tein level in patientswith an d those at risk for cardiov ascula r disease The mechani sms pro posed for thesea-tocoph erol actions include the inhibi tion of PKC, 5-lipoxygenas e, tyros ine kinase, as well

as cyclooxy genase- 2 (102) Unfor tunate ly, many of the studie s descri bed in this secti on werecarri ed out in tissue culture or are a resul t of e x vivo treatment s wi th a-tocoph erol Very fewmeasur ement s have been carri ed out showi ng changes in human s, so the healt h benefi ts ofthese tissue culture observat ions are lacki ng

NUTRITIONAL REQUIREMENT

The dieta ry referen ce intake s (DRIs) for Vitam in C, Vitamin E, Seleni um, and Carotenoidswer e publ ished in 2000 by the Pane l on Diet ary Anti oxidant s an d Related Comp ounds, FNB,Institut e of Med icine (2) The a-tocophe rol he alth benefi ts describ ed wer e primarily thoserelated to the prevention of deficiency symptoms (2) The estimated average requirement(EAR) was based on the amount of 2R-a-tocopherol intake that reversed erythrocyte hemolysis

in men who were vitamin E-deficient as a result of consuming a vitamin E-deficient diet forfive years (2) The EAR of 12 mg 2R-a-tocopherol was chosen because intakes at this levelprevented in vitro hydrogen peroxide-induced erythrocyte hemolysis The 2000 RDA for adults(both men and wom en  19 years) define d as 2R - a-tocophe rol is 15 mg =day (Tab le 4 1).The tolerable upper intake level (UL) was set at 1000 mg=day for vitamin E (any form ofsupplemental a-tocopherol) This was one of the few UL that was set using data in rats,because sufficient and appropriate quantitative data assessing long-term adverse effects ofvitamin E supplements in humans was not available

There have been several reports carrying out mathematical analyses of adverse outcomes

in clinical trials of vitamin E supplements A meta-analysis that combined the results of 19trials in normal subjects, as well as those with various diseases, including heart disease, end-stage renal failure, and Alzheimer’s disease, suggested 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 takevitamin E supplements (103) This report was highly criticized for using unorthodox meth-odology to reach their conclusions, given that simpler mathematical analyses did not find anystatistical relationship between all-cause mortality and vitamin E supplement dose Further-more, three other meta analyses that combined the results of randomized controlled trials

designe d to evaluat e the efficacy of vitamin E su pplementa tion for the preve ntion or ment of ca rdiovascu lar disease found no eviden ce that vita min E su pplementa tion up to 800

treat-IU =day significan tly increa sed or decreas ed cardiov ascula r diseas e mort ality or all-cau semort ality (104–10 6) Moreover, an inter vention tri al in 40,000 wom en, ha lf of whom too kvitamin E supplem ents (600 IU every other day) for ten years found no increa se in mort alitywith vita min E (107) Thus, it appears that the Miller et al (103) meta a nalysis overst ated therisks of vitamin E supplem ents

A DEQUACY OF VITAMIN E INTAKES IN NORMAL U.S P OPULATIONS

The amount of vitamin E con sumed by most U.S adults is suffici ent to preven t overtsympt oms of deficiency; howeve r, the actual quantities consu med by U.S adults, as asses sed

by various surveys (108–1 10), are closer to 8 mg than the requ ired 15 mg Estim ates are that92% of men and 98% of women did not meet the EAR for a-tocophe rol (111) These lowintake s may be real, or they may resul t from underreport ing of fat intake s Never theless, it isquite possible that man y pe ople do not consume a diet that contain s 15 mg a-tocophe rol;howeve r, sup plement intake is high (112)

FOOD SOURCES OF V ITAMIN E

The riches t dietary sources of vitamin E are edible vegeta ble oils (113) , because only plantssynthes ize vitamin E (14) RRR -a -tocophe rol is especially high in wheat germ, safflow er, an dsunfl ower oils Soybea n an d corn oils contai n predomi nantly g-tocop herol, as well as sometocotrienols Cottonseed oil, as well as palm oil, contain both a-tocopherol and g-tocopherol

in equal proportion In addition, palm oil contains large amounts of a-tocotrienol andg-tocotrienol (113) Vitamin E is found in relatively low concentrations in fruits and vegetables(Table 4.2)

Pregnancy Adult EARa 15 23 34 1000 1500 1100 Lactation Adult EAR plus average

amount secreted in human milk