by Ben Best
Vitamin E refers to a family of eight molecules having a chromanol ring (chroman ring with an alcoholic hydroxyl group) and a 12-carbon aliphatic side chain containing two methyl groups in the middle and two more methyl groups at the end. For the four tocopherols the side chain is saturated, whereas for the four tocotrienols the side chain contains three double-bonds, all of which adjoin a methyl group. The four tocopherols and the four tocotrienols have an alpha, beta, gamma and delta form — named on the basis of the number and position of the methyl groups on the chromanol ring. The alpha form has three methyl groups, the beta & gamma forms have two methyl groups and the delta for has only one methyl group.
Tocotrienols (found in high concentrations in palm oil) are many times more potent as anti-oxidants than are tocopherols, but they are poorly assimilated by digestion, are poorly distributed to tissues in blood and are rapidly metabolized and eliminated from the body. But tocotrienols are well-absorbed by the skin and thus are well suited for use as a Vitamin E cream [JOURNAL OF NUTRITION; Packer, L; 131:369S-373S (2001)].
The alpha form of tocopherol constitutes about 90% of the tocopherol in animal tissue, originally designated d-alpha-tocopherol on the basis of optical activity. There are actually three asymmetric carbon atoms in tocopherol, one at the 2-position of the chromanol ring, and the other two on the aliphatic chain, at the 4' and 8' positions — all being locations of methyl groups. The International Union of Pure and Applied Chemistry (IUPAC, which establishes naming conventions for chemicals) advocates an R & S system of stereoisomer designation, rather than the "d-" & "l-" prefixes (which indicate optical activity.. Therefore, the common natural form of alpha tocopherol has the IUPAC name 2R,4'R,8'R-alpha-tocopherol (RRR-alpha-tocopherol, for short) — "d-alpha-tocopherol" is now obsolete.
The first dietary role discovered for tocopherol was as an essential nutrient for normal development of an animal foetus — hence the name tos (Greek: childbirth), phero (Greek: to bring forth) and ol (alcohol). Tocopherol fertility restoration assays provided a basis for quantification, so-called "International Units" (IUs). (Different biological assays and IUs were established for all the fat-soluble vitamins.) The acetate of [dl]-alpha-tocopherol was arbitrarily assigned the value of one IU per milligram (mg). (Acetate and succinate derivatives are more stable, and hence more suited for storage and for use in supplements.) Natural alpha tocopherol (RRR-alpha-tocopherol) had an activity of 1.49 IU per mg, whereas synthetic alpha-tocopherol (a racemic mixture of all 8 = 23 stereoisomers) had an activity of one IU per mg. Beta, gamma and delta tocopherols had activities of 0.60 IU, 0.30 IU and 0.015 IU per mg, respectively. For alpha-tocopherol, the asymmetric carbon at the 2-position of the chromanol ring is the major (perhaps the only) determinant of the difference in biological activity.
However, there are many biological functions of tocopherol, and the different forms do not have the same relative activities for each function. For example, a review of cancer prevention by Vitamin E stated that gamma-tocopherol is the most potent form for preventing breast cancer [JOURNAL OF NUTRITION; Kline, K; 134:345S-346S (2004)]. Moreover, chemical assays became precise enough in the 1950s to make biological assays unnecessary. The United States Pharmacopeia (USP) no longer uses IU units. Therefore, the United States Food and Nutrition Board now quotes Recommended Daily Allowances (RDAs) in milligrams rather than in IUs. For all adults over the age of 14 (male or female) the RDA for natural RRR-alpha-tocopherol is 15 mg per day.
Alpha-tocopherol succinate is less hydrophobic than alpha-tocopherol or alpha-tocopherol acetate and is more readily taken-up by cells. The hydrophobic forms require a dose that is ten times greater to produce in order to produce the same effect as alpha-tocopherol succinate in cell culture studies [CANCER RESEARCH; Zu,K; 63(20):6988-6995 (2003)]. But neither the acetate nor succinate ester forms of alpha tocopherol ever reach cells when taken orally because they are both de-esterified to alpha-tocopherol in the intestine before being absorbed. The advantage of esterified forms as oral supplements is that they are more stable (resistant to oxidation) during storage because they are not antioxidants when esterified. (An ester is a salt formed by a carboxylic acid [-C(OH)=O] and an alcohol [-OH] — in this case the tocopherol is the alcohol).
Because only plants can synthesize Vitamin E, animals must get their Vitamin E by eating plants or by eating animals that have eaten plants. Although gamma-tocopherol is the predominant form of Vitamin E in the American diet (mainly from vegetable oil, but also from nuts), the liver preferentially loads LDL cholesterol with alpha-tocopherol for delivery to the body. As a result, alpha-tocopherol is at least 5 times more plentiful in the bloodstream than gamma-tocopherol. Gamma-tocopherol concentrates in certain tissues, constituting a third of total tocopherol in veins and nearly two-fifths the tocopherol in muscle.
Above 100 mg, Vitamin E supplements cannot raise plasma alpha-tocopherol concentration more than 2-3 fold. The alpha-tocopherol transfer protein in the liver governs assimilation, and new alpha-tocopherol preferentially replaces the old, which is excreted [AMERICAN JOURNAL OF CLINICAL NUTRITION; Traber,MG; 68(4):847-853 (1998)]. Experiments on guinea pigs indicate that newly administered alpha-tocopherol (radioactive) becomes equal to pre-existing alpha-tocopherol about six times faster in plasma than in muscle and about four times faster in muscle than in brain (107 days total) [LIPIDS; Burton,GW; 25(4):199-210 (1990)].
Currently, the biological activity of Vitamin E which attracts the most interest is the prevention of lipid peroxidation. Alpha-tocopherol is the most active tocopherol against peroxyl radicals (LOO.) and delta-tocopherol is the least active (alpha>beta=gamma>delta). The anti-oxidant activity of Vitamin E is based on the ease with which the hydrogen on the hydroxyl group of the chroman ring can be donated to neutralize a free radical (creating a more stabile tocopheroxyl radical). As with phospholipids, the polar chroman ring tends to stay near the edges of the membrane, whereas the hydrophobic core will be buried deep into the membrane. When a phospholipid tail becomes peroxidized by a free radical, the tail becomes more polar and migrates to the surfaces where it can meet the tocopherol chroman ring to be neutralized, while forming a tocopheroxyl radical. The tocopheroxyl radical can be reduced (restored) to tocopherol directly by Ubiquinol or Vitamin C — and then by glutathione or lipoic acid (via Vitamin C), which are in turn reduced by NADH or NADPH. Vitamin C and Vitamin E can compensate for decline in glutathione associated with aging, but excess Vitamin C and Vitamin E apparently reduces glutathione synthesis, thereby weakening cellular defense against oxidative stress [FREE RADICAL BIOLOGY & MEDICINE; Shang,F; 34(5):521-530 (2003)].
As a lipid-phase anti-oxidant which protects the membranes of cells and mitochondria, Vitamin E would be expected to boost the immune system and to protect against cancer. Vitamin E supplements significantly boosted immune response in a randomized control trial of healthy elderly subjects [JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION; Meydani,SN; 277(17):1380-1386 (1997)] and in young male Chinese [JOURNAL OF NUTRITION; Lee, CJ; 130:2932-2937 (2000)]. A randomized, double-blind placebo-controlled study of 200 IU supplementation with alpha-tocopherol in persons over 65 years of age showed a 20% reduction in incidence of the common cold [JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION; Meydani,SN; 292(7):828-836 (2004)].
Vitamin E has been demonstrated to extend the lifespan of Paramecium [EXPERIMENTAL GERONTOLOGY; Thomas,J; 23(6):501-512 (1988)] and nematode worms [MECHANISMS OF AGEING AND DEVELOPMENT; Harrington,LA; 43(1):71-78 (1988)], but for the nematodes the effect was apparently due to depressed metabolism rather than anti-oxidant activity.
Vitamin E supplementation has been shown to reduce cognitive impairment in rats, probably due to reduced oxidative stress in the cerebral cortex and hippocampus [ANNALS OF THE NEW YORK ACADEMY OF SCIENCES; Fukui, K; 959:275-284 (2002)]. Vitamin E reduced age-related cognitive decline in rats, but to a lesser extent than phytochemicals [JOURNAL OF NEUROSCIENCE; Joseph,JA; 18(19):8047-8055 (1998)]. RRR-alpha-tocopherol given to aged rats markedly improved retention of maximum memory function [JOURNAL OF NUTRITIONAL SCIENCE AND VITAMINOLOGY; Takatsu,H; 55(5):389-393 (2009)].
Alpha-tocopherol has been shown to reduce photoaging of skin caused by singlet oxygen caused by ultraviolet light (UVA) [JOURNAL OF BIOLOGICAL CHEMISTRY; 274(22):15345-15349 (1999)]. But carotenoids are superior to Vitamin E as quenchers of singlet oxygen. Mouse embryos resulting from fertilization by gamma-ray irradiated males showed nearly half the chromosomal abnormalities for males receiving 200 IU/kg one hour prior to irradiation [INTERNATIONAL JOURNAL OF RADIATION BIOLOGY; Mozdarani,H; 82(11):817-822 (2006)].
The main rationale for the use of Vitamin E to prevent atherosclerosis and coronary heart disease is based on the idea that tocopherol prevents oxidation of LDL cholesterol. LDL cholesterol that accumulates in the arterial sub-endothelial space can be oxidized and then ingested by macrophages that become the "foam cells" of fatty atherosclerosis. The resulting inflammatory reaction exacerbates the process. Alpha-tocopherol has been shown to reduce the LDL oxidation and to oppose the process in cell cultures and in animal studies [BIOCHEMICAL PHARMACOLOGY; de Nigris, F; 59:1477-1487 (2000) and JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION; Dutta, A; 22(4):258-268 (2003)].
The anti-atherosclerotic action of alpha-tocopherol has been demonstrated in rabbits [ JOURNAL OF LIPID RESEARCH; Schwenke, D; 43:1927-1938 (2002)] and in monkeys [ JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION; Verlangieri, AJ; 11(2):131-138 (1992)]. Epidemiological studies have supported the conclusion that high Vitamin E intake correlates with low risk of cardiovascular disease [ANNALS OF NUTRITION & METABOLISM; Wagner, KH; 48:169-188 (2004)]. An epidemiological study of over ten thousand elderly people (over age 66) found that those using Vitamin E alone had a relative risk of cancer and coronary artery disease mortality of 0.78 and 0.64, respectively. For those taking Vitamin C alone the values were 0.91 and 1.00. For those taking both Vitamin C and Vitamin E together, the values were 0.78 and 0.47, possibly indicating that the two vitamins independently had some anti-cancer benefit with no additive effect. And that Vitamin E was of significantly greater value against coronary artery disease than Vitamin C, but Vitamin C significantly improved the effectiveness of Vitamin E. Those taking general vitamin/mineral supplements (which generally include Vitamin C and Vitamin E) showed no less cancer or heart disease mortality than those not taking supplements [AMERICAN JOURNAL OF CLINICAL NUTRITION; Losonczy,KG; 64(2):190-196 (1996)]. Possibly the lack of benefit seen by general vitamin/mineral supplements is indicative of health problems in those attempting to compensate by taking the supplements.
But clinical trials that have attempted to prevent/treat cardiovascular disease with Vitamin E have shown no effect [JOURNAL OF GENERAL INTERNAL MEDICINE; Shekelle, PG; 19:380-389 (2004) and JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION; Lee,I; 294(1):56-65 (2005)]. In fact, a well-publicized meta-analysis of clinical trials concluded that Vitamin E supplements increase all-cause mortality [ANNALS OF INTERNAL MEDICINE; Miller, ER; 142:37-46 (2005)].
Better results were seen in a clinical trial that combined Vitamin E with Vitamin C [NUTRITION REVIEWS; Liu, L; 60(2):368-377 (2002)] — presumably because the Vitamin C provided a means to neutralize the tocopheroxyl radicals. But a more likely problem with the clinical trials was the use of alpha-tocopherol as a treatment of cardiovascular disease patients. Intervention studies that attempt to reduce fatal heart attacks among cardiovascular disease patients are more appropriately described as treatment programs rather than prevention programs (despite the attempt to prevent heart attack. Smokers are usually well-represented among the subjects used in these studies — and a pro-oxidant of concern in tobacco smoke is nitrogen dioxide and related nitrogenous compounds. When alpha-tocopherol reacts with nitrogen dioxide it forms a mutagenic nitrosating agent, whereas gamma-tocopherol effectively detoxifies [PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (USA); Cooney, RN; 90(5):1771-1775 (1993)]. Moreover, supplementation with alpha-tocopherol alone reduces blood levels of gamma-tocopherol, thereby reducing protection against oxidation by reactive nitrogen species [PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (USA); Christen, S; 94(7):3217-3222 (1997)]. Gamma-tocopherol, but not alpha-tocopherol, has been shown to be anti-inflammatory by inhibition of the COX-2 enzyme (independent of antioxidant activity) [PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (USA); Jiang,Q; 97(21):11494-11499 (2000)].
Tocopherol has the potential to act as a pro-oxidant rather than an anti-oxidant when co-antioxidants such as Vitamin C are not available to neutralize the tocopherol radical and when oxidative stress is mild. When oxidative stress is high, however, tocopherol radicals are more likely to neutralize (and be neutralized by) other radicals than to mediate lipid peroxidation, even if co-antioxidants are not available [JOURNAL OF LIPID RESEARCH; Kontush,A; 37(7):1436 (1996)]. Smokers given a high polyunsaturated diet (safflower oil) suffer oxidative damage that is considerably worsened by the addition of alpha-tocopherol [ARTERIOSCLEROSIS, THROMBOSIS, AND VASCULAR BIOLOGY; Weinberg,RB; 21(6):1029-1033 (2001)]. But for (nonsmoking) rats fed salmon oil, alpha tocopherol is antioxidant [INTERNATIONAL JOURNAL FOR VITAMIN AND NUTRITION RESEARCH; Flader,D; 73(4):275-283 (2003)]. Alpha tocopherol showed no effect on lipid peroxidation when given alone as a supplement to healthy persons [JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION; Meagher,EA; 285(9):1178-1182 (2001)]. But one thousand IU daily of alpha−tocopherol reduced post-exercise elevation in biomarkers of muscle damage and lipid peroxidation in men [FREE RADICAL BIOLOGY & MEDICINE; Sacheck,JM; 34(12):1575-1588 (2003)].
Vitamin E was shown to prevent cancer and DNA damage in laboratory rat kidneys subjected to iron-mediated peroxidation [CANCER RESEARCH; Zhang, D; 57:2410-2414 (1997)] and may be of value in prevention of colon cancer by the use of supplements [CANCER EPIDEMIOLOGY, BIOMARKERS & PREVENTION; White, E; 6:769-774 (1997)]. Independent of its antioxidant activity, Vitamin E can prevent cancer by cell-cycle inhibition, suppression of DNA synthesis and by inducing apoptosis. Vitamin E complements selenium by inducing apoptosis of cancer cells through distinctive mechanisms.
Alpha-tocopherol also reduces platelet aggregation through inhibition of Protein Kinase C (PKC), an effect independent of alpha-tocopherol's antioxidant activity [CIRCULATION; Freedman, JE; 94:2434-2440 (1996)]. PKC inhibition not only results in reduced platelet aggregation, but reduced nitric oxide production in endothelial cells, reduced vascular smooth muscle cell proliferation and reduced superoxide production by neutrophils & macrophages. Beta-tocopherol does not inhibit PKC and may interfere with the inhibition of PKC by alpha-tocopherol [FEBS LETTERS; Azzi,A; 519(1-3):8-10 (2002)].
A study of supplementation with 1000 IU daily of RRR-alpha-tocopherol for 12 weeks in healthy adults indicated that high doses of Vitamin E may antagonize Vitamin K [AMERICAN JOURNAL OF CLINICAL NUTRITION; Booth, SL; 80(1):143-148 (2004)]. But another study of healthy elderly people taking up to 800 IU (727 mg) per day of all-rac-alpha-tocopherol for 4 months showed no increase in bleeding time — nor any harmful effects whatsoever according to a large variety of blood assays and health measures [AMERICAN JOURNAL OF CLINICAL NUTRITION; Meydani, SN; 68:311-318 (1998)].
Increased amounts of tissue alpha-tocopherol results in decreased superoxide production in mitochondria [THE FASEB JOURNAL; Lass, A; 14:87-94 (2000)] and (in a study of mouse liver) a reduction of the aging-associated increase in mitochondrial size [JOURNAL OF ANTI-AGING MEDICINE; Agostinucci,K; 5(2):173-178 (2002)]. Although mitochondrial alpha-tocopherol concentration can be increased by increased alpha-tocopherol intake, increased CoEnzyme Q10 intake can also increase alpha-tocopherol concentration, even if mitochondrial concentration of CoEnzyme Q10 is not increased in the process [JOURNAL OF NUTRITION; Kamzalov,S; 133(10):3175-3180 (2003)]. Kidney & liver mitochondrial CoEnzyme Q10 concentration may increase, while heart, muscle & brain CoEnzyme Q10 concentration may remain unchanged [FREE RADICAL BIOLOGY & MEDICINE; Lass, A; 26(11/12):1375-1382 (1999)].
Vitamin E and CoEnzyme Q10 account for most of the anti-oxidant activity in biological lipids, notably in the membranes of cells and mitochondria. Lipid peroxidation is a chain reaction of free radical damage that is primarily interrupted by Vitamin E, far exceeding the free-radical scavenging by CoEnzyme Q10. CoEnzyme Q10 is more hydrophobic than Vitamin E and is therefore less mobile in cell membranes. But CoEnzyme Q10 functions to regenerate alpha-tocopherol by reducing (adding hydrogen to) the tocopheroxyl radical. CoEnzyme Q co-supplementation with Vitamin E eliminates the pro-oxidant potential of Vitamin E, resulting in greatly reduced lipid peroxidation [ARTERIOSCLEROSIS, THROMBOSIS AND VASCULAR BIOLOGY; Thomas,SR; 16(5):687-696 (1996)]. The CoEnzyme Q10 radical is readily regenerated (reduced) in mitochondria by the readily-available succinate [ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS; Lass, A; 352(2):229-236 (1998)].
Vitamin E is particularly protective against exercise-induced free radicals [AMERICAN JOURNAL OF PHYSIOLOGY 264:R992-R998 (1993)]. For a healthy non-smoking person seeking to boost immune system function, reduce cancer risk and reduce membrane lipid peroxidation damage during exercise (or protect mitochondria in general) through Vitamin E supplementation, a mixture of alpha-tocopherol and gamma-tocopherol supplements would be prudent — along with supplementation with CoEnzyme Q10 and Vitamin K.
High doses of Vitamin E can antagonize the actions of Vitamin K in animals — reducing blood coagulation and increasing vulnerability to bleeding & hemorrhage — but the effect can be reversed by Vitamin K injection [JOURNAL OF NUTRITION; March,BE; 103(3):371-377 (1973)]. Because of the potential for high doses of Vitamin E to antagonize Vitamin K in humans [AMERICAN JOURNAL OF CLINICAL NUTRITION; Booth,SL; 80(1):143-148 (2004)], persons taking high doses of Vitamin E would be prudent to also take Vitamin K. There appears to be little risk of excessive Vitamin K ingestion antagonizing Vitamin E because fully carboxylated glutamate residues on proteins cannot be over-carboxylated.
Like CoEnzyme Q10, Vitamin K is a biological quinone anti-oxidant which can regenerate Vitamin E from the tocopheroxyl radical [JOURNAL OF BIOLOGICIAL CHEMISTRY; Makai, K; 267(31):22277-22281 (1992)] and which has antioxidant properties which could make it useful for ischemia-reperfusion injury [THE JOURNAL OF NEUROSCIENCE; Li, J; 23(13):5816-5826 (2003)]. Alpha-tocopherol alone protects against ischemia-reperusion injury, and part of this protection may be due to reducing free-radical-induced phospholipase activity that can release arachidonic acid [AMERICAN JOURNAL OF PHYSIOLOGY; Massey,KD; 256(4 Pt 2):H1192-H1199 (1989)].