Vitamins and cardiovascular disease

S. Honarbakhsh; M. Schachter

doi:10.1017/S000711450809123X

Vitamins and cardiovascular disease

Published online by Cambridge University Press: 01 October 2008

S. Honarbakhsh and

M. Schachter

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S. Honarbakhsh*: Affiliation:
Department of Clinical Pharmacology, Faculty of Medicine, NHLI, ICL, St Mary's Hospital, LondonW2 1NY, UK
M. Schachter: Affiliation:
Department of Clinical Pharmacology, Faculty of Medicine, NHLI, ICL, St Mary's Hospital, LondonW2 1NY, UK
*: *Corresponding author: S. Honarbakhsh, fax +44 207 8866145, email shohreh.honarbakhsh@ic.ac.uk

Article contents

Abstract
Dosage, oxidative markers and isomers
The interaction of exogenous and endogenous vitamins: are we using the wrong vitamin?
Is combination therapy superior to single vitamin supplementation? Should we avoid β-carotene?
The role of vitamins in the progression and complication of atherosclerosis: should we start these vitamins earlier?
Directing vitamin use to subgroups
Discussion
Conclusion
References

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Abstract

CVD is a major cause of mortality and morbidity in the Western world. In recent years its importance has expanded internationally and it is believed that by 2020 it will be the biggest cause of mortality in the world, emphasising the importance to prevent or minimise this increase. A beneficial role for vitamins in CVD has long been explored but the data are still inconsistent. While being supported by observational studies, randomised controlled trials have not yet supported a role for vitamins in primary or secondary prevention of CVD and have in some cases even indicated increased mortality in those with pre-existing late-stage atherosclerosis. The superiority of combination therapy over single supplementation has been suggested but this has not been confirmed in trials. Studies have indicated that β-carotene mediates pro-oxidant effects and it has been suggested that its negative effects may diminish the beneficial effects mediated by the other vitamins in the supplementation cocktail. The trials that used a combination of vitamins that include β-carotene have been disappointing. However, vitamin E and vitamin C have in combination shown long-term anti-atherogenic effects but their combined effect on clinical endpoints has been inconsistent. Studies also suggest that vitamins would be beneficial to individuals who are antioxidant-deficient or exposed to increased levels of oxidative stress, for example, smokers, diabetics and elderly patients, emphasising the importance of subgroup targeting. Through defining the right population group and the optimal vitamin combination we could potentially find a future role for vitamins in CVD.

Keywords

Antioxidants Vitamin E Vitamin C β-Carotene Cardiovascular disease Heart disease

Information

Type: Review Article
Information: British Journal of Nutrition , Volume 101 , Issue 8 , 28 April 2009 , pp. 1113 - 1131

DOI: https://doi.org/10.1017/S000711450809123X [Opens in a new window]
Copyright: Copyright © The Authors 2008

CVD is believed to become the biggest cause of morbidity and mortality in men and women in the world in 2020⁽Reference Lopez and Murray¹⁾, emphasising the great need for retarding the increase in disease incidence. Individuals with a high dietary intake of fruit and vegetables have a clear reduction in the incidence of CHD⁽Reference He, Nowson and Lucas²^–Reference Liu, Lee and Ajani⁵⁾, stroke⁽Reference He, Nowson and MacGregor⁶^–Reference Vollset and Bjelke⁹⁾ and cardiovascular mortality⁽Reference Gaziano, Manson and Branch¹⁰^, Reference Liu, Manson and Lee¹¹⁾. Reactive oxygen species and free radicals have been implicated in the pathophysiology of CVD⁽Reference Cross, Halliwell and Borish¹²⁾, with vitamins E and C and β-carotene being hypothesised as the fundamental protective components in fruit and vegetables. It has also been hypothesised that flavonoids and fibre are also likely to be potential fundamental protective components in fruit and vegetables.

The body is equipped with antioxidative enzymes, such as glutathione peroxidase and superoxide dismutase, and vitamins including vitamins E and C and β-carotene which cooperate and in some cases act synergistically to provide protection against oxidative stress. Atherosclerosis is the underlying cause of CVD, involving the accumulation of modified LDL in the intima of the arterial wall⁽Reference Finking and Hanke¹³⁾ enabling plaque progression⁽Reference Steinberg, Parthasarathy and Carew¹⁴⁾ and the occurrence of cardiovascular events⁽Reference Berliner and Heinecke¹⁵⁾. LDL particles contain about 2700 fatty acids of which approximately half are polyunsaturated and are susceptible to oxidation⁽Reference Jialal and Devaraj¹⁶⁾.

The identification of the oxidative modification hypothesis of LDL⁽Reference Steinberg, Parthasarathy and Carew¹⁴⁾ and the strong correlation between the levels of oxidised LDL⁽Reference Stringer, Gorog and Freeman¹⁷⁾ and the ex vivo oxidative susceptibility of LDL⁽Reference Regnstrom, Nilsson and Tornvall¹⁸⁾ to the apparent extent of atherosclerosis provide a rationale for a role of oxidative stress in atherosclerosis. Oxidised LDL acts as a chemokine that stimulates the recruitment of circulating monocytes into the intimal space⁽Reference Quinn, Parthasarthy and Fong¹⁹^, Reference Quinn, Parthasarthy and Steinberg²⁰⁾ and inhibits the exit of resident macrophages⁽Reference Quinn, Parthasarthy and Steinberg²⁰⁾, enabling foam cell formation and cell-mediated LDL peroxidation. Oxidised LDL is cytotoxic⁽Reference Cathcart, Morel and Chisolm²¹^–Reference Morel, Hessler and Chisolm²³⁾ and also reduces NO bioavailability⁽Reference Cai and Harrison²⁴^–Reference Galle, Mulsch and Busse²⁶⁾, which results in endothelial dysfunction. In accordance with the response-to-injury hypothesis of atherogenesis, this results in the progression of the atherosclerotic lesion⁽Reference Steinberg, Parthasarathy and Carew¹⁴⁾ and consequent cardiovascular events⁽Reference Berliner and Heinecke¹⁵⁾. The role of these vitamins (vitamins E and C and β-carotene) is emphasised by their inhibitory action on the oxidative modification of LDL⁽Reference Diaz, Frei and Vita²⁷^, Reference Steinbrecher, Parthasarathy and Leake²⁸⁾ and their improvement of endothelial dysfunction⁽Reference Berliner and Heinecke¹⁵^, Reference Taddei, Virdis and Ghiadoni²⁹⁾. Their therapeutic role has been supported by animal studies⁽Reference Crawford, Kirk and Rosenfeld³⁰^–Reference Smith and Kummerow³³⁾ and has been further supported by changes in lipid peroxide levels⁽Reference Mezzetti, Zuliani and Romano³⁴⁾, ex vivo oxidisability of LDL⁽Reference Haidari, Javadi and Kadkhodaee³⁵^, Reference Chiu, Jeng and Shieh³⁶⁾, and plasma levels of these vitamins⁽Reference Luoma, Nayha and Sikkila³⁷^–Reference Esterbauer, Rotheneder and Striegl⁴¹⁾ being potential good predictors of future cardiac events and cardiovascular mortality.

Susceptibility to LDL peroxidation is dependent on the levels of these vitamins⁽Reference Esterbauer, Rotheneder and Striegl⁴¹⁾ and only once they are fully depleted is rapid oxidation possible⁽Reference Jessup, Rankin and De Whalley⁴²⁾. As a consequence these vitamins are frequently referred to, perhaps simplistically, as antioxidant vitamins. The role of these vitamins in reducing LDL oxidation has been most consistently shown for vitamin E while the data has been mixed for vitamin C and β-carotene. Therefore greater emphasis has been put on vitamin E in exploring a preventive or therapeutic role for these vitamins in CVD.

The role of these vitamins in CVD has long been emphasised mainly on the basis of their hypothesised antioxidant properties and the majority of trials have been initiated on this basis. However, in recent years research has greatly expanded in this area and studies now strongly support that the concept that these vitamins have other fundamental non-antioxidative properties, including actions on different aspects of the inflammatory responses that are involved in the pathogenesis of CVD. Each vitamin may have its own non-antioxidative properties, which will be discussed more in detail below, and as a consequence these vitamins target different aspects of the pathogenesis of CVD and as a result more emphasis can be put on the role of combination therapy. Through these properties a further novel role for these vitamins in CVD may be proposed. In the present review we will focus on putative antioxidant roles of these vitamins, but it should be emphasised that their non-antioxidative properties may be relevant to the modulation of CVD risk.

Vitamin E

Vitamin E is the main chain-breaking lipid-soluble vitamin in plasma and LDL⁽Reference Burton, Joyce and Ingold⁴³⁾, present in a complex of four isomers (α-tocopherol, γ-tocopherol, β-tocopherol and δ-tocopherol), of which α-tocopherol is biologically the most active⁽Reference Traber⁴⁴⁾. Supplementation with pharmacological doses ( ≥ 150 IU/d, ≤ 1200 IU/d) of vitamin E has been shown to reduce LDL peroxidation⁽Reference Princen, van Poppel and Vogelezang⁴⁵^, Reference Dieber-Rotheneder, Puhl and Waeg⁴⁶⁾ (1 mg vitamin E is equivalent to 1·49 IU vitamin E).

Atherosclerosis is now accepted to be a chronic inflammatory disease⁽Reference Diaz, Frei and Vita²⁷^, Reference Ross⁴⁷⁾ and vitamin E has shown to mediate anti-inflammatory effects beyond its antioxidative properties⁽Reference Islam, Devaraj and Jialal⁴⁸^–Reference Zapolska-Downar, Zapolski-Downar and Markiewski⁵¹⁾. Through these non-antioxidative properties vitamin E may target aspects of atherosclerosis beyond the oxidation of LDL, therefore extending its potential protective role in CVD. Vitamin E potentially reduces foam cell formation by decreasing monocyte recruitment⁽Reference Devaraj, Li and Jialal⁴⁹^, Reference Wu, Koga and Martin⁵²⁾, through reducing chemokine secretion⁽Reference Devaraj and Jialal⁵³⁾ and by reducing the expression of scavenger receptors on macrophages (CD36)⁽Reference Munteanu, Taddei and Tamburini⁵⁴^, Reference Devaraj, Hugou and Jialal⁵⁵⁾. Vitamin E can also potentially reduce the progression of atherosclerosis by reducing adhesion molecule expression⁽Reference Islam, Devaraj and Jialal⁴⁸^, Reference Zapolska-Downar, Zapolski-Downar and Markiewski⁵¹⁾, inhibiting smooth muscle cell proliferation⁽Reference Keaney, Simon and Freedman⁵⁶^, Reference Ozer, Palozza and Boscoboinik⁵⁷⁾ and platelet aggregation⁽Reference Freedman, Farhat and Loscalzo⁵⁸^, Reference Steiner⁵⁹⁾ and by enhancing NO bioavailability⁽Reference Keaney, Gaziano and Xu⁶⁰⁾. These effects have been shown to be partly mediated via non-antioxidant mechanisms causing inhibition of signalling pathways, particularly protein kinase C⁽Reference Murohara, Ikeda and Katoh⁶¹^, Reference Boscoboinik, Szewczyk and Hensey⁶²⁾, that have potentially been activated by oxidised LDL. Vitamin E has been shown to prevent oxidised LDL-induced NF-κB activation through suppressing protein kinase C⁽Reference Sugiyama, Kugiyama and Ogata⁶³⁾ and inhibiting IκB degradation⁽Reference Li, Saldeen and Mehta⁶⁴⁾, further reducing the inflammatory response that is mediated in CVD.

Another anti-atherogenic property of vitamin E is its ability to modulate gene expression, such as up-regulating endothelial NO synthase mRNA expression and consequently NO levels⁽Reference Goya, Sumitani and Otsuki⁶⁵⁾, hence protecting the endothelium. Vitamin E has been shown to prevent endothelial dysfunction through protecting the endothelium against reactive oxygen species and oxidised LDL⁽Reference Keaney, Guo and Cunningham⁶⁶⁾ and through stimulating endothelial cell proliferation⁽Reference Ulrich-Merzenich, Metzner and Schiermeyer⁶⁷^, Reference Kuzuya, Naito and Funaki⁶⁸⁾ and reducing endothelial apoptosis⁽Reference Uemura, Manabe and Yoshida⁶⁹^, Reference Li, Saldeen and Romeo⁷⁰⁾. These effects are mediated by mechanisms beyond that of inhibition of oxidation of LDL, which include inhibition of oxidised LDL-induced protein kinase C stimulation⁽Reference Keaney, Guo and Cunningham⁶⁶⁾, possibly via an activation of a phosphatase PP2A⁽Reference Azzi, Aratri and Boscoboinik⁷¹⁾, modulation of the Bcl-2 family of apoptosis-related proteins⁽Reference Haendeler, Zeiher and Dimmeler⁷²⁾, by inhibiting caspase-3 activity⁽Reference Uemura, Manabe and Yoshida⁶⁹⁾ and by inhibiting the oxidised LDL-induced up-regulation of angiotensin II receptor (AT1R) mRNA and protein. These properties have been further supported by animal studies⁽Reference Koga, Kwan and Zubik⁷³⁾.

These effects of α-tocopherol have only been confirmed by in vitro studies and animal studies but not yet in vivo. The importance of vitamin E in protecting against atherosclerosis has been further supported by the vitamin E-deficient mouse model, which suffered from increased levels of oxidative stress and atherosclerosis⁽Reference Terasawa, Ladha and Leonard⁷⁴⁾.

Vitamin C

The independent role of vitamin C in CVD has not been extensively assessed in clinical trials. However, as LDL oxidation occurs substantially in the sub-endothelial space⁽Reference Niki, Yamamoto and Komuro⁷⁵⁾, vitamin C may be most important in maintaining the reduced state of vitamin E. Water-soluble antioxidant vitamins, predominantly vitamin C, work to prevent the consumption of hydrophobic antioxidant vitamins such as vitamin E and β-carotene⁽Reference Jialal and Grundy⁷⁶⁾ and ensure their recycling⁽Reference Kagan, Serbinova and Forte⁷⁷⁾, therefore playing an important role in maintaining antioxidative protection. Therefore vitamin C can act synergistically with these other vitamins, enhancing the benefit achieved with supplementation. Like vitamin E, vitamin C has been shown to have additional non-antioxidant properties. Vitamin C has been shown in vivo to suppress endothelial apoptosis mediated by inflammatory cytokines and oxidised LDL⁽Reference Rossig, Hoffmann and Hugel⁷⁸⁾ and it has been shown to promote the proliferation of endothelial cells and the inhibition of vascular smooth muscle growth⁽Reference Ulrich-Merzenich, Metzner and Schiermeyer⁶⁷⁾via the extracellular signal-regulated kinase-signalling pathway⁽Reference Ulrich-Merzenich, Zeitler and Panek⁷⁹⁾. It has also been suggested that vitamin C has a role in preventing restenosis postangioplasty⁽Reference Tomoda, Yoshitake and Morimoto⁸⁰⁾. In fact the combination of vitamins C and E exhibited a stronger positive effect than vitamin C or vitamin E did on their own. Vitamin C has the ability to modulate gene expression and through down-regulating intercellular adhesion molecule-1 gene expression it can reduce monocyte adherence to the endothelium⁽Reference Rayment, Shaw and Woollard⁸¹⁾. Vitamin C has also been shown to enhance NO synthesis in endothelial cells⁽Reference Heller, Munscher-Paulig and Grabner⁸²⁾ and in vivo it has been shown to have sustained beneficial effects on endothelial-derived NO-dependent flow-mediated dilation⁽Reference Gokce, Keaney and Frei⁸³⁾. Vitamin C supplementation has also been shown to reduce vascular smooth muscle cell apoptosis and therefore prevent plaque instability in late-stage atherosclerosis⁽Reference Siow, Richards and Pedley⁸⁴⁾.

β-Carotene

β-Carotene is a fat-soluble vitamin present together with vitamin E in the lipid core of LDL particles⁽Reference Esterbauer, Rotheneder and Striegl⁴¹⁾. It is an excellent trapper of singlet oxygen and potentially a second-line antioxidative defence for LDL particles once vitamin E has been utilised⁽Reference Jessup, Rankin and De Whalley⁴²⁾. The role of carotenoids in oxidative protection has been inconsistent, data indicating neutral⁽Reference Gaziano, Manson and Branch¹⁰^, Reference Princen, van Poppel and Vogelezang⁴⁵^, Reference Reaven, Khouw and Beltz⁸⁵⁾, anti-⁽Reference Tsuchihashi, Kigoshi and Iwatsuki⁸⁶^, Reference Jialal, Norkus and Cristol⁸⁷⁾ and pro-oxidant⁽Reference Rautalahti, Albanes and Virtamo⁸⁸^, Reference Palozza, Calviello and Bartoli⁸⁹⁾ properties. The pro-oxidant effects have been proposed to be due to the tendency of β-carotene radicals reacting with oxygen to give rise to peroxyl radicals that mediate lipid peroxidation⁽Reference Tsuchihashi, Kigoshi and Iwatsuki⁸⁶⁾. Serum carotenoid levels have been inversely associated with atherogenic factors⁽Reference Hozawa, Jacobs and Steffes⁹⁰⁾, risk of atherosclerosis⁽Reference D'Odorico, Martines and Kiechl⁹¹⁾ and cardiovascular mortality⁽Reference Ito, Kurata and Suzuki⁹²⁾; however, these studies looked at the possible effect of a combination of carotenoids and did not assess the independent effect of β-carotene.

High dietary intake of vitamin E⁽Reference Knekt, Ritz and Pereira⁹³^–Reference Stampfer, Hennekens and Manson⁹⁷⁾, vitamin C⁽Reference Osganian, Stampfer and Rimm⁹⁸^, Reference Enstrom, Kanim and Klein⁹⁹⁾ and β-carotene⁽Reference Rimm, Stampfer and Ascherio⁹⁶^, Reference Osganian, Stampfer and Rimm¹⁰⁰^, Reference Klipstein-Grobusch, Geleijnse and den Breeijen¹⁰¹⁾ has been inversely associated with the incidence of CHD. High dietary intake of β-carotene has been associated with a reduced CVD mortality⁽Reference Buijsse, Feskens and Kwape¹⁰²⁾ and all-cause mortality⁽Reference Buijsse, Feskens and Schlettwein-Gsell¹⁰³⁾; however, this was restricted to elderly individuals.

The favourable safety profile of these vitamins⁽Reference Hathcock, Azzi and Blumberg¹⁰⁴^, Reference Kappus and Diplock¹⁰⁵⁾ has allowed several clinical trials to be conducted attempting to confirm their role. At this point the results have been inconsistent, with a few small trials suggesting a protective role while large-scale trials have concluded no benefit with vitamin supplementation in patients at high risk of CVD⁽Reference Wright, Lawson and Weinstein¹⁰⁶^–Reference Blot, Li and Taylor¹¹⁶⁾, or with pre-existing CVD⁽Reference Cook, Albert and Gaziano¹¹⁷^–Reference Brown, Zhao and Chait¹²⁰⁾.

There have been several explanations for this lack of correlation between observational studies and randomised controlled trials. The lack of benefit in randomised controlled trials could suggest that these vitamins are not the protective components in fruit and vegetables. As the results of observational studies can be as a consequence of confounding factors it is possible that other components of fruit and vegetables are the mediators of cardiovascular protection, such as flavonoids, fibre, etc.

However, the lack of benefit could also be a consequence of the differences in duration, vitamin dosages and target population between observational studies and randomised controlled trials.

Observational studies have been conducted on an average for 11 years while randomised controlled trials have continued for an average of 4 years, which can suggest that supplementation needs to be conducted for a longer period of time to gain benefit. Steinberg⁽Reference Steinberg¹²¹⁾ hypothesised that antioxidants were targeting early stages of atherosclerosis so that the average 4·5-year duration of the majority of trials was too short to achieve beneficial effects. However, none of the pre-existing trials have indicated any trend towards a protective role and the two trials conducted over more than 10 years⁽Reference Lee, Cook and Gaziano¹⁰⁸^, Reference Hennekens, Buring and Manson¹¹³⁾ have further disputed the role of antioxidants in CVD. Therefore before the trial duration is extended other areas should be addressed. The lack of detailed knowledge of the mechanism of oxidative modification has restricted us in defining an optimal antioxidant vitamin. The lack of efficient biomarkers for oxidative stress has not allowed us to assess in vivo effectiveness of these vitamins' antioxidant properties and define the optimal vitamin dosage. Whether the dosage of these vitamins plays a role in their beneficial effects is addressed in the present review.

The targeted population is still undefined; however, pre-existing evidence is suggestive of targeting subgroups such as smokers, diabetics and the elderly.

These vitamins have been shown to mediate effects beyond their antioxidative properties; however, at this point these have only been shown in vitro and not yet explored in in vivo studies. The present review will address the hypotheses that have been put forward to try to explain the lack of benefit with these vitamins in randomised controlled trials, provide further evidence regarding their role in CVD and explore what the future may entail for vitamin therapy in CVD.

Dosage, oxidative markers and isomers

The optimal vitamin dosage has not yet been defined. Nutritional doses of vitamin E (about 4–8 mg/d)⁽Reference Knekt, Ritz and Pereira⁹³^–Reference Rimm, Stampfer and Ascherio⁹⁶⁾ and vitamin E supplementation for at least 2 years with >100 IU/d⁽Reference Rimm, Stampfer and Ascherio⁹⁶^, Reference Stampfer, Hennekens and Manson⁹⁷⁾ with a maximum dose of 1000 mg/d⁽Reference Hathcock, Azzi and Blumberg¹⁰⁴⁾ have been beneficial in CHD. However, the majority of observational studies have shown disappointing findings in regards to supplemental intake of vitamin E ( ≤ 25 mg/d up to ≥ 250 mg/d)⁽Reference Knekt, Ritz and Pereira⁹³^, Reference Kushi, Folsom and Prineas⁹⁴⁾. Randomised clinical trials supplementing with 330–800 IU vitamin E per d have also been disappointing⁽Reference Lee, Cook and Gaziano¹⁰⁸^, Reference Hodis, Mack and LaBree¹¹⁰^, ¹¹⁵^, Reference de Gaetano¹²²^–Reference Lonn, Bosch and Yusuf¹²⁵⁾. The doses of these vitamins used in trials have been questioned, on the one hand for being suboptimal and on the other for being too high. Studies by Jialal et al. ⁽Reference Jialal, Fuller and Huet¹²⁶⁾ and Simons et al. ⁽Reference Simons, Von Konigsmark and Balasubramaniam¹²⁷⁾ and findings from observational studies support the concept that the dosages used in trials are not suboptimal. The use of mega-doses of these vitamins has been disputed due to their potential pro-oxidant⁽Reference Princen, van Poppel and Vogelezang⁴⁵^, Reference Upston, Terentis and Stocker¹²⁸^, Reference Podmore, Griffiths and Herbert¹²⁹⁾ and pro-atherogenic effects⁽Reference Keaney, Gaziano and Xu¹³⁰⁾ and their negative drug interactions⁽Reference Cheung, Zhao and Chait¹³¹^, Reference Corrigan¹³²⁾. Even though adverse effects are uncommon and shown to occur at doses well above those used in trials, it is possible that these override their beneficial effects, giving no net gained benefit.

The Vitamin E Atherosclerosis Prevention Study (VEAPS) trial⁽Reference Hodis, Mack and LaBree¹¹⁰⁾ indicated that a level of oxidative protection is needed to be achieved to gain anti-atherogenic effects, which is suggested by trials to be achieved with 800 IU RRR-α-tocopherol per d⁽Reference Fang, Kinlay and Beltrame¹³³^–Reference Stephens, Parsons and Schofield¹³⁵⁾. Vitamins' antioxidative effectiveness is assessed ex vivo or through plasma or urinary levels of oxidised biomarkers and it is not clear whether this accurately estimates arterial wall oxidation. These vitamins have been shown to reduce levels of oxidative stress in plasma but not in plaques⁽Reference Steinberg¹²¹⁾. Out of eighteen large-scale trials, only three assessed the effect that vitamin supplementation had on the level of oxidative stress (Table 1) ⁽Reference Hercberg, Galan and Preziosi¹⁰⁷^, Reference Lee, Cook and Gaziano¹⁰⁸^, Reference Hodis, Mack and LaBree¹¹⁰^–Reference Salonen, Nyyssonen and Salonen¹¹²^, Reference Omenn, Goodman and Thornquist¹¹⁴^–Reference Blot, Li and Taylor¹¹⁶^, Reference Waters, Alderman and Hsia¹¹⁸^–Reference Brown, Zhao and Chait¹²⁰^, Reference de Gaetano¹²²^–Reference Jialal, Fuller and Huet¹²⁶^, Reference Fang, Kinlay and Beltrame¹³³^–Reference Manson, Gaziano and Spelsberg¹³⁶⁾.

Table 1

Trials assessing antioxidant effectiveness*

na, Not been assessed in the trial.

* This Table looks at how intervention studies that are assessing the role of antioxidants in CVD have tried to assess the effectiveness of antioxidants, with some of them measuring antioxidant plasma levels and a few measuring the level of LDL oxidation.

Failure to achieve the oxidative threshold could be the underlying reason behind the disappointing findings of trials. Through identifying more accurate oxidative biomarkers we could assess whether these vitamins mediate their predicted antioxidative effects and identify dose–response curves for optimal oxidative and inflammatory protection.

It has been proposed that the vitamin isomer used in trials is relevant in regards to its effects. The trials that have concluded a positive effect with vitamin E all used RRR-α-tocopherol⁽Reference Salonen, Nyyssonen and Salonen¹¹²^, Reference Fang, Kinlay and Beltrame¹³³^–Reference Stephens, Parsons and Schofield¹³⁵⁾ and five out of nine trials that indicated neutral effects used all-rac α-tocopherol⁽Reference Hodis, Mack and LaBree¹¹⁰^, ¹¹⁵^, ¹¹⁹^, Reference de Gaetano¹²²^, ¹²⁴⁾. Stereoisomers differ structurally and as a result this can restrict their participation in signalling pathways and in other processes, which can result in them not mediating their non-antioxidative actions including the anti-inflammatory effects discussed previously. It is therefore possible that due to the vitamin E isomer that is used for supplementation in trials the non-antioxidative effects of vitamin E are not observed. All-rac α-tocopherol has a lower bioactivity than RRR-α-tocopherol⁽Reference Hoppe and Krennrich¹³⁷^, Reference Leth and Sondergaard¹³⁸⁾ and has been shown to lack anti-inflammatory properties⁽Reference Vega-Lopez, Kaul and Devaraj¹³⁹⁾ at dosages where this is achieved by RRR-α-tocopherol⁽Reference Upritchard, Sutherland and Mann¹⁴⁰⁾.

The interaction of exogenous and endogenous vitamins: are we using the wrong vitamin?

Traber⁽Reference Traber¹⁴¹⁾ hypothesised that single supplements may interfere with the uptake, transport, distribution and metabolism of other non-supplemented antioxidant nutrients. The disappointing results of clinical trials could therefore be a result of vitamins' negative interaction with other potentially protective vitamins. Even though studies have emphasised a role for α-tocopherol, γ-tocopherol has been shown to have an anti-atherogenic role⁽Reference Jiang, Christen and Shigenaga¹⁴²⁾ and a superior anti-inflammatory effect to that of α-tocopherol⁽Reference Jiang, Elson-Schwab and Courtemanche¹⁴³⁾. The main constituent of vitamin E supplementation is usually α-tocopherol, which has been implicated in reducing γ-tocopherol levels⁽Reference Dieber-Rotheneder, Puhl and Waeg⁴⁶^, Reference Huang and Appel¹⁴⁴^, Reference Handelman, Machlin and Fitch¹⁴⁵⁾ through competing for the same intestinal uptake mechanism⁽Reference Handelman, Machlin and Fitch¹⁴⁵⁾. Therefore the lack of benefits in trials could potentially be due to the gain from one vitamin causing the loss in protection mediated by another vitamin.

As we still lack knowledge regarding the mechanism of LDL oxidation in vivo the optimal vitamin in this context has not been defined. Total carotenoid intake has been associated with a reduced cardiovascular incidence and mortality⁽Reference Liu, Lee and Ajani⁵^, Reference Gaziano, Manson and Branch¹⁰⁾ and the lack of benefit with β-carotene is suggestive that this is the wrong carotenoid. Lycopene is a singlet oxygen scavenger which is part of the carotenoid family and has been predicted to be a stronger antioxidant vitamin than β-carotene⁽Reference Di Mascio, Kaiser and Sies¹⁴⁶⁾. High plasma levels of lycopene have been associated with a reduced risk of atherosclerosis⁽Reference Rissanen, Voutilainen and Nyyssonen¹⁴⁷^, Reference McQuillan, Hung and Beilby¹⁴⁸⁾ and CVD⁽Reference Sesso, Buring and Norkus¹⁴⁹^, Reference Rissanen, Voutilainen and Nyyssonen¹⁵⁰⁾. The effects of lycopene have not yet been assessed in large-scale trials.

Is combination therapy superior to single vitamin supplementation? Should we avoid β-carotene?

These vitamins show different efficacy depending on the type of oxidative stress and the body compartment in which it takes place. The lack of knowledge regarding where and how LDL undergoes oxidative modification has restricted us in defining the optimal vitamin type. As these vitamins each possess a specific role in the antioxidant defence system, through the use of a combination of vitamins the overall protection would potentially be broadened.

The potential superiority of combination therapy may be predicted from the following:

(1) Protective effects seen in observational studies with high dietary intake of fruit and vegetables containing several of these vitamins;
(2) Lack of benefit in randomised clinical trials with single compound supplementation;
(3) Pro-oxidant effect of these vitamins in the absence of required cofactors;
(4) Experimental data for the cooperative and synergistic effects of vitamins.

In fruit and vegetables there is a natural interaction between hydrophobic (for example, vitamin E) and hydrophilic (vitamin C) antioxidant vitamins that is lost with single vitamin supplementation and this could account for the lack of benefit. Supplementation with only one of these vitamins could result in an imbalance of endogenous antioxidants which weakens the antioxidant defence system and enables pro-oxidant effects to emerge⁽Reference Stocker, Bowry and Frei¹⁵¹⁾, as with tocopherol-mediated atherosclerosis seen with high doses of vitamin E⁽Reference Neuzil, Thomas and Stocker¹⁵²⁾. Through increasing the dietary intake of fruit and vegetables this results in increased levels of these vitamins in the ‘right environment’. Therefore through combination supplementation using doses of vitamins in physiological ratios we can optimise antioxidant status without resulting in an imbalance in the endogenous antioxidant levels. These vitamins have shown to act synergistically to mediate protection against oxidative stress. Vitamin C has been shown to regenerate vitamin E from its oxidised to its active state⁽Reference Kagan, Serbinova and Forte⁷⁷^, Reference Buettner¹⁵³^, Reference Packer, Slater and Willson¹⁵⁴⁾, to minimise its pro-oxidant effects⁽Reference Hirano, Kondo and Iwamoto¹⁵⁵⁾ and to cause synergistic inhibition of LDL peroxidation⁽Reference Sato, Niki and Shimasaki¹⁵⁶^, Reference Rifici and Khachadurian¹⁵⁷⁾. β-Carotene has also been shown to act synergistically with vitamin E⁽Reference Palozza and Krinsky¹⁵⁸⁾. It can then be hypothesised that this enhanced protection against oxidative stress should provide a greater anti-atherogenic effect and as a consequence reduce the incidence of clinical endpoints. However, clinical trials using a ‘cocktail’ of vitamins have not indicated any such positive effects⁽Reference Hercberg, Galan and Preziosi¹⁰⁷^, Reference Blot, Li and Taylor¹¹⁶^–¹¹⁹⁾. Jialal & Grundy⁽Reference Jialal and Grundy¹⁵⁹⁾ and Fuller et al. ⁽Reference Fuller, May and Martin¹⁶⁰⁾ concluded that combinations of these vitamins at doses similar to those used in trials (400–800 IU vitamin E per d, 1 g vitamin C per d and 30 mg β-carotene per d) did not provide further oxidative protection of LDL compared with a high dose of α-tocopherol (800 IU/d) on its own. This is therefore suggestive that the combination of these vitamins does not cause a greater reduction in lipid peroxidation than that attributable to single vitamin supplementation.

The majority of the ‘cocktail’ supplementations used in trials have included β-carotene⁽Reference Hercberg, Galan and Preziosi¹⁰⁷^, Reference Christen and Gaziano¹¹¹^, Reference Blot, Li and Taylor¹¹⁶^, ¹¹⁹^, Reference Brown, Zhao and Chait¹²⁰^, Reference Singh, Niaz and Rastogi¹⁶¹⁾ despite the pre-existing data disputing a role for β-carotene as an antioxidant vitamin, and even indicating pro-oxidant effects at the dosages used in trials⁽Reference Rautalahti, Albanes and Virtamo⁸⁸^, Reference Palozza, Calviello and Bartoli⁸⁹^, Reference Yang and Lowe¹⁶²⁾. Together with the negative effects seen in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study (ATBC)⁽Reference Wright, Lawson and Weinstein¹⁰⁶⁾ and the Beta-Carotene and Retinol Efficacy Trial (CARET)⁽Reference Omenn, Goodman and Thornquist¹¹⁴⁾ its use in CVD is highly questionable, particularly in smokers. The trials including β-carotene supplementation have overall failed to show a beneficial role in CVD (Table 2) ⁽Reference Hercberg, Galan and Preziosi¹⁰⁷^, Reference Christen and Gaziano¹¹¹^, Reference Blot, Li and Taylor¹¹⁶^, ¹¹⁹⁾; however, a combination of vitamins excluding β-carotene has indicated a beneficial role in atherosclerosis (Table 3) ⁽Reference Salonen, Nyyssonen and Salonen¹¹²^, Reference Arad, Spadaro and Roth¹⁶³⁾. In vivo the carotenoids do not appear alone but in a heterogeneous mixture, possibly acting synergistically⁽Reference Kiokias and Gordon¹⁶⁴^, Reference Stahl, Junghans and de Boer¹⁶⁵⁾. Through supplementation with only one carotenoid this can potentially lead to negative effects. Therefore the lack of overall protection with combination therapy could be as a result of a net negative balance between β-carotene pro-oxidant and vitamin E and C antioxidant effects. The beneficial effects in the Transplant-Associated Arteriosclerosis Trial⁽Reference Fang, Kinlay and Beltrame¹³³⁾ and the Antioxidant Supplementation in Atherosclerosis Prevention (ASAP) trial⁽Reference Salonen, Nyyssonen and Salonen¹¹²⁾ do, however, support a role for vitamins C and E in combination in preventing the progression of atherosclerosis. However, trials have shown contradictory results with the combined supplementation of vitamin C and vitamin E with regard to clinical endpoints, with one suggesting protective effects⁽Reference Arad, Spadaro and Roth¹⁶³⁾ and the other indicating neutral effects⁽Reference Cook, Albert and Gaziano¹¹⁷⁾.

Table 2

Intervention studies: combination of antioxidants including β-carotene⁽Reference Steinberg¹²¹^, Reference Stanner, Hughes and Kelly²⁵⁵^, Reference Clarke and Armitage²⁹²⁾*†

SU.VI.MAX, Supplémentation en Vitamines et Minéraux Antioxydants; HATS, HDL-Atherosclerosis Treatment Study.

* This Table includes intervention studies (double-blinded randomised controlled trials) that assess the effect of a combination of antioxidants (that include β-carotene) on CVD. It provides information regarding the structure of the intervention studies and their outcomes.

† 1 mg vitamin E per d is equivalent to 1·49 IU vitamin E per d.

Table 3

Intervention studies: combination of antioxidants excluding β-carotene⁽Reference Steinberg¹²¹^, Reference Stanner, Hughes and Kelly²⁵⁵^, Reference Clarke and Armitage²⁹²⁾*†

ASAP, Atorvastatin Simvastatin Atherosclerosis Progression; IMT, intima-to-media thickness; WACS, Women's Antioxidant Cardiovascular Study; WAVE, Women's Angiographic Vitamin and Estrogen.

* This Table includes intervention studies (double-blinded randomised controlled trials) that assess the effect of a combination of antioxidants (that exclude β-carotene) on CVD. It provides information regarding the structure of the intervention studies and their outcomes.

† 1 mg vitamin E per d is equivalent to 1·49 IU vitamin E per d.

The role of vitamins in the progression and complication of atherosclerosis: should we start these vitamins earlier?

The role of these vitamins in preventing the progression of atherosclerosis and destabilisation of plaques has not been fully confirmed. Oxidised LDL has been shown to stimulate smooth muscle proliferation⁽Reference Zhao, Seng and Zhang¹⁶⁶^, Reference Qiao, Zhang and Xia¹⁶⁷⁾ and platelet aggregation⁽Reference Siess, Zangl and Essler¹⁶⁸⁾ and to be an independent marker for the destabilisation of plaques⁽Reference Anselmi, Garbin and Agostoni¹⁶⁹⁾. These vitamins have in vitro been shown to reduce platelet aggregation⁽Reference Ryszawa, Kawczynska-Drozdz and Pryjma¹⁷⁰^, Reference Jandak, Steiner and Richardson¹⁷¹⁾ and modulate smooth muscle phenotype⁽Reference Azzi, Aratri and Boscoboinik⁷¹⁾, potentially playing a role in retarding the progression of late-stage atherosclerosis, hence attempts to use them in secondary prevention. However, the neutral effects seen in secondary prevention trials may be indicative of the wrong timing of supplementation. Animal studies and observational studies have indicated a therapeutic role through assessing their effect on early lesions while in trials the primary endpoints have been the incidence of major vascular events. Steinberg & Witztum⁽Reference Steinberg and Witztum¹⁷²⁾ suggested that antioxidant vitamins are only protective when given before the development of disease, prioritising a role for them in primary prevention.

A meta-analysis of secondary prevention trials concluded that there was a lack of anti-atherogenic effect of vitamin supplementation⁽Reference Bleys, Miller and Pastor-Barriuso¹⁷³⁾ and individuals with late-stage atherosclerosis and pre-existing CVD actually had increased cardiac and all-cause mortality with vitamin supplementation⁽¹¹⁹^, Reference Manson, Gaziano and Spelsberg¹³⁶^, Reference Rapola, Virtamo and Ripatti¹⁷⁴^, Reference Miller, Pastor-Barriuso and Dalal¹⁷⁵⁾. This negative effect on fatal and non-fatal CHD is not seen in individuals without pre-existing CHD⁽Reference Virtamo, Rapola and Ripatti¹⁷⁶⁾ and the use of vitamins in these individuals has even suggested a 30 % reduction in overall mortality⁽Reference Hayden, Welsh-Bohmer and Wengreen¹⁷⁷⁾. The negative effects of these vitamins on late-stage atherosclerosis may be due to their limiting effect on ischaemic pre-conditioning⁽Reference Sun, Tang and Park¹⁷⁸⁾, negative interaction with drugs commonly taken by these patients such as nitrates, warfarin and diuretics⁽Reference Hayden, Welsh-Bohmer and Wengreen¹⁷⁷⁾ and their pro-oxidant effects⁽Reference Bowry and Stocker¹⁷⁹⁾ that can destabilise the plaque. These findings are suggestive that vitamin supplementation may have an adverse effect on plaque-related complications and, if so, its use should be restricted to those with early stages of disease, excluding individuals with late-stage atherosclerosis. However, this is difficult in practice. In Western populations atherosclerosis begins early in life, implying that such supplementation should be initiated in childhood and continued for decades. At the same time most adults, certainly those with overt CVD, will have late atherosclerotic lesions.

Directing vitamin use to subgroups

Jialal et al. ⁽Reference Jialal, Freeman and Grundy¹⁸⁰⁾ concluded that LDL preparations from different individuals showed different susceptibility or resistance to oxidation. Studies have indicated inter-individual variation in the response seen with antioxidants⁽Reference Esterbauer, Puhl and Dieber-Rotheneder¹⁸¹⁾, suggesting that individuals exposed to increased levels of oxidative stress or who were antioxidant deficient would gain more benefit⁽Reference Halliwell¹⁸²⁾. Vitamin E has been shown to have a variable antioxidant effect that is dependent on the rate of lipid peroxidation⁽Reference Azzi, Boscoboinik and Marilley¹⁸³⁾ and supplementation studies with vitamin E have indicated no significant effect on lipid peroxidation in vivo in healthy individuals⁽Reference Meagher, Barry and Lawson¹⁸⁴^, Reference Patrignani, Panara and Tacconelli¹⁸⁵⁾. These results dispute the role of vitamin E supplementation in individuals with normal baseline levels of antioxidants and oxidative stress (who then appear to be ‘non-responders’). As the majority of trial participants meet their RDA of these so-called antioxidant vitamins and with none of the large clinical trials assessing baseline levels of oxidative stress it is possible that the inclusion of ‘non-responders’ dilute the overall beneficial effect that is seen with responders, accounting for the disappointing overall findings. Clinical trials targeting individuals with an abnormal antioxidant status have shown more consistent benefits⁽Reference Salonen, Nyyssonen and Kaikkonen¹⁰⁹^, Reference Fang, Kinlay and Beltrame¹³³^, Reference Boaz, Smetana and Weinstein¹³⁴^, Reference Singh, Niaz and Rastogi¹⁶¹⁾, indicating a role for subgroup targeting.

The Cambridge Heart Antioxidant Study (CHAOS) trial⁽Reference Stephens, Parsons and Schofield¹³⁵⁾ concluded that there was a significant reduction in cardiovascular events with α-tocopherol supplementation. Brown⁽Reference Brown¹⁸⁶⁾ concluded that a large number of these patients had a 3·5-fold increase in frequency for a polymorphism in the endothelial NO synthase gene that made them more prone to endothelial dysfunction and of greater need for vitamin E, hence further supporting subgroup targeting. However, it is still hard to accurately identify individuals exposed to increased oxidative stress due to the lack of efficient biomarkers for oxidative stress.

Patients with cardiovascular risk factors are exposed to greater amounts of oxidative stress⁽Reference Meagher and Rader¹⁸⁷⁾, which contributes to endothelial dysfunction⁽Reference Block, Dietrich and Norkus¹⁸⁸^, Reference Panza, Casino and Kilcoyne¹⁸⁹⁾. The enhanced level of oxidative stress is partly due to their reduced dietary intake of these so-called antioxidant vitamins⁽Reference Singh, Niaz and Bishnoi¹⁹⁰⁾ and this could possibly be responsible for the increased rate of atherosclerosis seen in these patients. The use of vitamins could retard the development of cardiovascular risk factors and reduce the risk of CVD.

Vitamin supplementation has been shown to improve endothelium-dependent dilatation in smokers⁽Reference Heitzer, Just and Munzel¹⁹¹⁾ and in hypercholesterolaemic⁽Reference Neunteufl, Kostner and Katzenschlager¹⁹²⁾, hypertensive⁽Reference Plantinga, Ghiadoni and Magagna¹⁹³⁾ and diabetic patients⁽Reference Skyrme-Jones, O'Brien and Berry¹⁹⁴⁾.

Subgroup 1: smokers

Smoking is associated with an increased progression of atherosclerosis⁽Reference Poredos, Orehek and Tratnik¹⁹⁵⁾ and of heart disease⁽Reference Glantz and Parmley¹⁹⁶⁾, possibly mediated by exposure to increased levels of oxidative stress⁽Reference Harats, Ben-Naim and Dabach¹⁹⁷^–Reference Morrow, Frei and Longmire¹⁹⁹⁾. In smokers, plasma ascorbic acid, α-tocopherol and β-carotene levels are significantly depleted⁽Reference Mezzetti, Lapenna and Pierdomenico²⁰⁰^–Reference Faure, Preziosi and Roussel²⁰²⁾, partly as a consequence of increased utilisation⁽Reference Ayaori, Hisada and Suzukawa²⁰³^, Reference Frei, Forte and Ames²⁰⁴⁾, reduced regeneration of ascorbic acid⁽Reference Lykkesfeldt, Loft and Nielsen²⁰⁵⁾ and their poorer diet⁽Reference Stryker, Kaplan and Stein²⁰⁶⁾. Smokers have also been shown to have a down-regulated enzymic antioxidant defence system with reduced levels of catalase and glutathione peroxidase⁽Reference Hemalatha, Venkatesan and Bobby²⁰⁷⁾ making them further prone to oxidative damage.

Supplementation with a combination of vitamins re-establishes a normal antioxidant status⁽Reference Lykkesfeldt, Christen and Wallock²⁰⁸⁾ and reduces oxidative stress⁽Reference Fuller, Grundy and Norkus²⁰⁹^–Reference Kim and Lee²¹¹⁾ in smokers. The Antioxidant Supplementation in Atherosclerosis Prevention (ASAP) study⁽Reference Salonen, Nyyssonen and Salonen¹¹²⁾ concluded a greater anti-atherogenic benefit in smokers than non-smokers. These vitamins have also on the other hand been shown to mediate a pro-oxidant effect with increased levels of oxidative stress⁽Reference Yang and Lowe¹⁶²⁾ and the likelihood of this is enhanced in smokers⁽Reference Princen, van Poppel and Vogelezang⁴⁵^, Reference Truscott²¹²⁾. However, the use of a combination of vitamins excluding β-carotene may prevent the increased likelihood of pro-oxidant effects and the negative findings encountered in the Beta-Carotene and Retinol Efficacy Trial (CARET)⁽Reference Omenn, Goodman and Thornquist¹¹⁴⁾ and the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) study⁽¹¹⁵⁾.

Subgroup 2: obese and overweight patients

In obese and overweight individuals fat-soluble vitamins can potentially become retained in visceral tissue, which can cause reduced serum levels of these vitamins. It has been shown that obese children have significantly reduced circulating levels of vitamin E and β-carotene⁽Reference Strauss²¹³^, Reference Decsi, Molnar and Koletzko²¹⁴⁾ and reduced LDL β-carotene and vitamin E levels⁽Reference Kuno, Hozumi and Morinobu²¹⁵⁾ compared with normal-weight children. As a result these individuals may be more prone to oxidative stress with an increased likelihood of endothelial dysfunction and LDL oxidation. This can partly account for the increased risk of CVD in obese or overweight individuals and supports a therapeutic role for supplementation with these vitamins in these individuals. Vitamin E supplementation in obese and overweight individuals has been shown to improve the metabolic profile (HbA1c, serum malondialdehyde levels and erythrocyte glutathione peroxidase activity were reduced)⁽Reference Ble-Castillo, Cleva-Villanueva and Diaz-Zagova²¹⁶⁾, increase antioxidant levels and reduce pro-oxidants⁽Reference Ble-Castillo, Cleva-Villanueva and Diaz-Zagova²¹⁶^, Reference Sutherland, Manning and Walker²¹⁷⁾. Randomised controlled trials are required to assess whether this reduction in oxidative stress reduces the development of CVD in obese or overweight individuals.

As obesity is a major risk factor for CVD, a large number of individuals in secondary prevention trials are likely to be overweight or obese. As the majority of trials do not directly assess the antioxidative actions of these vitamins in vivo and obese individuals are prone to the retention of vitamins in adipose tissue, it is plausible that these vitamins do not mediate their predicted effect in these individuals, possibly partly accounting for the lack of benefit seen in trials. This further emphasises the importance of identifying accurate biomarkers to assess vitamins' antioxidant effectiveness in vivo.

Subgroup 3: hypercholesterolaemic patients

Studies have shown that hypercholesterolaemic individuals have increased plasma lipid peroxide levels⁽Reference Davi, Alessandrini and Mezzetti²¹⁸^, Reference Yalcin, Sabuncu and Kilinc²¹⁹⁾ and possess LDL that is more susceptible to oxidation⁽Reference Cominacini, Pastorino and Garbin²²⁰^, Reference Lavy, Brook and Dankner²²¹⁾. As α-tocopherol activity has been shown to be inversely related to cholesterol content in plaques⁽Reference Parker, Sabrah and Cap²²²⁾, these individuals are also prone to have a diminished antioxidant status. This suggests that these individuals would benefit from vitamin supplementation.

Vitamin supplementation has been shown to alter lipid profile, mediating a reduction in total cholesterol, TAG and LDL levels⁽Reference Rezaian, Taheri and Mozaffari²²³⁾, and positively correlating with HDL levels in individuals without diagnosed disease⁽Reference Simon and Hudes²²⁴^, Reference Hallfrisch, Singh and Muller²²⁵⁾. The use of vitamins in hypercholesterolaemia can be questioned as they have been shown to blunt the beneficial effect of simvastatin/niacin⁽Reference Brown, Zhao and Chait¹²⁰⁾. However, α-tocopherol has been excluded as a potential cause of this response⁽Reference Singh, Otvos and Dasgupta²²⁶⁾. Nonetheless vitamin E supplementation in hypercholesterolaemic patients has resulted in a small but significant decrease in HDL-cholesterol levels and therefore caution still needs to be taken in regards to vitamin E supplementation⁽Reference Leonard, Joss and Mustacich²²⁷⁾.

Statins have been shown to reduce vitamin E, β-carotene and ubiquinol-10 levels⁽Reference Jula, Marniemi and Huupponen²²⁸⁾ and it has therefore been suggested that they may worsen the antioxidant status. This could possibly be due to statins reducing the circulating LDL fraction and therefore the delivery of these vitamins. This fact further emphasises a probably beneficial role of vitamin supplementation in hypercholesterolaemic patients.

In recent studies it has been shown that patients taking 10 mg atorvastatin per d gain an increase in plasma level of vitamin E (+42 %; P < 0·01)⁽Reference Cangemi, Loffredo and Carnevale²²⁹⁾ and dual therapy with vitamins and statins has appeared to provide greater cardiovascular protection than statins on their own⁽Reference Blum, Milman and Shapira²³⁰⁾. The lack of negative interaction between these agents further emphasises a beneficial role of supplementation with these vitamins in hypercholesterolaemic patients.

Subgroup 4: hypertensive patients

It has also been hypothesised that oxidative stress plays a role in the pathogenesis of hypertension and hypertension-induced damage through reducing NO levels and inducing endothelial dysfunction⁽Reference Landmesser and Harrison²³¹⁾. Hypertensive patients have been shown to be exposed to increased levels of lipid peroxidation and to have abnormal antioxidant status⁽Reference Russo, Olivieri and Girelli²³²⁾. Observational trials have shown an inverse correlation between fruit and vegetable intake⁽Reference Appel, Moore and Obarzanek²³³⁾, serum levels of putative antioxidant vitamins⁽Reference Chen, He, Hamm and Batuman²³⁴^, Reference Salonen, Salonen and Ihanainen²³⁵⁾ and the development of high blood pressure. These vitamins have been shown in in vitro studies to play a role in the aetiology of hypertension by restoring NO activity and endothelial function⁽Reference Plantinga, Ghiadoni and Magagna¹⁹³^, Reference On, Kim and Sohn²³⁶^, Reference Tse, Maxwell and Thomason²³⁷⁾. Vitamin E (400 IU/d) and vitamin C (1000 mg/d) supplementation resulted in beneficial effects on endothelium-dependent vasodilatation and arterial stiffness⁽Reference Plantinga, Ghiadoni and Magagna¹⁹³⁾ and in significantly lower systolic, diastolic and mean arterial blood pressure levels compared with the placebo group⁽Reference Rodrigo, Prat and Passalacqua²³⁸⁾. Hypertensive patients have been more consistently shown to possess a reduction in ascorbic acid levels than those of any other antioxidant vitamins⁽Reference Fotherby, Williams and Forster²³⁹⁾, possibly indicating an advantage of vitamin C supplementation over the other vitamins in these patients. Dietary intake⁽Reference Ness, Khaw and Bingham²⁴⁰⁾ and plasma levels⁽Reference Bates, Walmsley and Prentice²⁴¹⁾ of ascorbic acid have been inversely related to blood pressure in some studies but not all⁽Reference Czernichow, Bertrais and Blacher²⁴²^, Reference Miller, Appel and Levander²⁴³⁾; in view of the lack of long-term benefit⁽Reference Kim, Sasaki and Sasazuki²⁴⁴⁾ further research is required.

Subgroup 5: diabetic patients

Diabetic patients are exposed to increased levels of lipid peroxidation⁽Reference Gopaul, Anggard and Mallet²⁴⁵⁾ as a result of LDL glycation⁽Reference Sobal, Menzel and Sinzinger²⁴⁶⁾ and their increased levels of the small dense LDL subfraction⁽Reference Anderson, Gowri and Turner²⁴⁷⁾, contributing to their high risk of macrovascular complications⁽Reference Uusitupa, Niskanen and Siitonen²⁴⁸⁾. Vitamin E supplementation with doses that are greater than 800 IU/d in type 1 and 2 diabetic patients have been shown to reduce the oxidisability of LDL⁽Reference Fuller, Chandalia and Garg²⁴⁹^, Reference Reaven, Herold and Barnett²⁵⁰⁾ and improve endothelial function⁽Reference Skyrme-Jones, O'Brien and Berry¹⁹⁴^, Reference Ting, Timimi and Boles²⁵¹⁾. Supplementation with high-dose α-tocopherol has been associated with a reduced incidence of CHD⁽Reference Costacou, Zgibor and Evans²⁵²⁾ and microvascular complications⁽Reference Bursell, Clermont and Aiello²⁵³⁾ in diabetic users compared with non-users. Supplementation with 400 IU vitamin E per d also resulted in a significant reduction in cardiovascular events compared with a placebo group⁽Reference Milman, Blum and Shapira²⁵⁴⁾. However, its long-term effects have not been confirmed by trials⁽Reference Stanner, Hughes and Kelly²⁵⁵^, Reference Lonn, Yusuf and Hoogwerf²⁵⁶⁾, possibly due to the use of lower vitamin dosages than those that have indicated short-term benefit in the small supplementation studies. At this point the available data are still too sparse to suggest the recommendation of vitamins to diabetic patients and more emphasis should be placed on targeting other diabetic-associated atherogenic factors.

Subgroup 6: patients with end-stage renal failure

IHD remains a leading cause of death in end-stage renal failure patients. Vitamin supplementation has been beneficial to patients with end-stage renal failure⁽Reference Boaz, Smetana and Weinstein¹³⁴^, Reference Jha, Flather and Lonn²⁵⁷⁾ through reducing their increased levels of oxidative stress⁽Reference Handelman, Walter and Adhikarla²⁵⁸^, Reference Dasgupta, Hussain and Ahmad²⁵⁹⁾. The Secondary Prevention with Antioxidants of Cardiovascular disease in End-stage renal disease (SPACE) trial showed a 70 % reduction in myocardial infarct rates in haemodialysis patients with pre-existing CVD when supplemented with high-dose vitamin E⁽¹²⁴^, Reference Jha, Flather and Lonn²⁵⁷⁾.

Subgroup 7: cardiac transplant or acute myocardial infarction patients

Atherosclerosis is a major complication of transplantation that limits the prolonged benefit of the transplant⁽Reference Mullins, Cary and Sharples²⁶⁰⁾.

Vitamin supplementation has been beneficial to cardiac transplant⁽Reference Fang, Kinlay and Beltrame¹³³⁾ and acute myocardial infarction patients⁽Reference Singh, Niaz and Rastogi¹⁶¹^, Reference Jaxa-Chamiec, Bednarz and Drozdowska²⁶¹⁾ in reducing their increased levels of oxidative stress⁽Reference Pechan, Danova and Olejarova²⁶²^, Reference Singh, Niaz and Sharma²⁶³⁾, making these vitamins a possible novel treatment for improving survival in these patients. The Indian Experiment of Infarct Survival⁽Reference Singh, Niaz and Rastogi¹⁶¹⁾ and the Myocardial Infarction and Vitamins (MIVIT) pilot⁽Reference Jaxa-Chamiec, Bednarz and Drozdowska²⁶¹⁾ trial both confirmed a role for these vitamins in preventing post-myocardial-infarction complications and cardiac events. They have also been implicated in reducing the rejection of allogenic grafts⁽Reference Slakey, Roza and Pieper²⁶⁴⁾, further emphasising a role in transplant patients.

The increased risk of congestive heart failure in vitamin E-supplemented post-myocardial-infarction patients⁽Reference Lonn, Bosch and Yusuf¹²⁵^, Reference Marchioli, Levantesi and Macchia²⁶⁵⁾ indicates, however, the need of further trials. The authors of these trials hypothesised that these negative findings were due to pro-oxidant generation mediated by vitamin E.

Subgroup 8: elderly individuals

Elderly individuals are exposed to increased levels of oxidative stress⁽Reference Patrignani, Panara and Tacconelli¹⁸⁵⁾. Cohort studies⁽Reference Buijsse, Feskens and Kwape¹⁰²^, Reference Buijsse, Feskens and Schlettwein-Gsell¹⁰³^, Reference Losonczy, Harris and Havlik²⁶⁶⁾ and trials⁽Reference Lee, Cook and Gaziano¹⁰⁸⁾ have both indicated benefit with supplementation in the elderly. In a subgroup analysis of the Women's Health study, only individuals above the age of 65 years gained a reduction, of 26 %, in cardiovascular events⁽Reference Lee, Cook and Gaziano¹⁰⁸⁾. The Atherosclerosis Risk in Communities (ARIC) study concluded an age-relationship between dietary intake and carotid atherosclerosis, with supplementation only showing benefit in women above the age of 55 years⁽Reference Kritchevsky, Shimakawa and Tell²⁶⁷⁾. This therefore suggests that vitamin supplementation would be of benefit to elderly individuals.

Discussion

Antioxidant research has so far failed to confirm a role for vitamin E, vitamin C and β-carotene in the primary or secondary prevention of CVD. A total of nine primary and eleven secondary prevention trials, including approximately 150 000 and 60 000 participants respectively, have been disappointing. If there is a role for these vitamins in CVD, why is it that we have not identified it through trials? It has long been known that a high intake of fruit and vegetables is associated with a reduced incidence of CVD and it was initially hypothesised that vitamin E, vitamin C and β-carotene were the fundamental protective components that mediated this effect; as a consequence a range of studies was initiated to confirm their role.

Observational studies

It has long been believed that observational studies show that a high dietary intake of these vitamins is associated with a reduced risk of CVD and that there is a discrepancy between these studies and trials. However, even though this is true in regards to vitamin E, this is not the case when it comes to vitamin C and β-carotene. Observational studies have shown an inverse correlation between dietary intake of vitamin E (about 4–8 mg/d) and the incidence of CHD⁽Reference Knekt, Ritz and Pereira⁹³^–Reference Knekt, Reunanen and Jarvinen⁹⁵⁾. But the majority of studies have indicated no benefit with increased dietary intake of vitamin C⁽Reference Ito, Kurata and Suzuki⁹²^, Reference Kushi, Folsom and Prineas⁹⁴^, Reference Rimm, Stampfer and Ascherio⁹⁶^, Reference Osganian, Stampfer and Rimm⁹⁸^, Reference Buijsse, Feskens and Kwape¹⁰²⁾ and β-carotene⁽Reference Knekt, Ritz and Pereira⁹³^, Reference Riemersma, Wood and Macintyre²⁶⁸^, Reference Iannuzzi, Celentano and Panico²⁶⁹⁾ and those that have indicated a beneficial role have not adjusted for vitamin E intake⁽Reference Knekt, Reunanen and Jarvinen⁹⁵⁾ and hence its effects. The lack of benefit with a high dietary intake of β-carotene (about 890–5500 μg/d) and vitamin C (about 50–170 mg/d)⁽Reference Knekt, Ritz and Pereira⁹³⁾ is suggestive that β-carotene and vitamin C are not the relevant protective components in fruit and vegetables, therefore making one question whether they have a protective role in CVD.