Nutraceuticals and Atherosclerosis: Human Trials

Authors

  • Lina Badimon,

    1.  Cardiovascular Research Center, CSIC-ICCC, Hospital de la Santa Creu i Sant Pau and IIB-Santpau
    2.  CIBEROBN Instituto de Salud Carlos III
    3.  Cardiovascular Research Chair, (IUAB-HSCSP-Fundacion Jesus Serra), Barcelona
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  • Gemma Vilahur,

    1.  Cardiovascular Research Center, CSIC-ICCC, Hospital de la Santa Creu i Sant Pau and IIB-Santpau
    2.  CIBEROBN Instituto de Salud Carlos III
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  • Teresa Padro

    1.  Cardiovascular Research Center, CSIC-ICCC, Hospital de la Santa Creu i Sant Pau and IIB-Santpau
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Correspondence
Lina Badimon, Cardiovascular Research Center, C/Sant Antoni Ma Claret 167, 08025 Barcelona, Spain.
Tel.: +34-93-556-58-80;
Fax: +34-93-556-55-59;
E-mail: lbadimon@csic-iccc.org

SUMMARY

The high prevalence of obesity, atherosclerosis, and cardiovascular disease (CVD) is largely attributable to the contemporary lifestyle that is often sedentary and includes a diet high in saturated fats and sugars and low ingestion polyunsaturated fatty acids (PUFAs), fruit, vegetables, and fiber. Epidemiological studies have confirmed a strong association between fat intake, especially saturated- and transfatty acids, plasma cholesterol levels, and rate of coronary heart disease (CHD) mortality. In counterpart, beneficial cardiovascular effects have been reported in populations consuming the “healthy” Mediterranean-type diet. Indeed, many nutrients and phytochemicals in fruits, vegetables, and wine, including fiber, vitamins, minerals, antioxidants, have shown to be independently or jointly responsible for the apparent reduction in CVD risk. Therefore, in patients with overt CVD, efforts have focused on combining both drug treatments and nutrition interventions. Undoubtedly, the advances in the knowledge of both the disease processes and healthy dietary components have provided new avenues to develop pharmaceutical and/or dietary strategies to halt the development of vascular disease. In this regard, within the last years, pioneering nutritional strategies, such as nutraceuticals, have been developed aimed at reducing the main atherosclerotic risk factors and promoting cardiovascular health. Furthermore, a growing body of clinical evidence has demonstrated positive cardiovascular effects associated with dietary fibers, cholesterol-lowering natural agents, olive oil, ω-3 PUFAs, antioxidants, and polyphenols intake. Moreover, monounsaturated fatty acids intake has shown to modulate the expression of key atherosclerotic-related genes. Yet, in the case of antioxidants, some large clinical trials have failed to confirm such atheroprotective effects. Furthermore, there might be interactions between these natural food supplements and cardiovascular medications that cannot be overlooked. Hence, there is a need for a better understanding and more scientific evidence of the relative contribution of major nutraceutical constituents to the inhibition of the progression of atherosclerosis and its clinical consequences.

Introduction

Atherosclerosis is the underlying cause of cardiovascular disease (CVD), a leading cause of morbidity and mortality in the Western countries. Although the pathophysiological mechanisms behind atherosclerosis are not completely understood, it is widely recognized that lipid deposition, oxidative stress, vascular inflammation, smooth muscle cell differentiation, and endothelial dysfunction play an integral role in the formation, progression, and rupture of the atherosclerotic plaque [1].

Epidemiological evidence has supported a protective role for diets low in saturated fat and rich in fruits and vegetables as well as a moderate wine consumption against the development and progression of CVD. Beneficial effects are encountered in populations consuming the Mediterranean-type diet. Many nutrients and phytochemicals in fruits, vegetables, and wine, including fiber, vitamins, minerals, antioxidants, could be independently or jointly responsible for the apparent reduction in CVD risk. The advances in the knowledge of both the disease processes and healthy dietary components have provided new avenues to develop pharmaceutical and/or dietary strategies to halt the development of vascular disease. In this context, the expansion and growing popularity of nutraceuticals aimed at promoting heart health can be viewed as an example. Concerning atherosclerosis prevention, nutraceuticals include foods, functional foods, and/or dietary supplements that may provide prevention and/or treatment to the onset of the disease and the occurrence of cardiovascular events.

Healthy Fats: Monounsaturated and Polyunsaturated Fatty Acids

Current recommendations on dietary fat emphasize quality rather than quantity [2]. Metabolic studies have long established that the dietary fatty acid composition, but not the total amount of fat, predicts serum cholesterol levels. Fatty acids can be divided into four general categories: saturated, monounsaturated, polyunsaturated, and transfats. Saturated fatty acids (SFA) and transfatty acids are related with elevated cardiovascular risk whereas monounsaturated fatty acids (MUFA, ω-9) and polyunsaturated fatty acids (PUFA, ω-3, ω-6) are associated with a decreased risk of coronary heart disease (CHD) [3]. Dietary fat quality also influences the activity of enzymes involved in the desaturation of fatty acids in the body.

Monounsaturated Fatty Acids

Ecological studies have suggested an inverse association between MUFA intake and total mortality, as well as with CHD death [4]. To this respect, the Seven Countries Study yielded the first convincing epidemiologic evidence that mortality from CHD was particularly low in Mediterranean countries where olive oil, which is rich in MUFA, is the main dietary source of fat [5]. However, olive oil besides having a high level of MUFA fat (>75% oleic acid, cis18:1, n-9) contains other minor components with biological properties, which make it more than a MUFA fat [6]. In fact, although data concerning olive oil intake and primary endpoints for CVD are scarce, olive oil has shown to improve the major risk factors for CVD (lipoprotein profile, blood pressure, glucose metabolism, and antithrombotic profile) as well as to positively modulate endothelial function, inflammation, and oxidative stress [6].

The protective effect of MUFA against CHD was also supported by a regression analysis of data from the Nurses’ Health Study of 80,082 women followed up for >14 years [7]. In contrast, however, two prospective studies found a positive association between intake of MUFA and CHD risk [8,9], perhaps because they did not adjust their results for important confounding variables as other types of fat simultaneously present on the diet (e.g., the beef, a major source of MUFA and SFA in the American diet). In fact, other epidemiological studies that have controlled for a number of potentially confounding variables also have reported protective effects of MUFA against CHD [10]. We have recently reported, in asymptomatic high cardiovascular risk subjects that intake of traditional Mediterranean diet supplemented with virgin olive oil actively modulates the expression of key genes involved in vascular inflammation, foam cell formation, and thrombosis toward an antiatherothrombotic profile [11].

Results of a meta-analysis of 60 controlled trials published between 1970 and 1998 supported the beneficial effects of a high MUFA-diet on serum lipid levels and LDL oxidation [12–15]. Besides these atheroprotective effects, we have also reported in healthy subjects, that MUFA-enriched diets prevent smooth muscle cells DNA synthesis likely reducing plaque-related smooth muscle cells proliferation [16]. Additionally, several intervention studies in human have supported that intake of MUFA-rich has additional non-lipid-related advantages, including favorable effects in preventing arrhythmias, lowering heart rate, blood pressure, platelets activity, coagulation, and fibrinolysis (Figure 1) [17–19]. Cross-over feeding trials performed in diabetic patients provided first level of evidence of the benefits of MUFA-rich diets in front of carbohydrate-rich diets not only for healthy but also for diabetic individuals. Thus, MUFA-rich diets exert a positive influence in body weight, glucose profiles, and serum lipid levels in patients with type-2 diabetes [20]. In metabolic studies, substitution of MUFA instead of carbohydrate for SFA calories has been linked to a decrease of total- and LDL-cholesterol and an increase in high-density lipoproteins (HDL)-cholesterol plasma levels [6,21], as well as a reduction in the total cholesterol/HDL cholesterol ratio [22].

Figure 1.

Effects of monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) in reducing cardiovascular risk and the potential underlying mechanisms likely involved in such atheroprotective effects. TG: triglycerides; vWF: Von Willebrand Factor, FVII: coagulation factor VII; HDL: high-density lipoproteins; LDL: low-density lipoproteins.

Polyunsaturated Fatty Acids

Alpha linolenic acid (ALA) and linoleic acid (LA) belong to the omega-3 (ω-3) and omega-6 (ω-6) series of PUFA, respectively. LA and ALA are essential fatty acids that can be converted into long-chain PUFAs, such as arachidonic acid and eicosapentaenoic acid (EPA)/docosahexaenoic acid (DHA), respectively [23]. Food sources of ω-3 and ω-6 are seafood and fatty fish, most vegetable oils, cereals, and walnuts. Among PUFA, ω-3 fatty acids show great promise in primary and secondary prevention of CVD [24]. The two major ω-3 fatty acids that have been associated with cardiovascular benefit are EPA and DHA. Generally, very little ALA is converted to EPA, and even less to DHA, and therefore direct intake of the latter two is optimal. Data on the effects of ALA on CVD outcomes are limited. In cohort studies, low ALA intake has been associated with risk of fatal CHD [25,26] and sudden cardiac death [27]. In a recent study in a Costa Rica population [1819 cases with a first nonfatal acute myocardial infarction (AMI) and 1819 population-based controls], ALA intake and plasma levels predicted a better prognosis, independent of fish and EPA/EDA intake, in the post-AMI population [28].

Up to now, more than 25 published trials have evaluated the risk of CHD as a function of ω-3 PUFA plasma levels. Taken together, these studies showed that intake of fish oil is associated with CHD risk reduction [29]. A recent meta-analysis with pooled data from 19 observational studies [24] supported the notion that fish consumption is an important component of lifestyle modification for the prevention of total and fatal CHD. In addition, the authors of a meta-analysis of 11 prospective cohort studies (encompassing 222,364 persons with an average of 11.8 years of follow-up) concluded that each 20 g per day increase in fish intake is associated with a 7% lower risk of CHD mortality [30]. In support to these studies, the most compelling evidence for cardiovascular benefits of ω-3 PUFA comes from three large randomized trials (DART [31], GISSI-Prevention trial [32], JELIS [33]) that included 2033 men with recent myocardial infarction (MI), 11,323 post-MI patients, and 18,645 patients in primary (14,981) or secondary (83,664) prevention, respectively. In the DART trial, patients were randomly assigned to different dietary advice groups, whereas in the GISSI-Prevention trial, patients were randomly assigned to receive an EPA plus DHA supplement or placebo on an open-label basis. These studies have documented that ω-3 PUFA lower cardiovascular risk in primary and more especially in secondary prevention of CHD. Thus, the investigators of the DART study concluded that a modest intake of fatty fish (2 or 3 portions per week) may reduce mortality in men who have recovered from MI. The reduction of CHD mortality was lowered by 62% in the subgroup receiving a fish oil capsule containing 450 mg of EPA plus DHA daily.

Other studies, however, have not shown favorable results. Burr et al. [34] based on a trial of 3114 men with angina, suggested that patients treated with fish oil capsules had a higher risk of sudden cardiac death than untreated control subjects [34]. A Norwegian study by Nilson et al. [35] did not show a benefit of ω-3 PUFA supplementation in post-MI patients and in the recently presented OMEGA trial, consisting of 3851 patients with AMI from 104 centers in Germany, EPA/DHA did not show any further benefit on primary or secondary prevention when given together with other specific treatments (e.g., aspirin, clopidogrel, statins, beta-blockers, and angiotensin-converting enzyme inhibitors) [36].

Several mechanisms have been proposed to explain how ω-3 PUFA might beneficially influence CVD. These include preventing arrhythmias, lowering heart rate and blood pressure, decreasing platelet aggregation, improving vascular reactivity, and lowering plasma triglyceride levels [19,37,38]. The latter is accomplished by decreasing the production of hepatic triglycerides and increasing the clearance of plasma triglycerides, a mechanism probed for EPA and DHA, not for ALA [29].

Cholesterol-Lowering Natural Agents: Sterols/Stanols and Red Yeast Rice

Plant sterols (i.e., phytosterols’) and stanols (saturated form of sterols) are natural constituents of plants structurally related to cholesterol. Plant sterols/stanols reduce cholesterol absorption in the intestinal gut thereby reducing plasma LDL concentrations. Plant sterols/stanols are abundant in vegetable oils and olive oil, but also in fruits and nuts. However, recent advancements in food technology have seen the emergence of food products such as margarine, milk, yoghurt, and cereal products being enriched with plant sterols/stanols and promoted as a food that can help lower serum cholesterol. A meta-analysis of 41 trials showed that intake of 2 g/day of stanols or sterols reduced LDL concentrations by about 10–11%[39] and that there were little additional effects at doses higher than 2.5 g/day. Furthermore, HDL and/or very low-density lipoproteins were generally not affected by stanols/sterols intake. Yet, effects of sterols/stanols on LDLs have been found to be additive to diets and/or cholesterol-lowering drugs. As such, eating foods low in saturated fat and cholesterol and high in stanols/sterols has shown to further reduce LDL by 20% and adding sterols/stanols to statin medication seems more effective than doubling the statin dose [40]. On the other hand, similar efficacy has been observed between plant sterols and plant stanols when they are esterified, which is the form added to foods [41]. However, the food form may substantially affect LDL reduction. Serum LDL-cholesterol was significantly lowered when plant sterols were added to milk (15.9%) and yoghurt (8.6%), but significantly less when added to bread (6.5%) and cereal (5.4%) [42]. Nevertheless, routine prescription of plant sterols/stanols has shown to be an effective strategy in the management of hypercholesterolemic patients in the clinical setting. In fact, the National Cholesterol Education Program Expert Panel (NCEP ATP III) recommends, since 2001, phytosterol-enriched functional foods as part of an optimal dietetic prevention strategy in primary and secondary prevention of CVDs [43]. Moreover, the American Heart Association [44] and the European Current Dietary Guidelines[45] support plants sterols as a therapeutic option for individuals with elevated cholesterol levels. Yet, it is unclear whether phytosterols have a positive effect on atherosclerosis progression and ultimate CVD [39,46]. Recent released guidelines have been more critical with food supplementation with phytosterols and have drawn attention to significant safety issues based on large epidemiological studies [47]. As such, results of the PROCAM-study showed that patients afflicted with MI or sudden cardiac death had increased plant sterol concentrations [48]. Upper normal levels of plant sterols were also associated with a 3-fold increase of risk for coronary events among men in the highest tertile of coronary risk according to the PROCAM-algorithm. Similar data are available for the plant sterol campesterol from the MONICA/KORA-study. In this prospective study, campesterol correlated directly with the incidence of AMI [49].

Another natural compound capable of reducing cholesterol levels is red yeast rice (Monascus purpureus). This fermented rice contains numerous monacolins that are naturally occurring HMG-CoA reductase inhibitors. Indeed, numerous studies have suggested a beneficial lipid lowering effect from commercial preparations of this traditional supplement. For instance, a meta-analysis involving 9625 patients in 93 randomized trials, 3 different commercial preparation of red yeast rice produced a mean reduction in total-cholesterol, LDL-cholesterol, triglyceride, and a mean rise in HDL-cholesterol [50]. More recently, a double-blind, multicenter trial in China has demonstrated, in 4870 patients with a previous MI and high total cholesterol levels after 4.5 years follow-up, that xuezhikang administration (a commercial red yeast rice preparation) at a dose of 0.6 g twice daily was associated with a reduction in the incidence of major coronary events, including nonfatal MI and death from CHD compared to placebo (5.7% vs. 10.4%) [51]. Subsequent subgroup analyses of this trial have further supported these observations by confirming a reduction in cardiovascular outcomes among diabetics and in the elderly [52]. In addition, Xuezhikang has also shown, in small clinical trials, to effectively improve endothelial function in patients with coronary artery disease (CAD) [53]. Finally, an extract of red yeast rice has recently demonstrated to be tolerated as well as pravastatin and achieved a comparable reduction of low-density lipoprotein cholesterol in a population previously intolerant to statins [54].

Cereal Grains and Dietary Fiber

High intake of dietary fiber (e.g., nondigestible polysaccharides, naturally occurring resistant starch and oligosaccharides, and lignins in plants) is associated with a reduced cardiovascular risk. Observational studies have consistently shown that subjects consuming relatively large amounts of dietary fiber have significantly lower rates of CHD [55], stroke [56], and peripheral vascular disease [57]. In a pooled analysis of 10 prospective cohorts [58], each 10 g/day increment of energy-adjusted total dietary fiber was associated with a 14% decrease in risk of coronary events and a 27% decrease in risk of coronary death. However, in the Health Professionals Follow-Up Study, only cereal fiber, not fruit or vegetable fiber, was inversely associated with risk of total stroke [55]. Also Mozaffarian et al. [73] based on the results of a prospective cohort study conducted over 8.6 years in a population of 3588 individuals (men/women) aged 65 years or older at baseline, reported an inverse association of cereal fiber consumption late in life with risk of total stroke and ischemic stroke and a trend toward lower risk of ischemic heart disease. Differing from the above results, the advice to increase cereal fiber intake did not affect recurrent myocardial infarction or mortality (coronary or all-cause mortality) in the DART study [31]. Similarly, a pooled analysis of different studies suggest that cereal fiber by itself has low or no significant influence in CHD risk whereas a stronger inverse association was reported between wholegrain intake and risk for CHD [74]. However, based on the results of the prospective Iowa Women's Health Study, Jacobs and coworkers demonstrated that a similar amount of total cereal fiber had different associations with total mortality, depending on whether the fiber came from foods that contained primarily wholegrain or refined grain [75]. These results highlight the “wholegrain hypothesis”[74] that argues that health benefits stem from more than just the fiber. The wholegrain is nutritionally more important because it delivers a whole package of phytoprotective substances that might work synergistically to reduce cardiovascular risk.

Observational studies have consistently shown that major cardiovascular risk factors as hypertension, diabetes, obesity, and dyslipemia are also less common in individuals with highest levels of fiber consumption, as recently reviewed by different authors [76,77]. To this respect, a recent intervention study (PREDIMED feeding trial substudy) [78] has shown that the increase of dietary fiber intake associates with significant reductions in body weight, waist circumference, blood pressure, fasting glucose, and an increase in HDL cholesterol in a high-risk cohort of subjects with either type 2 diabetes or at least 3 CHD risk-factors (current smoking habit, hypertension, HDL cholesterol<40 mg/dL, body mass index >25 kg/m2).

Polyphenols

Several studies, although not all, have found an inverse association between polyphenol consumption and CVD mortality (Figure 2). In fact, such beneficial effects may partly help to explain the protective CVD effects achieved by foods and beverages containing polyphenols (tea, vegetables, fruits, wine, etc.) [86].

Figure 2.

Clinical studies linking polyphenol consumption and CVD outcome.

The beneficial effects derived from polyphenols appear to be mediated via a plethora of biochemical pathways and signaling mechanisms acting either independently or synergistically. Indeed, polyphenols have shown in in vivo studies to exert antiatherosclerotic effects in the early stages of atherosclerosis development (e.g., decrease LDL oxidation); improve endothelial function and increase nitric oxide release (potent vasodilator); modulate inflammation and lipid metabolism (i.e., hypolipidemic effect); improve antioxidant status; and, protect against atherothrombotic episodes including myocardial ischemia and platelet aggregation. Figure 3 summarizes the speculated mechanisms by which polyphenols may provide cardiac and vascular protection, highlighting the common protective mechanism by which polyphenols exert atheroprotection.

Figure 3.

Postulated mechanisms by which polyphenols exert cardiac and vascular protective effects focusing on atherosclerosis prevention, a common pathological feature. MCP-1- monocyte chemoattractant protein-1; NO: nitric oxide; IL: interleukin, TNF: tumor necrosis factor; I/R: ischemia reperfusion; SMC: smooth muscle cells; LDL: low-density lipoproteins; ET: endothelin; PGI2: prostacyclin; tPA: tissue plasminogen activator; NFKB: nuclear factor kappa B.

In addition, human clinical trials, albeit few and small, have also supported a benefit of polyphenols consumption on cardiovascular risk factors. For instance, patients suffering from CAD have shown an improvement in endothelial function and on the coronary microcirculation [87,88]. Similarly, red wine consumption has shown to prevent the acute impairment of endothelial function that occurs following cigarette smoking or consumption of high-fat meal [89,90] and to modulate monocyte migration in healthy subjects [91]. Indeed, it must be considered that some of the protective effects of wine appeared to be linked to ethanol, per se. However, several studies have indicated, when comparing different beverage types, that wine seems to exert more protective effects than other forms of alcohols supporting a protective role for polyphenolic compounds. As to other polyphenol-related nonalcoholic drinks, it has been recently reported in patients with CAD and carotid artery stenosis, that pomegranate juice consumption for 18 and 36 months, respectively, slowed atherosclerosis progression, assessed by carotid-intima thickness, likely through the potent antioxidant characteristics of promegranate polyphenols [92,93].

Antioxidant-Vitamin Supplementation

According to the oxidative-modification hypothesis in which, as stated above, reactive oxygen species and free radicals play a major role in the pathophysiology of atherosclerosis, supplementation with antioxidants (vitamins A, C, E, folic acid, β-carotene, selenium, zinc) was expected to protect against atherosclerosis [13]. In fact observational prospective human cohort studies have shown that a high dietary intake of fruit and vegetables is associated with a reduction in the incidence of CHD [107], stroke [14], and cardiovascular mortality in general [108]. Moreover, epidemiologic studies have reported that high dietary intake of foods rich in vitamin E [109], vitamin C [110], and β-carotene [111], have been inversely associated with the incidence of CAD.

Different results are, however, obtained with vitamin and antioxidant supplementations. Evidence from the U.S. Preventive Services Task Force has reported that, although observational studies have shown an inverse correlation between dietary intake of vitamin E and the incidence of CAD, no such protective effects are reported for vitamin C and β-carotene supplementations [112]. In fact, a potential explanation for such vitamin E-specific protective effects may derive from its fat-soluble nature as well as its integration within LDL particles. As a consequence there has been a major emphasis in conducting vitamin E supplementation randomized controlled trials attempting to confirm their protective role in both primary and secondary prevention. Three large primary prevention trials [79–81] and four large-scale secondary prevention trials [32,82–84] with vitamin E supplementation including 43,169 and 16,993 patients, respectively, have been performed until now. However, a majority of these prospective randomized controlled clinical trials with vitamin supplements have been disappointing [113]. Indeed, only one trial has shown a reduction in myocardial infarction and cardiac events whereas all the others have shown no effect or detrimental effects (Table 1). Similarly, controversy exists regarding the potential benefit associated with antioxidant supplementation in general. A total of 22 trials (n = 134,590 subjects) of which 6 are primary, 12 secondary, and 3 both primary and secondary prevention assessing the effects of antioxidant supplementation on the risk of CVD, coronary restenosis, and the progression of atherosclerotic lesions have been reported [114]. As seen in Figure 4 a majority of the results (14 trials) have been disappointing failing to demonstrate any significant benefit with vitamin C, E, and β-carotene supplementation. Furthermore, within five trials, such antioxidant supplementation was associated with increased all-cause mortality and two have shown higher risk of fatal CHD (ATBC and CARET) [79,96].

Table 1.  Large randomized controlled trials of vitamin E supplementation
Primary Prevention of Cardiovascular Disease____________________
Alpha-Tocopherol Beta-Carotene (ATBC) Cancer Prevention Study[79]
 Number patients: 29,133 men (smokers)
 Follow- up: 5–8 years follow-up
 Treatment: alpha-tocopherol (50 IU per day), beta carotene (20 IU per day), or both alpha-tocopherol and beta carotene or placebo
 Outcome: no differences in the incidence of MI, cardiovascular events, cardiovascular mortality
Heart Outcomes Prevention Evaluation (HOPE) trial[80]
 Number patients 2545 women and 6996 men ≥55 years of age who had cardiovascular disease or diabetes in addition to one other risk factor
 Follow-up: 4.5 years
 Treatment: 400 IU of vitamin E daily or placebo
 Outcome: no apparent effect on myocardial infarction, stroke, and death from cardiovascular causes
Primary Prevention Project[81]
 Number patients: 2583 women and 1912 men with one or more of the following risk factors: hypertension, hypercholesterolemia, diabetes, obesity, family history of premature myocardial infarction, or >64 years
 Follow- up: 3.6 years
 Treatment: vitamin E (300 IU/day) and aspirin 100 mg/day
 Outcome: no effects on primary prevention
Secondary Prevention of Cardiovascular Disease____________________
Rapola et al. Alpha-Tocopherol Beta-Carotene (ATBC) Cancer Prevention Study[82]
 Number patients: 1795 men (smokers) with angina pectoris aged 50–69 years
 Follow- up: 5.5 years follow-up
 Treatment: alpha-tocopherol (50 IU per day), beta carotene (20 IU per day), or both alpha-tocopherol and beta carotene or placebo
 Outcome: no evidence of beneficial effects for alpha tocopherol or beta carotene supplements
Rapola et al. Alpha-Tocopherol Beta-Carotene (ATBC) Cancer Prevention Study[83]
 Number patients: 1862 men (smokers) aged 50–69 years with previous myocardial infarction
 Follow-up: 5.3 years follow-up
 Treatment: alpha-tocopherol (50 IU per day), beta carotene (20 IU per day), or both alpha-tocopherol and beta carotene or placebo
 Outcome: risk of fatal coronary heart disease increased in the groups that received either beta-carotene or the combination of alpha-tocopherol and beta-carotene
Primary Prevention Project[84]
 Number patients: 2002 patients with established ischemic heart disease (angiographically)
 Follow- up: median of 510 days (3–981 days)
 Treatment: alpha-tocopherol 800 IU daily for 546 patients; 400 IU daily for 489 patients; placebo.
 Outcome: alpha-tocopherol treatment substantially reduces the rate of nonfatal MI, with beneficial effects apparent after 1 year of treatment.
Gruppo Italiano per lo Studio della Sopravvivenza nell’Infarto miocardico(GISSI group)[32]
 Number patients: 11,325 recent (<3 months) myocardial infarction
 Follow-up: median of 510 days (3–981 days)
 Treatment: vitamin E (300 UI/day, n = 2830), n-3 PUFA (1 g/day; n = 2836), both (n = 2830), or placebo (n = 2828)
 Outcome: vitamin E had no benefit. Supplementation with n-3 PUFA led to a clinically important and statistically significant benefit.
Heart Protection Study Collaborative Group[85]
 Number patients: 20,536 men and female (high CVD risk)
 Follow- up: median of 5.5 years
 Treatment: simvastatin (40 mg) or combination of vitamins E (600 mg), C (250 mg), and β-carotene (20 mg).
 Outcome: Antioxidant vitamins are safe although no this regimen did not produce any significant reductions in the 5-year mortality from, or incidence of, any type of vascular disease, cancer, or other major outcome.
Figure 4.

Clinical trials based on antioxidant supplementation, which have reported lack of protection, benefit or detrimental effects on the risk of suffering cardiovascular disease, coronary restenosis, and progression of atherosclerotic lesions.

The controversial results reported in clinical trials investigating the role of antioxidants in CVD may be attributed to several factors. Table 2 examines potential explanations that have been postulated, which may help to explain the lack of correlation between observational studies and randomized controlled trials.

Table 2.  Hypothesis suggested to explain the lack of benefit with vitamins
I.Define the optimal dosage
II.Identify more accurate oxidative biomarkers
III.Use the appropriate vitamin isomer
IV.Interference or competition between vitamins
V.Combination vitamin administration may be superior to single supplementation. Especially if combining hydrophobic (Vit E) and hydrophilic (Vit C) vitamins
VI.Adequate timing of supplementation. Secondary prevention may be too late so it should be restricted to early stages of the atherosclerotic disease
VII.Interindividual variation in the response to antioxidants. Potential causes:
 a. Presence of cardiovascular risk factors
 b. Smoking, obesity, hypercholesterolemia, diabetes, end-stage
 renal failure
 c. Acute myocardial infarction, cardiac transplant
 d. Elderly individuals
VIII.Causative role of oxidative stress in atherosclerosis could be an epiphenomenon
IX.Inadequate study design to translate observations studies observations to randomized clinical trials

Acknowledgments

This work was supported by PNS 2006–10091 (to LB) from the Spanish Ministry of Science; Lilly Foundation (to LB), CIBER OBN06 (to LB), and FIS PI071070 (to TP). We thank Fundacion Juan Serra, Barcelona, for their continuous support. G.V. is recipient of a grant from the Spanish Ministry of Science and Innovation (RyC, MICINN).

Conflict of Interest

The authors declare no conflict of interests.

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