Natural Antioxidants and Hypertension: Promise and Challenges

Authors

  • Tinoy J. Kizhakekuttu,

    1.  Department of Medicine, Cardiovascular Medicine Division and Department of Pharmacology, Medical College of Wisconsin, Milwaukee, WI, USA
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  • Michael E. Widlansky

    1.  Department of Medicine, Cardiovascular Medicine Division and Department of Pharmacology, Medical College of Wisconsin, Milwaukee, WI, USA
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Correspondence
Michael E. Widlansky, M.D., MPH, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, FEC Suite E5100, Milwaukee, WI 53226, USA.
Tel.: 414-955-6755;
Fax: 414-456-6203;
E-mail: mwidlans@mcw.edu

Abstract

Hypertension reigns as a leading cause of cardiovascular morbidity and mortality worldwide. Excessive reactive oxygen species (ROS) have emerged as a central common pathway by which disparate influences may induce and exacerbate hypertension. Potential sources of excessive ROS in hypertension include nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, mitochondria, xanthine oxidase, endothelium-derived NO synthase, cyclooxygenase 1 and 2, cytochrome P450 epoxygenase, and transition metals. While a significant body of epidemiological and clinical data suggests that antioxidant-rich diets reduce blood pressure and cardiovascular risk, randomized trials and population studies using natural antioxidants have yielded disappointing results. The reasons behind this lack of efficacy are not completely clear, but likely include a combination of (1) ineffective dosing regimens, (2) the potential pro-oxidant capacity of some of these agents, (3) selection of subjects less likely to benefit from antioxidant therapy (too healthy or too sick), and (4) inefficiency of nonspecific quenching of prevalent ROS versus prevention of excessive ROS production. Commonly used antioxidants include Vitamins A, C and E, L-arginine, flavanoids, and mitochondria-targeted agents (Coenzyme Q10, acetyl-L-carnitine, and alpha-lipoic acid). Various reasons, including incomplete knowledge of the mechanisms of action of these agents, lack of target specificity, and potential interindividual differences in therapeutic efficacy preclude us from recommending any specific natural antioxidant for antihypertensive therapy at this time. This review focuses on recent literature evaluating naturally occurring antioxidants with respect to their impact on hypertension.

Introduction

Hypertension is the most important cardiovascular risk factor worldwide, contributing to half of prevalent coronary heart disease and approximately two-thirds of prevalent cerebrovascular disease [1]. While a multitude of genetic and environmental factors contribute to this complex disease, excessive reactive oxygen species (ROS) have emerged as a central common pathway by which disparate influences may induce and exacerbate hypertension [2]. Furthermore, a significant body of epidemiological [3] and clinical trial data [4,5] suggest that diets known to contain significant concentrations of naturally occurring antioxidants appear to reduce blood pressure and may reduce cardiovascular risk.

In light of these data, there is significant interest in identifying key, naturally occurring antioxidants to both prevent and treat hypertension. This review focuses on the recent literature evaluating naturally occurring antioxidants with respect to their impact on hypertension.

Role of Oxidative Stress in the Pathogenesis of Hypertension

ROS are generated by multiple cellular sources, including NADPH oxidase, mitochondria, xanthine oxidase, uncoupled endothelium-derived NO synthase, cycloxygenase, and lipoxygenase (Table 1) [6,7]. The dominant initial ROS species produced by these sources is superoxide (O2). Superoxide is a short-lived molecule that can subsequently undergo enzymatic dismutation to hydrogen peroxide. Superoxide can oxidize proteins and lipids, or react with endothelium-derived nitric oxide (NO) to create the reactive nitrogen species peroxynitrite. Peroxynitrite and other reactive nitrogen species can subsequently oxidize proteins, lipids, and critical enzymatic cofactors that may further increase oxidative stress [8,9]. Hydrogen peroxide produced by enzymatic dismutation of O2 can be further converted to highly reactive hydroxyl radicals (via Fenton chemistry) that can cause DNA damage [10]. The balance between superoxide production and consumption likely keeps the concentration of O2 in the picomolar range and hydrogen peroxide in the nanomolar range [11]. These homeostatic levels of ROS appear to be important in normal cellular signaling [12–14] and normal reactions to stressors [15,16].

Table 1.  Potential sources of excessive reactive oxygen species in hypertension
 • NADPH oxidase
 • Mitochondria
 • Xanthine oxidase
 • Endothelium-derived NO synthase
 • Cycloxygenase 1 and 2
 • Cytochrome P450 epoxygenase
 • Transition metals (e.g., iron)

While multiple diverse factors likely contribute to the development of hypertension, the pathogenesis of this disease appears related, at least in part, to the development of a state of excessive oxidative stress. Local excessive superoxide production in the kidneys, central nervous system (CNS), and vasculature, along with inflammatory activation, are central findings in hypertension models [17,18]. Animal studies demonstrate the development of hypertension with associated increases in oxidative stress and impaired vasodilation in rats exposed to a high-salt and oxidant-containing diet [19]. Furthermore, infusion of superoxide dismutase lowers oxidative stress and blood pressure in these animal models [20–22]. Divergent animal models of hypertension, including spontaneous hypertension [23], salt-sensitive hypertension [24,25], renovascular hypertension [26,27], and obesity-related hypertension [28] are all associated with excessive oxidative stress. These data suggest, regardless of etiology, excessive ROS is a common factor in the pathogenesis and morbidity of hypertension.

Roles and Interactions of Sources of Oxidative Stress in Hypertension

As delineated in Table 1, multiple diverse sources of ROS generation are relevant to the pathogenesis of hypertension. The prominent role of excessive superoxide produced by NADPH oxidase under angiotensin II stimulation in the development of hypertension has recently been extensively reviewed [17]. Interestingly, data emerging over the past several years indicate that hypertension-related excessive ROS levels are most likely secondary to regulatory interactions between the major sources of ROS themselves involving ROS in signaling processes.

Mitochondria produce excessive ROS in the setting of spontaneous hypertension [29,30] as well as in hypertensive states characterized by salt sensitivity and elevated endothelin-1 levels [31]. Mitochondria may produce superoxide through multiple mechanisms, including the electron transport chain complexes (I, II, and III), monoamine oxidase A and B, and Krebs cycle enzymes [32]. Interestingly, extensive regulatory cross-talk between NADPH oxidase and mitochondria appears to modulate superoxide production from both sources [33,34]. For example, overexpression of thioredoxin-2, an important mitochondrial-based antioxidant thiol, blunts angiotensin II-induced hypertension [35] and lowers basal blood pressure in transgenic mice [36].

NADPH oxidase and mitochondria also interact with endothelium-derived NO synthase (eNOS) through ROS production, modulating overall NO bioavailability and superoxide production from eNOS. eNOS has been localized to the outer mitochondrial membrane in endothelial cells [37]. Beyond quenching NO through direct reaction to create peroxynitrite, superoxide from NADPH oxidase or mitochondria can oxidize tetrahydrobiopterin (BH4), a necessary eNOS cofactor, leading to eNOS uncoupling and superoxide production from eNOS [38]. Furthermore, uncoupling of eNOS leads to reduced NO bioavailability and excessive mitochondrial ROS production [39].

Xanthine oxidase, which generates superoxide by converting hypoxanthine to xanthine, is upregulated by NADPH oxidase under oscillatory shear conditions [40]. Xanthine oxidase may also contribute to excessive ROS production in salt-sensitive hypertension, although its relative contribution compared to mitochondria and NADPH oxidase remains to be fully elucidated [41]. Prior work also demonstrates potential roles for lipoxygenase, cycloxygenase, cytochrome P450 epoxygenase, and transitional metals in overall cellular superoxide production, but further study is necessary to better delineate the roles of these sources of ROS in hypertension.

While the causal intrinsic and extrinsic factors governing the development of hypertension are very likely multifactorial, genetic polymorphisms associated with sources of oxidative stress in hypertension may modulate an individual's potential for elevated ROS and the development of hypertension [42–50]. Overall, while there may be a hierarchy of the relative contributions of each ROS source in hypertension, the measured combined local concentrations of superoxide and peroxynitrite most likely reflect a combination of genetic susceptibility, coordinated ROS generation from multiple sources, local environmental influences on sources of ROS production, and overall intrinsic antioxidant defense mechanisms [51].

Selecting Natural Antioxidants to Treat Hypertension

Randomized trials employing nonpharmacological dietary interventions emphasizing fruits, vegetables, whole grains, and nuts have shown impressive blood pressure lowering results in both hypertensive and normotensive subjects [52–54]. Similar interventions demonstrated to reduce cardiovascular morbidity and mortality continue to maintain interest in the potential of isolating specific compounds enriched in these diets that may be responsible for the overall dietary benefits [55].

The dietary components in these studies are high in compounds known to have antioxidant properties leading many to ascribe the benefits of these diets to their increased content of natural antioxidants. However, prior randomized trials and population studies in healthy populations and patients at high risk for cardiovascular events that have employed combinations of some of these natural antioxidants as dietary supplements have, for the most part, shown disappointing results [56–63]. The reasons behind these disappointing results are not completely clear, but likely include a combination of: (1) ineffective dosing regimens, (2) the potential pro-oxidant capacity and other potentially deleterious effects of some of these compounds under certain conditions [64,65], (3) selection of subjects less likely to benefit from antioxidant therapy (too healthy or too sick) [66], and (4) inefficiency of nonspecific quenching of prevalent ROS versus primary prevention of excessive ROS production [67,68].

When considering antioxidant therapy for hypertension, lessons from prior disappointing attempts to reduce blood pressure and cardiovascular risk with antioxidant therapy should be considered. The profile of an ideal agent is outlined in Table 2. The importance of patient selection is being increasingly recognized in light of emerging data suggesting that antioxidant supplementation in healthy subjects may blunt the protective benefits of aerobic exercise training, suggesting ROS generation can be beneficial under certain circumstances [69].

Table 2.  Optimal profile of natural antioxidant agent for antihypertensive therapy
 • Good oral bioavailability
 • Patient-friendly dosing regimen (once or twice daily dosing)
 • Concentrates locally in relevant tissues (kidney, brain, and/or vasculature)
 • Limited potential for pro-oxidative role and secondary cell signaling that may limit effectiveness
 • Inhibits the production of ROS rather quenching ROS post-production
 • Good safety profile with limited side effects
 • Efficacious for hypertension originating from disparate etiologies
 • No adverse interactions with the metabolism of potential concomitant antihypertensive pharmacological therapy
 • Has pleiotropic effects that go beyond blood pressure lowering and translate into prevention of/reversal of/slower progression of end-organ damage

Antihypertensive Profile of Common Natural Antioxidants

Antioxidant Vitamins

Vitamin A Precursors and Derivatives

Vitamin A precursors and derivatives are retinoids that consist of a beta-ionone ring attached to an isoprenoid carbon chain. Foods high in vitamin A include liver, sweet potato, carrot, pumpkin, and broccoli leaf. Initial interest in vitamin A-related compounds focused primarily on beta-carotene, given initial promising epidemiological data with respect to its cardioprotective effects and a correlation of higher plasma levels to lower blood pressure in men [70]. However, concerns about beta-carotene's pro-oxidative potential came to light with a report suggesting adverse mitochondrial effects of beta-carotene cleavage products [71]. Furthermore, adverse mortality data with respect to beta-carotene has limited interest in this compound as an effective antihypertensive agent [72].

Recently, interest in vitamin A derivatives has turned to lycopene. Itself a potent antioxidant [73], lycopene is found concentrated in tomatoes. One small study has shown a reduction in blood pressure with a tomato extract-based intervention (containing a combination of potential antioxidant compounds including lycopene) in patients with stage I hypertension [74], although second study showed no effect in prehypertensive patients [75].

Ascorbic Acid (Vitamin C)

L-ascorbic acid is a six-carbon lactone and, for humans, is an essential nutrient. In Western diets, commonly consumed foods that contain high levels of ascorbic acid include broccoli, lemons, limes, oranges, and strawberries. The toxicity potential of this compound is low, although an increased risk of oxalate renal calculi may exist at higher doses (exceeding 2 g/day) [76].

The initial purported mechanisms for the potential benefits of ascorbate supplementation were centered on quenching of single-electron-free radicals. Subsequent research has demonstrated that the plasma concentration of ascorbate required for this mechanism to be physiologically relevant is not attainable by oral supplementation [77]. However, vitamin C can concentrate in local tissues to levels an order of magnitude higher than that of plasma. At this level, ascorbate may effectively compete for superoxide and reduce thiols [78,79]. Recent data also suggest potential suppressive effects of ascorbate on NADPH oxidase activity [80,81]. Ascorbate appears to have limited prooxidant ability [82].

Ascorbate's antihypertensive efficacy has been evaluated in multiple small studies. Many [83–86], but not all [87], show modest reductions in blood pressure in both normotensive and hypertensive populations. These data also suggest that supplementation has limited effect on systemic antioxidant markers [85] and that little additional blood pressure benefits are seen beyond a 500 mg daily dose. Large-scale randomized trial data specific to ascorbate supplementation and its effects on hypertension are currently lacking. Data from Heart Protection Study (HPS) suggest no significant mortality benefit from antioxidant supplementation including 250 mg/day of ascorbate [57]. However, the relatively low dose of ascorbate, use of combination therapy, and high-risk patient population studied in HPS leave unanswered key questions of appropriate dosing and target.

α-Tocopherol (Vitamin E)

Vitamin E is a generic term for a group of compounds classified as tocopherols and tocotrienols [88]. While there are four isomers in each compound class of Vitamin E, the overwhelming majority of the active form is α-tocopherol [89]. Dietary sources high in vitamin E include avocados, asparagus, vegetable oils, nuts, and leafy green vegetables.

Vitamin E is a potent antioxidant that inhibits LDL and membrane phospholipid oxidation [90]. Interestingly, inflammatory cells and neurons have binding proteins for α-tocopherol, the actions of which may include inhibition of NADPH oxidase, lipoxygenase, and cyclooxygenase [91]. However, studies demonstrating vitamin E's pro-oxidant capacity under certain cellular conditions suggest that local condition may influence vitamin E's redox activity [65].

Initial excitement for vitamin E supplementation was based on the reduction of cardiovascular events seen in the Cambridge Heart Antioxidant Study [60]. However, follow-up studies have been largely disappointing [92–94]. While one small study that used vitamin E in combination with zinc, vitamin C, and beta-carotene showed a modest, significant reduction in blood pressure over 8 weeks of therapy [95], other small studies [96,97] show either no effect or a pressor effect from vitamin E supplementation. Furthermore, the more definitive Heart Outcomes Prevention Evaluation trial failed to show either blood pressure or mortality benefit for patients at high risk for cardiovascular disease [92].

L-Arginine

L-arginine is an amino acid and the main substrate for the production of NO from eNOS in a reaction that is dependent on BH4[98]. Potential dietary sources include milk products, beef, wheat germ, nuts, and soybeans. Reduced levels of BH4 lead to uncoupling of NO synthesis, with oxygen as terminal electron acceptor instead of L-arginine, resulting in the generation of superoxide by eNOS [99–101]. Low cellular levels of L-arginine have been demonstrated in human hypertension [102,103]. While L-arginine deficiency itself does not appears to lead to uncoupling of eNOS [104], low levels of L-arginine may lead to reduced levels of bioavailable NO, which could contribute to hypertension. Thus, L-arginine supplementation could theoretically reduce blood pressure by allowing for a restoration of normal NO bioavailability, perhaps overcoming overall L-arginine deficiency as well as more successfully competing for the eNOS active site with circulating asymmetric dimethylarginine, a competitor of L-arginine that may be increased in the setting of hypertension [105].

This concept is supported by studies demonstrating an antihypertensive effect of L-arginine supplementation in salt-sensitive rats [106], healthy human subjects [107], hypertensive diabetics [108], patients with chronic kidney disease [109], and diabetic patients in combination with N-acetylcysteine, a precursor of glutathione [108]. L-arginine's antihypertensive effect may be mediated in part by its suppressive effects on angiotensin II and endothelin-1, and its potentiating effects on insulin [110].

However, recent concerns about potential deleterious increases in homocysteine in the setting of L-arginine supplementation have been raised [111]. The majority of L-arginine is processed into creatine, which leads to increased homocysteine levels [112]. Homocysteine can increase oxidative stress [113]. A recent study confirms this mechanism is relevant to L-arginine metabolism in humans [114], suggesting a potential mechanism for neutralizing the eNOS-related antioxidant effects of L-arginine.

Flavonoids

Flavonoids are polyphenolic compounds commonly found in concentrated amounts in fruits, vegetables, and beverages, including apples, berries, grapes, onions, pomegranate, red wine, tea, cocoa, and dark chocolate. The exact structure and composition of the flavonoid compounds vary between food sources, and flavonoid content can be altered based on the manner of food preparation [115]. Interest in flavonoids as antioxidant therapy for cardiovascular disease originates from epidemiological data suggesting improved cardiovascular outcomes in individuals with high levels of intake of food and beverages with high flavonoid content [115,116] as well as cellular work suggesting a strong antioxidant effect of these compounds [117–120].

However, the limited oral bioavailability of flavonoids suggests cell signaling mechanisms, rather than free radical quenching activity, is more likely to be root of sustained cardiovascular benefits from flavonoids [120]. This concept is consistent with studies demonstrating that flavonoids can inhibit NADPH oxidase through ACE inhibition [121,122], increase eNOS-specific NO production through the estrogen receptor [123], and alter COX-2 expression [124]. Studies investigating the antihypertensive effects of flavonoids are inconclusive. While multiple small studies of short-duration dark-chocolate therapy have demonstrated blood pressure lowering effects in hypertensives [125–128], studies in normotensive and prehypertensive individuals have demonstrated no benefit [75,129]. Furthermore, tea intake may, at least temporarily, increase blood pressure in certain populations [130,131]. The specific flavonoids and combination of flavonoids that exert the largest beneficial effects remain unknown.

Mitochondria-Related Antioxidants

Coenzyme Q10 (CoQ)

CoQ (2,3 dimethoxy-5 meth-6-decaprenyl benzoquinone) is derived from mevalonic acid and phenylalanine, and can be supplemented by oral intake. This compound is a key component of the electron transport chain, accepting electrons from Complexes I and II and the glyceraldehyde-3-phosphate shuttle. CoQ levels have been shown to be lower in older adults known to have a greater prevalence of hypertension [132]. The mechanism of action of CoQ is not likely to be secondary to a superoxide scavenger effect given CoQ's hydrophobic properties. CoQ may reduce mitochondrial superoxide production by increasing the efficiency of electron transfer from Complexes I and II down the mitochondrial electron transport chain [133]. Coenzyme Q may also exert an antioxidant effect by reducing lipid peroxidation at the level of the plasma membrane [134].

Early data from noncontrolled studies in human hypertension demonstrate reductions in blood pressure with CoQ supplementation [135,136]. Furthermore, small randomized studies using a CoQ dose of 100–120 mg daily have demonstrated significant reductions in blood pressure with minimal side effects in patient with Stage II hypertension [137–140]. Interestingly, a new, mitochondrial-targeted formulation of CoQ has demonstrated antihypertensive efficacy in a hypertensive rat model [141].

α-Lipoic Acid (LA) and Acetyl-L-Carnitine (ALCAR)

LA is a dithiol compound synthesized from octanoic acid in mitochondria. The in vivo and in vitro effects of LA have been thoroughly reviewed elsewhere [142,143]. LA has moderate oral bioavailability [144]. While LA is a potent in vitro antioxidant, the limited plasma concentrations achievable with supplementation and rapid clearance of LA suggest free radical scavenger and antioxidant recycling activities are unlikely to be the primary in vivo activities of LA. Participation in mitochondrial-associated metabolic pathways, in cell signaling that may improve coupling of eNOS, and in antiinflammatory actions are among the potential beneficial effects of LA supplementation [142,145]. Work in a diabetic rat and multiple different hypertensive rat models have shown the potential for LA supplementation to reduce blood pressure [146–149].

ALCAR is a key compound in the transport of fatty acids into mitochondria for beta-oxidation. The antioxidant mechanism of ALCAR supplementation appears to be secondary to reductions in mitochondrial ROS production in synergy with concomitant LA therapy [150]. The exact intramitochondrial mechanism of ALCAR's effects are not clear, and prior work in older rats demonstrates ALCAR's potential to be pro-oxidative when used alone [151]. Further data suggest ALCAR may be of particular benefit in diabetics with hypertension secondary to their low carnitine levels [152] and elevated circulating free fatty acid levels [153,154].

Human data with respect to the antihypertensive effects of these compounds are limited to two small studies, which have shown some promising results. Consistent with animal data, combined ALCAR and LA therapy reduced systolic blood pressure in coronary artery disease patients with hypertension and/or metabolic syndrome at the time of enrollment [155]. Further, 32 type 2 diabetic subjects supplemented with 2 g/day of acetyl-L-carnitine showed significantly lower blood pressure and improved insulin sensitivity [156].

Other Potential Natural Antioxidant Agents

Garlic [157], glutamate [158], N-acetylcysteine [159], sour milk [160,161], and vitamin D [162,163] all have shown antihypertensive effects through antioxidant mechanisms that may involve inhibition of sources of excessive ROS. Further work remains to be done to establish the mechanisms and efficacy of these interventions.

Conclusions and Future Directions

A summary of our findings with respect to the above interventions is contained in Table 3. Critical evaluation of these data reveal several issues and limitations related to our current knowledge of natural antioxidant compounds and their potential antihypertensive efficacy that obviate our ability to recommend any individual agent at this time (Table 4). First, the majority of these agents have been discovered to have potential mechanisms of action that were initially unanticipated, including the potential for deleterious, pro-oxidative effects. A greater understanding of the mechanisms of action of the above agents may allow providers to better target therapies to appropriate populations. Second, while interventions such as tomato extract and dark chocolate may hold promise, the identity of the compounds or mix of compounds responsible for the antihypertensive effects of these interventions remain unknown and need to be identified before lycopene or individual flavonoid compounds can be recommended as supplements for antihypertensive therapy. Third, small, single-center trials often enrolling less than 100 subjects comprise the majority of studies found related to novel antioxidant therapy for hypertension, leaving open concerns with respect to publication bias. In addition, the vast majority of these studies made no systemic measurements of total antioxidant capacity, making it a bit difficult to determine whether changes in antioxidant capacity accompanied the observed reductions in blood pressure. With the exception of Vitamins A and E (which cannot be recommended at this time), data from larger randomized clinical trials aimed at blood pressure lowering and optimally also measuring cardiovascular endpoints and antioxidant effects would help more clearly distinguish which of the above agents, if any, may be reasonable to recommend as antihypertensive agents as well as help determine if antioxidant actions may be responsible for any ameliorative effects. Thus, despite some interesting findings, the recommendations of the American Heart Association of a dietary strategy rich in fruits and vegetables appears to continue to be the best strategy for nonpharmacological therapy in hypertension [164]. Further work is clearly necessary in order to identify which natural antioxidants have efficacy and their mechanisms of action.

Table 3.  Profiles of commonly used natural antioxidants for hypertension
SupplementAntioxidant capacity?Pro-oxidant capacityLowers BP in small
clinical trials
Lowers BP in large randomized controlled studies
Vitamin A
 Beta-carotone++
 Lycopene (tomato extract)+?+?
Vitamin C++?
Vitamin E++±
L-Arginine+++?
Flavonoid containing food/beverages+?± (positive for dark chocolate, negative for tea)?
Acetyl-L-carnitine (ALCAR)?±+?
α-Lipoic acid++ (in combination with ALCAR)?
Table 4.  Current limitations precluding recommendation of specific natural antioxidant supplements for hypertension
 • Incomplete knowledge of the mechanism(s) of action of these agents
 • Lack of target specificity of these agents
 • Lack of large, randomized control trials to determine true antihypertensive efficacy for many of these agents
 • Lack of data on “hard” outcomes (e.g., death, cardiovascular events)
 • Potential differences in therapeutic efficacy in different hypertensive populations

Acknowledgments

Dr. Widlansky is supported by grant 1K23HL089326. Dr. Kizhakekuttu is supported by a Ruth L. Kirschstein NIH T32 training grant (HL007792-15).

Conflict of Interest

The authors declare no conflict of interests.

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