The Potential Role of Antioxidants in the Treatment of Peripheral Arterial Disease: A Systematic Review

Peripheral arterial disease (PAD) has a worldwide prevalence and is a significant cause of cardiovascular morbidity and mortality. Due to its high prevalence and higher rates of ischemic cardiovascular and lower-extremity events, its treatment is essential. Increased levels of oxidative stress cause disease. This review aimed to evaluate different studies of antioxidant treatments for PAD patients. A systematic search for relevant studies was performed on the PubMed, SCOPUS, and ScienceDirect databases, and 18 studies fulfilled the inclusion criteria. In total, 16.6% of the studies used natural antioxidants, and 83.3% used synthetic antioxidants. The reviewed studies show that natural antioxidants were completely effective in treating PAD, and synthetic antioxidants showed effective results in only 53% of the studies. A less-than-optimal pro-oxidant–antioxidant balance does not improve the symptoms of PAD. In conclusion, antioxidants in their natural forms are more effective for PAD patients, and ensuring the optimal pro-oxidant–antioxidant balance is an effective method for managing treatment with antioxidants.


Introduction
Peripheral arterial disease (PAD) is a significant cause of cardiovascular morbidity and mortality. An estimated 230 million people are affected worldwide [1]. Despite the high prevalence, PAD remains underrated, underrecognized, and undertreated [2]. Higher rates of ischemic cardiovascular and lower-extremity events, and increased prescription medication, combined with outpatient and inpatient care, can result in an increased burden on healthcare services and add to health expenses [3]. Furthermore, significant amputations can have socio-economic and psychological consequences.
PAD prevalence increases with age and affects 12-14% of the general population. Two main complications in elderly patients with PAD, and also considered to be among the leading causes of morbidity and mortality, are coronary artery disease (CAD) and cerebrovascular disease (CVD) [4]. PAD is recognized to have a greater-than-20% predictive value for coronary heart disease (CHD), mainly manifesting as a significant coronary event in the next ten years. It has been shown in patients over 50 years of age that there is a coexisting incidence of CAD in 68% of patients, while 42% have a chance of having a stroke [4,5].
Although no specific treatment has been developed for the definitive improvement of PAD, supportive therapeutic protocols are routinely used to reduce symptoms in patients with PAD. The treatment provided to a patient with PAD depends on the clinical history and the underlying disease that triggered the PAD [6]. To date, many mechanisms have been proposed for the causes of PAD. One of the most relevant causes is high levels of oxidative stress. Elevated levels of oxidants have been reported in patients with PAD, although there has been some evidence of antioxidant dysfunction in patients.
Furthermore, the balance between oxidative stress and antioxidants can be impaired in PAD [7]. High levels of oxidative stress can lead to endothelial damage, stimulate the formation of atherosclerotic plaques by LDL oxidation, and increase the levels of free radicals [8]. Therefore, some studies on PAD have considered antioxidant therapies in patients with PAD [9]. Different results have been shown for the effect of antioxidants, which can be discussed. This systematic review aimed to collect information about the impact of antioxidants in treating PAD.

Literature Search
In this systematic review, recommendations stated in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. The systematic review protocol was registered on PROSPERO (registration ID 345836). A comprehensive search of the PubMed, SCOPUS, and ScienceDirect databases was conducted from 20 September 2020 to 20 December 2021. It was found that the most important sources of antioxidants are (1) in natural and dietary forms and (2) in the form of synthetic and medicinal supplements. Diets containing vitamins E, A, and C, beta-carotene, lycopene, and selenium are the most important sources of natural antioxidants. Additionally, masoprocol, pramipexole, allopurinol, pentoxifylline, melatonin, dimethyl sulfoxide, and acetylcysteine are among the synthetic and medicinal forms. Additionally, a search was performed using relevant keywords or title headings, including 'peripheral arterial disease', 'antioxidant therapy', 'supplementation', 'therapeutic effect', and 'complications'. Full details of the search strategy can be found in Table 1. Finally, reference lists of related review articles were manually searched. Table 1. Full details of the search strategy terms.

Study Selection
The inclusion and exclusion criteria established for the review are indicated in Table 2. For studies to be included in the review, all the studies had to meet all the inclusion criteria. Two authors, SK and HS, separately screened the articles based on the inclusion criteria. Thus, complete reports were received for all studies that were unclear or appeared to meet the inclusion criteria. Additionally, the conflicts that occurred were resolved through analysis until an agreement was reached. When agreement could not be reached, a third reviewer (BF) participated.

Data Extraction and Quality Assessment
The reviewers independently extracted and recorded data, including the author and year of publication, population characteristics such as the sample size, antioxidant intervention (supplementation/dietary and medicinal forms), duration of follow-up, status after intervention, and quality control score as determined by the two independent reviewers.
The studies were assessed using a study design and sampling method appropriate for the research, an adequate sample size considering the prevalence of PAD, approved criteria used for the evaluation of the results, unbiased analysis of the outcomes, an adequate response rate, reporting of the statistics with confidence intervals, and detailed descriptions of the study subjects. Finally, the quality assessment showed that the selected studies could be approved in terms of all the mentioned criteria.

Search Results
The process of choosing relevant studies is summarized in Figure 1. The search strategy identified 210 articles; additionally, 17 articles were further identified by searching through the reference lists of relevant reviews. After duplicates were excluded, 135 articles were selected by title and abstract for their eligibility. From these, 92 articles were evaluated based on their full texts, with 74 articles excluded due to unreliable study designs, patient populations, interventions, outcome measures, sample sizes smaller than 10, or no full text being available. Consequently, 18 articles were included in this review.
Overall, no relationship between the dose and duration of antioxidant treatment was reported [10,23]. In 11 (61.1%) studies, the results for antioxidant therapy were reported to show it to be significantly effective [10][11][12]17,20,[22][23][24][25][26][27], and in 7 (38.8%) studies, no significant differences from the placebo group were observed [13][14][15][16]18,19,21]. The results are summarized in Table 3. additionally, 17 articles were further identified by searching through the reference lists of relevant review. After duplicates were excluded, 135 articles were selected by title and abstract for their eligibility. From these, 92 articles were evaluated by their full texts, with 74 articles excluded due to unreliable data. Consequently, 18 articles were included in this review.   There was a significant improvement in arterial blood flow and antithrombotic effect

Discussion
Oxidative stress plays a vital role in the development of PAD, but not in isolation, as other risk factors including hypertension, diabetes, smoking, hypercholesterolemia, obesity, and physical inactivity also contribute to the development of symptoms of PAD [7,28]. However, it has been shown that increased levels of oxidative stress can be an important mechanism in the pathophysiology of these risk factors, leading to increased levels of oxidized LDL (ox-LDL), increased thrombus formation, endothelial dysfunction, and the development of atherosclerotic plaques [7,8]. Therefore, it can be deduced that antioxidants may be effective in PAD treatment [29,30].
The current study was designed as a systematic review investigating the effect of antioxidant therapy in PAD patients. Two forms of antioxidants were studied-natural antioxidants (in the form of fruits and vegetables) and synthetic antioxidants (such as dietary supplements or medications).
The reviewed studies show that natural antioxidants have been thoroughly effective in treating PAD (100%), but fewer studies have used this method [10][11][12]. This beneficial effect is not consistently seen with synthetic antioxidants, and only half (53%) of the studies have shown effective results [17,20,[22][23][24][25][26][27]. Based on these data, it could be assumed that antioxidants in their natural form may be more effective than synthetic forms in treating PAD. In addition, fruits and vegetables are rich sources of minerals along with antioxidants. On the other hand, the presence of minerals such as sodium, potassium, selenium, magnesium, zinc, copper, and calcium together with antioxidants and an optimal balance with them has been shown to increase the effect of antioxidants [31].
Furthermore, in 38.8% of the reviewed studies, there was no significant effect on the improvement in patients using antioxidants [13][14][15][16]18,19,21]. Furthermore, based on the reviewed studies, these also showed that the dose and duration of antioxidant therapy did not significantly affect its effectiveness.
Overall, studies on the effect of antioxidants causing improvements in PAD patients show some contradictory outcomes. On the contrary, this contradiction regarding the antioxidant effects for PAD treatment may be due to the oxidative effect of antioxidants. It has been shown that, if the level of antioxidants in the body exceeds the required level, they produce oxidative effects, and excess levels of antioxidants can lead to increased levels of oxidative stress [32]. Therefore, although the therapeutic perspective on antioxidant supplements has been optimistic, they should not be administered indiscriminately and/or excessively. The intrinsic antioxidant system in cells can prevent damage from the effects of oxidants. Therefore, checking the pro-oxidant-antioxidant balance can be an effective method for managing antioxidant treatment. The measurement of oxidant and antioxidant capacity has been shown to help in understanding the balance between them. Therefore, prescribing antioxidants may be more effective in patients after considering the pro-oxidantantioxidant balance/ratio [32,33].
Finally, due to the multifactorial nature of PAD, using antioxidants as a definitive treatment alone may not be effective [6]. Therefore, using additional therapies according to the patient's underlying disease (anticoagulants, antiplatelet agents, antihypertensives, antidiabetic agents, and statins) along with the appropriate amount of antioxidants seems to be more effective [6,34]. Due to the positive feedback between the initiator (cause) of pathogenesis and its consequences, there is an interaction between the oxidative stress level and thrombosis, leading to the elevation of both [35]. This is also true about the interaction between inflammation and thrombosis [36]. Therefore, interactions at multiple critical points need to be identified and addressed. Although oxidative stress is the hub of this network, we need to consider other strategic issues that can trigger a feedback loop and break the cycle started by the initiator compound. As a result, combination therapy can be more comprehensive and effective [6,35,36]. Additionally, potassium and calcium in some fruits and vegetables, such as oranges, can regulate blood pressure. Based on this, it can be assumed that natural antioxidants in fruits and vegetables can be more efficacious [31]. The review also shows a lack of further similar studies on the effectiveness of antioxidants in populations at risk of developing PAD (such as older adults and people with unhealthy lifestyles) in preventing PAD.

Conclusions
In conclusion, though there is optimism regarding using antioxidants in treating PAD, the outcomes do not support the optimism. Using antioxidants in the natural forms and vitamins in the form of fruits and vegetables is likely to be more effective for PAD patients.
Finally, two essential factors must be considered to be more effective in treating PAD. First, evaluating the optimal pro-oxidant-antioxidant balance can be an effective method for managing treatment with antioxidants. In other words, prescribing antioxidants at the appropriate dose makes antioxidants more effective and strongly suggests that custom therapies need to be devised. Secondly, using the optimal amount of antioxidants for treating the underlying disease in PAD patients, together with other agents such as anticoagulants, antiplatelet agents, antihypertensives, hypoglycemic agents, antidiabetic agents, statins, and other cholesterol-balancing drugs, is a strong recommendation.
Author Contributions: S.K., data analysis, writing-original draft preparation, and visualization; H.S., data analysis; M.P., supervision and review; B.F., conceptualization, data analysis, and writingreview and editing; A.S., conceptualization, supervision, and review. All the authors have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.