Coenzyme Q10 Supplementation Improves Adipokine Levels and Alleviates Inflammation and Lipid Peroxidation in Conditions of Metabolic Syndrome: A Meta-Analysis of Randomized Controlled Trials

Evidence from randomized controlled trials (RCTs) suggests that coenzyme Q10 (CoQ10) can regulate adipokine levels to impact inflammation and oxidative stress in conditions of metabolic syndrome. Here, prominent electronic databases such as MEDLINE, Cochrane Library, and EMBASE were searched for eligible RCTs reporting on any correlation between adipokine levels and modulation of inflammation and oxidative stress in individuals with metabolic syndrome taking CoQ10. The risk of bias was assessed using the modified Black and Downs checklist, while the Grading of Recommendations Assessment, Development and Evaluation (GRADE) tool was used to evaluate the quality of evidence. Results from the current meta-analysis, involving 318 participants, showed that CoQ10 supplementation in individuals with metabolic syndrome increased adiponectin levels when compared to those on placebo (SMD: 1.44 [95% CI: −0.13, 3.00]; I2 = 96%, p < 0.00001). Moreover, CoQ10 supplementation significantly lowered inflammation markers in individuals with metabolic syndrome in comparison to those on placebo (SMD: −0.31 [95% CI: −0.54, −0.08]; I2 = 51%, p = 0.07). Such benefits with CoQ10 supplementation were related to its ameliorative effects on lipid peroxidation by reducing malondialdehyde levels, concomitant to improving glucose control and liver function. The overall findings suggest that optimal regulation of adipokine function is crucial for the beneficial effects of CoQ10 in improving metabolic health.


Introduction
Metabolic diseases are acknowledged to greatly affect the quality of life to those affected in addition to be the leading cause of death worldwide [1,2]. Overnutrition and sedentary lifestyle are currently known to be some of the major factors driving the rapid rise in metabolic diseases [3,4], in both developed and developing countries [5,6]. In fact, the past few decades have seen a drastic increase in the number of overweight and obese individuals [7]. Of major concern is that obesity, together with other metabolic complications such as hypertension, type 2 diabetes (T2D), and non-alcoholic fatty liver disease (NAFLD) can significantly accelerate the risk of heart failure, leading to reduced lifespan of the general population [4,8,9]. Consequently, there has been a great interest in understanding the pathophysiological mechanisms associated with the development of metabolic complications such as T2D and NAFLD [10]. This is especially important to identify molecular mechanisms for therapeutic target to alleviate metabolic complications.
Complex pathological mechanisms have thus far been linked with the development and aggravation of metabolic syndrome. For instance, a state of dyslipidemia, which is characterized by abnormally elevated serum levels of triglycerides and cholesterol, has been shown to promote enhanced ectopic lipid accumulation and subsequent injury to various body organs [4,11,12]. Increased organ and arterial lipid accumulation can induce insulin resistance and cause endothelial dysfunction, mostly through exacerbating inflammation and oxidative stress. Indeed, elevated pro-inflammatory cytokines such as C-reactive protein (CRP), interleukin (IL)-6, including lipid peroxidation makers, such as malondialdehyde (MDA) levels, have been identified in patients with dyslipidemia [13][14][15]. Consequently, our group and others are scrutinizing literature on the therapeutic value of various compounds in protecting against metabolic diseases, especially their impact on adipose tissue function and ameliorative properties on inflammation and oxidative stress [16][17][18][19].
Coenzyme Q 10 (CoQ 10 ), also referred to as ubiquinone, is attracting much interest due to its envisaged health benefits [20,21]. Interestingly, CoQ 10 is present in all eukaryotic cells, and it forms a crucial part of the electron transport chain. Notably, its deficiency is associated with the generation of oxidative stress and myocardial damage in the diabetic state [22]. Whereas, dietary intake of CoQ 10 is directly linked with enhanced levels of ubiquinol-10 within the human body, which is further crucial for blocking lipid peroxidation by protecting against low-density lipoprotein oxidation in conditions on metabolic syndrome [23,24]. Beyond its impact on lipid peroxidation, published evidence from randomized clinical trials (RCTs) shows that CoQ 10 remains important in ameliorating inflammation and improving adipokine function in conditions of metabolic syndrome [25][26][27]. Although other meta-analysis of RCTs has been conducted on the effects of CoQ 10 on markers of inflammation or oxidative stress [28][29][30], including assessing adiponectin levels [31], such information remains limited for being too specific and does not collectively give a better picture on the influence of this quinone on conditions of metabolic syndrome. Therefore, this meta-analysis assesses essential evidence on the modulatory effects of CoQ 10 on adipokine function, including its impact on markers of inflammation and lipid peroxidation in individuals with metabolic syndrome.

Selection and Characteristic Features of Included RCTs
Approximately, 24 RCTs were retrieved from relevant online databases reporting on the impact of CoQ 10 on adipokine levels in individuals with metabolic syndrome. Subsequently, six RCTs met the inclusion criteria as displayed in Figure 1. Other studies were excluded for reasons such as being out adipokine levels in individuals without the metabolic syndrome were excluded from the meta-analysis [32].
The general characteristic features of the included studies are summarized in Table 1. In brief, all included RCTs were published in peer-reviewed journals, each year between 2014 and 2018. Briefly, this study was composed of a total of 318 participants with an average age of 48 years, and the majority of these being female (73%). The majority of participants included in the current meta-analysis were those with T2D (n = 176), NAFLD (n = 82), and hypertension (n = 60). The prominent doses of CoQ10 used were 100 or 200 mg/daily for a minimum of four weeks, up to three months (Table 1).

Risk of Bias and Quality of RCTs
The risk of bias and quality of six included studies was assessed by Kabelo Mokgalaboni (K.M.) and Vuyolwethu Mxinwa (V.M.), according to a modified Downs and Black's guideline, as previously mentioned. The overall median score range of the included studies was 22 (19)(20)(21)(22)(23)(24), with five of them rated excellent (21-24 scores) and one rated as good (19 scores). In addition, KM used the Cohen's Kappa interrater to assess the degree of agreement in terms of quality assessment scores of the included studies [33]. The levels of agreement between the two reviewers (K.M. and V.M.) were scored as moderate to perfect. Furthermore, all included studies scored high in reporting bias 9 (9-10) out of ten possible scores (overall agreement 90%, Kappa = 0.89), external validity 2 (1-3) out of three possible scores (overall agreement 62%, Kappa = 0.52), internal validity 6 (5-6) out of seven possible scores, (overall agreement 79.6%, Kappa = 0.83) and selection bias with an average median scores of 5 (3-6) out of a six possible scores (overall agreement 80.95%, Kappa = 0.76).

Publication Bias
Publication bias was assessed using funnel plots for each measured outcome. Notably, visual inspection of funnel plots showed symmetry in all reported outcomes. The general characteristic features of the included studies are summarized in Table 1. In brief, all included RCTs were published in peer-reviewed journals, each year between 2014 and 2018. Briefly, this study was composed of a total of 318 participants with an average age of 48 years, and the majority of these being female (73%). The majority of participants included in the current meta-analysis were those with T2D (n = 176), NAFLD (n = 82), and hypertension (n = 60). The prominent doses of CoQ 10 used were 100 or 200 mg/daily for a minimum of four weeks, up to three months (Table 1).

Risk of Bias and Quality of RCTs
The risk of bias and quality of six included studies was assessed by Kabelo Mokgalaboni (K.M.) and Vuyolwethu Mxinwa (V.M.), according to a modified Downs and Black's guideline, as previously mentioned. The overall median score range of the included studies was 22 (19)(20)(21)(22)(23)(24), with five of them rated excellent (21-24 scores) and one rated as good (19 scores). In addition, KM used the Cohen's Kappa interrater to assess the degree of agreement in terms of quality assessment scores of the included studies [33]. The levels of agreement between the two reviewers (K.M. and V.M.) were scored as moderate to perfect. Furthermore, all included studies scored high in reporting bias 9 (9-10) out of ten possible scores (overall agreement 90%, Kappa = 0.89), external validity 2 (1-3) out of three possible scores (overall agreement 62%, Kappa = 0.52), internal validity 6 (5-6) out of seven possible scores, (overall agreement 79.6%, Kappa = 0.83) and selection bias with an average median scores of 5 (3-6) out of a six possible scores (overall agreement 80.95%, Kappa = 0.76).

Publication Bias
Publication bias was assessed using funnel plots for each measured outcome. Notably, visual inspection of funnel plots showed symmetry in all reported outcomes.

CoQ 10 Supplementation Improved Adipokine Levels
It is now well-established that adipose tissue functions as a vital organ for the secretion of adipose-derived factors or rather adipokines which play a key role in the regulation of inflammation [37]. In fact, one of these adipokines is adiponectin, an anti-inflammatory metabolite that has been shown to improve insulin sensitivity [38,39]. Interestingly, pooled estimates showed that CoQ 10 supplementation in individuals with metabolic syndrome increased adiponectin levels when compared to those on placebo (SMD: 1.44 [95% CI: −0.13, 3.00]; I 2 = 96%, p < 0.00001) (Figure 2A). On the other hand, leptin has been long identified as a pro-inflammatory metabolite that exacerbates inflammation in conditions of metabolic syndrome [40]. Remarkably, a meta-analysis of included studies revealed significant reduction of leptin levels in individuals on CoQ 10 supplementation when compared to those on placebo (SMD: −0.59 [95% CI: −0.98, −0.21]; I 2 = 0%, p = 0.37) ( Figure 2B).

CoQ10 Supplementation Reduced Markers of Lipid Peroxidation
It is well-recognized that individuals with metabolic syndrome have increased levels of oxidative stress, especially lipid peroxidation [41]. The latter is also a relevant target of CoQ10 in light of its critical role as a lipophilic antioxidant [41]. Interestingly, CoQ10 supplementation reduced MDA levels, as a marker of lipid peroxidation, when compared to placebo (SMD: −1.57 [95% CI: −3.60, 0.47]; I 2 = 97%, p < 0.00001) ( Figure 4).

CoQ 10 Supplementation Ameliorated Pro-Inflammatory Markers
Consistent with the dysregulation in adipokine levels, metabolic syndrome has been associated an aggravation in inflammation [38,39], and this state that is characterized by increased pro-inflammatory cytokines and acute phase reactants such as CRP, IL-6, and tumor necrosis factor alpha (TNF-α) [13,14]. Interestingly, pooled estimates of inflammation markers showed a significant decrease in individuals on CoQ 10

CoQ10 Supplementation Reduced Markers of Lipid Peroxidation
It is well-recognized that individuals with metabolic syndrome have increased levels of oxidative stress, especially lipid peroxidation [41]. The latter is also a relevant target of CoQ10 in light of its critical role as a lipophilic antioxidant [41]. Interestingly, CoQ10 supplementation reduced MDA levels, as a marker of lipid peroxidation, when compared to placebo (SMD: −1.57 [95% CI: −3.60, 0.47]; I 2 = 97%, p < 0.00001) (Figure 4).

CoQ 10 Supplementation Reduced Markers of Lipid Peroxidation
It is well-recognized that individuals with metabolic syndrome have increased levels of oxidative stress, especially lipid peroxidation [41]. The latter is also a relevant target of CoQ 10 in light of its critical role as a lipophilic antioxidant [41]. Interestingly, CoQ 10 supplementation reduced MDA levels, as a marker of lipid peroxidation, when compared to placebo (SMD: −1.57 [95% CI: −3.60, 0.47]; I 2 = 97%, p < 0.00001) (Figure 4).

CoQ10 Supplementation did not Impact Liver Function in Individuals with NAFLD
Briefly, NAFLD remains one of the major characteristic features of metabolic syndrome [43], and this condition is persistent with the rise in hepatic enzymes such as alanine transaminase and aspartate transaminase (AST). Here, pooled estimates showed a small effect size in the liver function among individuals on CoQ10 supplements versus those on placebo (SMD: 0.33 [95% CI: −0.94, 1.61]; I 2 = 93%, p < 0.00001) ( Figure 6). Despite substantial levels of heterogeneity, the test for subgroup differences showed no significant subgroup effect (p = 0.09). The overall summary of findings on the beneficial effects of CoQ10 supplementation in individuals with metabolic syndrome is provided in Table 2.

CoQ10 Supplementation did not Impact Liver Function in Individuals with NAFLD
Briefly, NAFLD remains one of the major characteristic features of metabolic syndrome [43], and this condition is persistent with the rise in hepatic enzymes such as alanine transaminase and aspartate transaminase (AST). Here, pooled estimates showed a small effect size in the liver function among individuals on CoQ10 supplements versus those on placebo (SMD: 0.33 [95% CI: −0.94, 1.61]; I 2 = 93%, p < 0.00001) ( Figure 6). Despite substantial levels of heterogeneity, the test for subgroup differences showed no significant subgroup effect (p = 0.09). The overall summary of findings on the beneficial effects of CoQ10 supplementation in individuals with metabolic syndrome is provided in Table 2.

CoQ 10 Supplementation Did Not Impact Liver Function in Individuals with NAFLD
Briefly, NAFLD remains one of the major characteristic features of metabolic syndrome [43], and this condition is persistent with the rise in hepatic enzymes such as alanine transaminase and aspartate transaminase (AST). Here, pooled estimates showed a small effect size in the liver function among individuals on CoQ 10 supplements versus those on placebo (SMD: 0.33 [95% CI: −0.94, 1.61]; I 2 = 93%, p < 0.00001) ( Figure 6). Despite substantial levels of heterogeneity, the test for subgroup differences showed no significant subgroup effect (p = 0.09). The overall summary of findings on the beneficial effects of CoQ 10 supplementation in individuals with metabolic syndrome is provided in Table 2.  ⨁⨁⨁⨁$$ HIGH * The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: Confidence interval; MD: mean difference; OR: odds ratio; NE: not estimable GRADE Working Group grades of evidence. High certainty: We are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: We are moderately confident in the effect estimate: The true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different.

Discussion
Emerging research supports the beneficial effects of CoQ 10 against metabolic complications [32,34,35,44,45]. Several published RCTs suggest that CoQ 10 can regulate adipokine levels, including those of leptin and adiponectin, to impact metabolic function. As a prime example, Gholami et al. 2018 [36] reported that CoQ 10 supplementation in women with T2D was effective in increasing the concentrations of adiponectin or the adiponectin/leptin ratio, including reducing MDA levels which could translate to improved insulin sensitivity and amelioration of oxidative stress. Although Mehrdadi and colleagues [35] recorded a decline in adipolin levels with CoQ 10 supplementation, this dietary compound could improve glucose homeostasis by reducing HbA1c concentrations. Perhaps suggesting that the anti-adipogenic effects of CoQ 10 [46] are important in improving glucose homeostasis though not affecting adipolin in these patients. Like adiponectin, adipolin can impact the metabolic syndrome by attenuating atherosclerosis and improving insulin sensitivity, as reviewed elsewhere [47]. In another RCT [34], CoQ 10 could lower lipid peroxidation by reducing MDA levels, albeit not significantly affecting FPG, HbA1c or adiponectin levels in patients with T2D. Apparently, dose selection could explain such limitation since diabetic individuals were treated with a dose of 100 mg CoQ 10 , which was much lower than the other two RCTs [35,36], which showed beneficial effects with 200 mg dose of ubiquinone on modulating glucose metabolism.
Nevertheless, the overall pooled estimates in the current study showed that CoQ 10 supplementation significantly increased adiponectin levels in individuals with metabolic syndrome, which is concomitant to improved glucose control, and reduced levels of leptin and pro-inflammatory markers such as IL-6, CRP, and TNF-α. Apparently, circulation levels of adipokines such as leptin and adiponectin can act as biomarkers to evaluate obesity-associated complications, including low-grade inflammation [48]. While leptin is considered a crucial link between the compromised metabolic state and exacerbation of inflammation [40], enhanced adiponectin levels remain essential for effective regulation of glucose and lipid metabolism to improve metabolic function [38]. Interestingly, certain pharmacological interventions such as stragaloside II and isoastragaloside I have been found to selectively enhance adiponectin secretion in primary adipocytes without any apparent effects on other adipokines [49]. Thus, making therapeutic regulation of adipokine levels, including associated pro-inflammatory responses, as explored in the current study, an interesting strategy to protect against the metabolic syndrome.
Beyond controlling hyperglycemia and makers of lipid peroxidation or inflammation in those with T2D, other RCTs support the beneficial effects of CoQ 10 in patients with NAFLD. Briefly, Farsi and colleagues [27] demonstrated that CoQ 10 supplementation in patients with NAFLD limits liver damage by lowering the actions of AST and gamma-glutamyl transpeptidase, in connection with reducing the levels of CRP and TNF-α. Interestingly, such findings are consistent with elevated serum levels of adiponectin in patients with NAFLD [27]. Although these benefits could be observed, pooled estimates showed that CoQ 10 did not show a significant effect in improving liver function in those with NAFLD. A possible reason for not observing significant effects with CoQ 10 supplementation could be the limited number of RCTs included in the current meta-analysis.
Looking at the impact of other adipokine markers, Farhangi and colleagues [25] demonstrated that CoQ 10 could significantly reduce serum levels of AST and MDA when compared to the placebo group. Here, changes in serum FPG were a significant predictor of fluctuations in serum adipokines such as vaspin, chemerin and pentraxin-3. Although not widely explored as adiponectin and leptin, adequate regulation of serum levels of other adipokines such as vaspin, chemerin, and pentraxin 3 has been associated with improved insulin sensitivity and vascular function [50][51][52]. Alternatively, Nesami and co-workers [26] found CoQ 10 supplementation to be effective in decreasing some pro-inflammatory factors like IL-6 and CRP, in association with increasing adiponectin levels in patients with mild hypertension. Thus, broadly indicating that by effectively regulating adipokine levels, CoQ 10 can greatly impact metabolic health in connection to ameliorating inflammation and oxidative stress in patients with T2D, NAFLD, or hypertension, as increasingly discussed [28][29][30]53]. Therapeutic benefits of CoQ 10 supplementation are essential important since its deficiency in the body or enhanced oxidation status has been linked with compromised metabolic function [24,54,55]. In fact, Gholami and colleagues [36] demonstrated that at the end of the intervention, CoQ 10 supplementation significantly improved its serum levels, including that of adiponectin while reducing that of MDA in patients with T2D. Thus, inferring that increased intake of CoQ 10 could lead to its enhanced endogenous levels, which are vital for the amelioration of oxidative stress, especially lipid peroxidation products. Also highlighting the importance of understanding the link between CoQ 10 intake and its delivery in vivo, as discussed elsewhere [56].

Materials and Methods
The current meta-analysis was prepared in accordance with the Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) guidelines [57]. Subsequently, supplementary file S1 provides a PRISMA checklist for this meta-analysis.

Strategy to Search RCTs
The systematic search for eligible studies was conducted using electronic databases such as MEDLINE, Cochrane Library, and EMBASE from inception up to 29 February 2020 by two independent investigators (Phiwayinkosi V. Dludla, P.V.D.; and Tawanda M. Nyambuya, T.M.N.). In cases of disagreement, a third investigator (Bongani B. Nkambule, B.B.N.) was consulted for adjudication. The primary search was limited to RCTs, reporting the impact of CoQ 10 supplementation on adipokine levels, glucose control, liver function, including makers of inflammation and oxidative stress in individuals with metabolic syndrome. Within the same cohort, individuals receiving placebo were used a comparative control. For an optimal search strategy, Medical Subject-Heading (MeSH) and text words such as "coenzyme Q 10 ", "metabolic syndrome", "adipokine", "inflammation", and their matching synonyms and connected words or phrases were modified for each database used. In addition, a manual search was also performed through references of articles, abstracts, preprint platforms (to identify unpublished studies), which is essential to identify relevant evidence. There were no language restrictions applied in the search strategy, whilst EndNote version 10 (Clarivate Analytics, Philadelphia, USA) was used to manage the reference list and eliminate duplicates, as previously reported [58].

Inclusion and Exclusion Criteria
The current meta-analysis encompassed RCTs evaluating the impact of CoQ 10 on adipokine levels in correlation with basic metabolic parameters in adults (>18 years) with metabolic syndrome. In brief, included RCTs were those that evaluating the use of CoQ 10 as an intervention, contained the comparison group on placebo, and reported on adipokine levels in individuals with metabolic syndrome. Animal or in vitro studies were not included, while other exclusions included books, cohort or observational studies, letters, and case reports. Review articles were only scanned for RCTs. Furthermore, studies not reporting on the effects of CoQ 10 on adipokine levels in patients without metabolic syndrome were excluded.

Data Extraction and Assessment of Quality
For data extraction, P.V.D. and T.M.N independently assessed all relevant articles and cautiously selected those that met the inclusion criteria. Any disagreements were resolved by referring a third investigator (B.B.N.). The primary outcome of the meta-analysis was to establish the impact of CoQ 10 supplementation on adipokine levels in conditions of metabolic syndrome. Another significant objective was to understand whether the modulation of adipokine levels by CoQ 10 supplementation related to improved inflammatory response and amelioration of oxidative stress markers. To achieve this, pertinent data items from each RCT included, such as name and year of publication, the country where the study was conducted, sample and gender distribution, in addition to CoQ 10 dosage and intervention period were extracted. Moreover, the risk of bias was assessed by two investigators (V.M. and K.M.) using the modified Downs and Black checklist, which is suitable for both randomized and non-randomized studies [59,60]. Any differences were determined by referring the third investigator (T.M.N.). The same investigators were also tasked with evaluating the quality of evidence across the selected RCTs by using the Grading of Recommendations Assessment Development and Evaluation (GRADE) approach [61].

Statistical Analysis
Statistical analysis was performed using RevMan software (version 5.0; Cochrane Collaboration, Oxford, UK). Higgin's I 2 statistics was used to test for statistical heterogeneity [62]. The random or fixed-effects models were used depending on the levels of statistical heterogeneity, as previously discussed [63]. Cohen's d method was employed to interpret effect sizes, whereby a standardized mean difference of 0.2, 0.5. and 0.8 were regarded as small, medium, and large effect, respectively [64]. This method makes use of standardized mean difference (SMD) and not actual mean differences to measure effect sizes. Briefly, SMD is calculated as per the Cochrane guidelines, where it is recommended when estimating the effect size of pooled studies which report a similar outcome using a variety scales (SI units) [65]. While a p-value <0.05 was considered statistically significant. The inter-rater reliability was evaluated for both the encompassed studies and risk of bias by means of Cohen's kappa. For example, a kappa value of <0.00 was considered as poor strength of agreement, 0.00-0.20 as minor agreement, 0.21-0.40 as fair agreement, 0.41-0.60 as reasonable agreement, 0.61-0.80 as significant agreement, and 0.81-1.00 as perfect agreement [33].

Study Limitations
The limitation of the current meta-analysis includes the low number of RCTs included, which significantly reduces the confidence in the level of the. reported findings, especially those on its beneficial properties against NAFLD. Thus, suggesting that additional RCTs are necessary to improve our understanding of the correlation between adipokine regulation and metabolic abnormalities in patients with NAFLD. Another limitation, most included RCTs did not detect serum levels of CoQ 10 post-intervention, which is a crucial aspect to assess to determine its bioavailability and its therapeutic potential.

Conclusions
Coenzyme Q 10 (CoQ 10 ) is a dietary compound with established antioxidant properties and is increasingly investigated for its ameliorative effects against various metabolic complications. Evidence from other RCTs has been synthesized to inform on the impact of CoQ 10 on inflammation, oxidative stress, or insulin resistance [28][29][30][66][67][68], including its modulation of adiponectin [31]. However, the current meta-analysis provides novel findings on the broad effects of this compound on the regulation of various adipokines on diverse metabolic complications. Indeed, summarized evidence suggests that optimal regulation of adipokine markers such as adiponectin and leptin is crucial for the beneficial effects of CoQ 10 in attenuating oxidative stress and inflammation in conditions on metabolic syndrome.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.