Non-communicable diseases (NCDs) account for 71% of the deaths worldwide with rising prevalence in lower- and middle-income countries (Huang, 2009; Robinson et al., 2013). NCDs have been ranked as one of the top ten global threats in 2019 by World Health Organization (Khowaja et al., 2007; Robinson et al., 2013). Metabolic syndrome (MetS) is a type of NCD with worldwide prevalence ranging from less than 10% to as much as 84% (Rhee et al., 2010) with the burden being greater in South Asian countries (Sever et al., 2003; Su and Li, 2011).

MetS is characterized by a cluster of three or more features including hyperglycaemia, hypertriglyceridemia, a low level of high-density lipoprotein cholesterol (HDL-C), blood pressure and central obesity (Bodeker and Kronenberg, 2002; Anderson and Taylor, 2012). A person who has at least three out of five of these characteristics is labelled as MetS patient. The following criteria should be met for MetS (AuH, 1998; Anderson and Taylor, 2012): waist circumference more than 35 and 40 inches in women and men, respectively (central obesity); triglycerides (TGs) 150 mg/dl or greater, HDL-C less than 50 and 40 mg/dl in women and men, respectively, blood pressure (BP) of 130/85 mm Hg or higher, fasting blood glucose (FBG) of 100 mg/dl or greater. Besides the above mentioned abnormalities, underlying initiators of MetS are inflammation, oxidative stress and insulin resistance (Ma et al., 2009; Aziz et al., 2013; Amin et al., 2015a). Together these factors pose a three- and five-fold greater risk for cardiovascular disease (CVD) and type II diabetes mellitus (T2DM) respectively (Zimmermann et al., 2007), along with high mortality rate (Gilani and Rahman, 2005).

MetS has multiple aetiologies and therefore no single drug can be effective in reversing this situation. The main stay of prevention and management of individuals at risk is life-style modification. However, those who have high levels of risk factors are the recipients for pharmacological treatment which is aimed towards individual symptoms’ management (AuH, 1998; Devalaraja et al., 2011; Mohamed, 2014). Multiple drugs including drugs to lower the blood glucose level, TGs, and blood pressure (Robinson et al., 2013) may be needed for a long time resulting in drug related complications, low compliance rate and high cost of care (Khowaja et al., 2007; Huang, 2009). Alternately, some researchers suggest to advocate life-style modification as the first line therapy for prevention of a chronic disease, rather than using pharmacological therapies such as metformin in pre-diabetes (Rhee et al., 2010) and statins in mild to moderate dyslipidemia (Sever et al., 2003). Endorsing only life-style modifications is challenging for the physicians especially among high-risk patients such as in obese patients, since compliance to dietary modification, and physical activity is difficult to attain (Samir et al., 2011). Therefore, it is imperative to explore innovative therapies which are cost-effective and acceptable, with fewer adverse effects, in order to reduce the risk of cardiovascular diseases (CVD) through addressing the risk factors.

According to World Health Organization (WHO), up to 80% of the Asian population relies on complementary and alternative/Traditional medicine (CAM/T) for their primary healthcare, possibly because more than 80% of people in developing nations can barely afford basic medical needs (Su and Li, 2011). Interestingly, almost half of the population in the developed world also uses CAM/T therapies (Bodeker and Kronenberg, 2002). Amongst the most common complementary modalities used by individuals with CVD risk factors are natural products (Anderson and Taylor, 2012) that have evidently contributed in the development of modern medicine for cardiovascular disorders (AuH, 1998). MetS requires multiple factors to be addressed simultaneously, therefore polyherbal combinations can offer a safe and more effective therapeutic option. Research has revealed that the multi-component properties of polyherbal combinations make them suitable for treating complex diseases and offer great potential for exhibiting synergistic actions. Evaluation of literature from individual effects of potential polyherbal combinations paves the path for deriving new combinations.

Synergistic therapeutic actions of polyherbal formulations are possible through underlying mechanisms such as regulation of same or different targets in various pathways hence in combination enhance efficacy, regulation of enzymes and transporters to improve oral drug bioavailability, neutralize adverse effects and overcome drug resistance mechanisms. Synergism is observed when multiple chemical constituents are present in single or in combination of herbs (Amin et al., 2015a), which are potential therapeutic options for various disease targets. This forms the basis of polyherbal therapies (Ma et al., 2009; Aziz et al., 2013) and is considered rational and more efficacious in multi-targeted diseases (Zimmermann et al., 2007). The effect-enhancing and side-effect neutralizing properties of polyherbal combinations (Gilani and Rahman, 2005) prompted us to review the literature on the efficacy of polyherbal combinations in metabolic syndrome, the incidence of which is rising globally. This will help researchers to identify various effective polyherbal combinations in MetS, which may help in the drug development process, as well as provide future direction towards research on prevention and cure of a menace like metabolic syndrome. Although synergistic therapeutic interactions of herbal ingredients have been frequently reported, to the best of our knowledge, none of the reports have offered review of polyherbal formulations in MetS. Individual herb reviews related to MetS were limited to functional foods (Mohamed, 2014) and exotic fruits (Devalaraja et al., 2011). Hence, in this review, we present recent literature reporting herb synergisms and efficacy of various polyherbal formulations in MetS. We have identified the herb to be good if it manages to modulate at least 3 out of 5 MetS criteria.

The selection parameter, applied filters, as well as output of all the searches, are summarised in Figure 1A. In Figure 1B the summary of identified results is presented according to PRISMA guidelines (Page et al., 2019; Maraolo, 2021).

The total reference shortlisted were 109, out of which duplications and or articles which could not be retrieved were removed (n = 15) and number of articles to review were 94. Out of total 94 articles, 26 were divided as clinical trials and remaining 68 articles were either based on animal studies or in-vitro assays. These articles were further shortlisted by reviewing their basic theme and it was identified that some of the articles did not have the objective to manage MetS or were aimed at identifying new targets for management of Met S such as use of probiotics or correlating with gut microbiota (Ni et al., 2018) or the basic target for those studies were to cater different disease, although parameters for MetS were being met. Hence, out of 68 animal studies, filtered animal studies were identified to be 24 which matched our main objective of MetS. The taxonomic classification of polyherbal combinations used both in animal and clinical studies are summarized in Tables 1, 2, respectively. The meta-analysis of animal studies is summarized in Table 3. To further analyze the quality of studies, a semiquantitative scale was used, the details of which are presented separately as Table 4. The maximum score was 8, and references have been aligned from highest score to lowest score.

Out of 26 clinical trial articles, 15 articles matched our main objective, and their meta-analysis is presented in Table 5 according to SPIDER model with references. Supplementary Table S1 is attached to shows the analysis by SPICE protocol along with information about other targets met besides the 5 parameters of MetS. Table 6 summarizes the qualitative scoring based on a checklist as mentioned in analysis section along with the online link available for the same. Out of 15 polyherbal combinations that were reviewed three formulations were able to modify 4 MetS parameters clinically. They include Yiqi Huazhuo Gushen herbal formula (Tian-zhan et al., 2019), Yiqi Huaju Qingli Formula (Wang et al., 2013), Sesame oil and vitamin E (Farajbakhsh et al., 2019). Six polyherbal combinations were able to reduce three out of 5 standard MetS parameters. The combinations included, Curcuma longa and Nigella sativa (Amin et al., 2015b), Diabegon (Yadav et al., 2014), modified Lingguizhugan decoction (MLD)+ weekend fasting (Yang et al., 2014a), Dahuang Huanglian Xiexin Decoction (Yu et al., 2018), combination of Nutraceuticals (Rozza et al., 2009) and Altilix supplement containing chlorogenic acid and luteolin (Castellino et al., 2019).

MetS is a cluster of metabolic abnormalities that appear as a pre-diseased state and predisposes to CVD risk even before overt disease such as diabetes or hypertension develops. Catering those risk factors at this stage could prevent incidence of CVD. Hence, clinicians need to target multiple risk factors simultaneously. As the incidence of MetS is rising, there is a need to identify therapeutic modalities that could address multiple disease targets, offer better compliance, and reduce risk of adverse effects (Reilly and Rader, 2003; Keith et al., 2005). Polyherbal formulations could mutually enhance pharmacological synergy on the targeted disease and often exhibit pharmacological and therapeutic superiority in comparison to isolated single constituents.

The current review focuses on studies published from 2005–2020, reporting the efficacy of polyherbal therapies in MetS. This is attributed to either the action of bioactive ingredients from different herbs on the same molecular target forming a multiple-drug-one-target model (additive effect) and/or the functionally diverse targets but with potentially clinically relevant associations forming a multiple-drug-multiple-target-one-disease (synergistic effect) (Lu et al., 2012; Wang et al., 2012). In the current review, we identified 25 animal based studies in which polyherbal formulations were used in animal models of Mets. We categorised them as good and not very good, based on the modulation of MetS parameters. Studies which were able to modulate 4-5 parameters were considered as very effective, whereas studies that modulated three or less than 3 parameters were marked as not so good. This, however, does not reflect on the quality of review. For the quality of review, we devised an 8-question checklist and marked one point for meeting the criteria and 0 for no meeting the criteria. The overall score was 8.

From the effect point of view, different combinations were identified as very effective in animal based studies. They included combination of Curcuma longa, Salacia reticulate, Gymnema sylvestre, Emblica officinalis, Terminalia chebula (Thota et al., 2014), Glycyrrhizae uralensis Fischer, Rheum undulatum Linne, Prunus persica Linne, Cinnamomum cassia Presl and Natrii Sulfas (Sung et al., 2014), Rhizoma coptidis, Radix scutellariae, Cortex phellodendri and Fructus gardeniae (Li et al., 2013), Red ginseng and Polygoni Multiflori Radix (Kho et al., 2016) and modified lingguizhugan decoction (Yao et al., 2017a). These combinations modulated all the five parameters of MetS including reduction in body weight/obesity, BP, TG, and fasting blood glucose (FBG) and increase in HDL. Additionally, combination of soybean meal and probiotics (Bifidobacterium longum) (Mounts et al., 2015), Fu Fang Zhen Zhu Tiao Zhi formula (Hu et al., 2014), Curcuma Longa and Nigella Sativa (Amin et al., 2015a) and mixed extracts of Artemisia iwayomogi and Curcuma longa (Lee et al., 2015a) improved 4/5 MetS parameters and can be further considered for clinical trials.

These studies however exhibited certain limitations. For example, Lee et al. (2015a), comprehensively studied effect of Artemisia iwayomogi and Curcuma longa extract on metabolic markers along with fine mechanistic details but did nto use positive controls in their study. Similarly, Yao et al. (2017a) did not use positive controls in their study when studying effect of modified Lingguizhugan decoction (MLD) and only selected one dose for intervention. Hence, dose dependent effect couldn’t be assessed. Besides, they did not study the effect mediated by MLD alone and only showed results of MLD with dietary restriction and exercise; additional group of MLD should have been added for confidently claiming the effect of MLD in the study. Amin et al., presented their findings comprehensively about use of combined Curcuma longa and Nigella sativa in MetS models but despite of mention of measuring body weight fortnightly, there were no results about effect on body weight (Amin et al., 2015a).

Some studies showed reduced effect on Met S parameters, but their focus was more on mechanistic details. For instance, study by Gao et al. (2015) on effect of Erchen decoction (ECD) exhibited effect on 3 parameters of MetS including FBG, TG and body weight and abdominal circumference. One of the appreciable aspects of this study is that the researchers reported abdominal circumference and body weight simultaneously. Limited animal studies consider abdominal circumference, which is the actual predictor of MetS. Additionally, molecular mechanisms of ECD on diabetic parameters have been elaborated at genetic level, where expression of CDK5 regulatory subunit associated protein 1 like 1 (CDAK1) has been shown and correlated with improved islet cell function. Since this preparation did not have effect on LDL and HDL, combining it with antidyslipidemic herb, such as Curcuma longa and/or Nigella sativa coupled with low dose of ECD may be a good combination for future studies. Like this, extracts of Salvia miltiorrhiza and Gardenia jasminoides (Tan et al., 2013), showed effect on 3 parameters of Met S, but gave an elaborate mechanism for their antiobesity effect including enhanced leptin expression. Amongst the studies reported in this review, limited studies assessed BP (Thota et al., 2014; Amin et al., 2015a); whereas, most of them did not assess blood pressure in their models, and therefore the studies which have either met 3 or 4 out of 5 parameters of MetS are majorly the ones which did not assess BP in their animal models (Mounts et al., 2015). One of the reasons for this could be that BP monitoring in animals is technically challenging, and assessing it for number of animals, which usually are 40–50 altogether, is highly tedious and time consuming.

The other part of our review focussed on clinical trials in the last 15 years which used polyherbal formulations for the management of MetS. Amongst the combinations reviewed the most effective considered were the ones which met maximum MetS parameters. The maximum parameters modified were 4 out of 5 by 3 combinations including Yiqi Huazhuo Gushen herbal formula (Tian-zhan et al., 2019), Yiqi Huaju Qingli Formula (Wang et al., 2013), and Sesame oil and vitamin E combination (Farajbakhsh et al., 2019). However, these studies were assessed for short period of time ranging from 8 to 12 weeks, which may be helpful in determining the acute effect but not long-term effect and side-effects.

From this perspective a study by Yadav et al. (2014) is worth mentioning who studied the effects of herbal combination “Diabegon” till 1.5 years and monitored the effect on liver and kidney parameters, which showed no toxic effects on these organs. In fact, the combination reduced uric acid and effectively reduced FBG, TG and increased HDL, although BP was not monitored. Another worthy study in this regard was controlled clinical trial which used Keishibukuryogan, a traditional Japanese (Kampo) formula (Nagata et al., 2012) in MetS patients in a cross over design. Although, it did not reduce any MetS parameters, its main outcome was improvement in endothelial function which has a preventive role towards atherosclerosis. Such study designs should be adopted for formulas which have shown promising results in small scale studies.

Some studies design was flawed and therefore the effects could not be validated. For instance, study by Yang et al. (2014a) on MLD along with weekend fasting tested the combination on MetS patients but no comparative control was used. We could not determine whether the effect was due to MLD or weekend fasting. Aims of the study were also not clearly written in the write-up. Similarly, a combination of nutraceuticals with dietary interventions very efficiently reflected the improvement in MetS parameters to an extent that the patients no longer fulfilled the MetS criteria after treatment (10/15) (Rozza et al., 2009). Nevertheless, with such a small sample size, the magnitude of impact could not be extrapolated and needs to be studied further. Some clinical studies assessed only limited parameters of MetS and therefore in terms of effectiveness those combinations are considered as not so good. Nevertheless, that’s not completely true, because the authors did not measure the remaining parameters (He et al., 2007; Panahi et al., 2014; Panahi et al., 2015). Reason for this could be that the main objective of those studies was to explore additional mechanisms of MetS. For instance, Panahi et al., (Panahi et al., 2015) report curcuminoids to reduce 2 out of 5 MetS parameters because they assessed only BP and BMI. Their main finding was anti-inflammatory and antioxidant activities, whereas antidyslipidemic effect was reported in their preceding study (Panahi et al., 2014).

The current review has certain limitations. One of the factors to be considered for future reviews should be to differentiate the polyherbal combinations according to different ethnicities and cultures in which the herb is famously used such as Asian, Chinese and Japanese traditional medicine. The current review can be used by researchers for idnetifying different polyherbal combinations by considering which herbs could simultaneously target many or all risk factors for MetS. For future studies some known anti-obesity and/or antihypertensive herbs shall be considered as an add-on with those polyherbal combinations that predominantly exhibited anti-hyperglycaemic and anti-dyslipidemic effect, to be able to manage multiple MetS parameters simultaneously. This is one of the advantages of such reviews that researchers could identify the missing targets and add herb accordingly for future studies.