Human gut microbiome: Therapeutic opportunities for metabolic syndrome—Hype or hope?

Abstract Shifts in gut microbiome composition and metabolic disorders are associated with one another. Clinical studies and experimental data suggest a causal relationship, making the gut microbiome an attractive therapeutic goal. Diet, intake of probiotics or prebiotics and faecal microbiome transplantation (FMT) are methods to alter a person's microbiome composition. Although FMT may allow establishing a proof of concept to use microbiome modulation to treat metabolic disorders, studies show mixed results regarding the effects on metabolic parameters as well as on the composition of the microbiome. This review summarizes the current knowledge on diet, probiotics, prebiotics and FMT to treat metabolic diseases, focusing on studies that also report alterations in microbiome composition. Furthermore, clinical trial results on the effects of common drugs used to treat metabolic diseases are synopsized to highlight the bidirectional relationship between the microbiome and metabolic diseases. In conclusion, there is clear evidence that microbiome modulation has the potential to influence metabolic diseases; however, it is not possible to distinguish which intervention is the most successful. In addition, a clear commitment from all stakeholders is necessary to move forward in the direction of developing targeted interventions for microbiome modulation.

In human subject research, an association between the gut microbiome composition and obesity has widely been demonstrated.
A causal relationship was first suggested by showing that a relative decrease in Bacteroides is associated with obesity, whereas a diet leading to weight loss leads to an increase in Bacteroides. 5ta from in vitro systems and animal models suggest that diet or drugs (e.g.antibiotics) alter the microbiome and that this dysbiosis is mechanistically involved in disrupting molecular metabolism as well as signalling through bacterial metabolites (e.g.bile acids).This impacts energy intake and leads to metabolic disorders.For example, diets high in fat and glucose lead to increased gut permeability, translocation of bacterial products, low-grade inflammatory response and insulin resistance. 6,7Bile acids are bidirectionally interacting with the microbiome-the microbiome influences bile acid composition and bile acids shape the microbiome.In obesity, altering bile acid composition via a reduction of microbial diversity through an antibiotic leads to increased insulin resistance. 8Altered microbiome composition, leading to altered bile acid metabolism and signalling might also contribute to fibrogenesis, liver injury and tumorigenesis in non-alcoholic fatty liver disease. 9Host genetics also play a role since immune control of the microbiome is beneficial to microbial populations that constrain lipid metabolism to prevent metabolic syndrome. 10However, these findings cannot be readily translated to treat or prevent human obesity yet, in part due to limitations in the experimental systems, thereby preventing the establishment of clear causality.Much of the current findings in the research of obesity and microbiome remain at the level of associations.
In this scoping review, we aimed to summarize the clinical evidence on the potential of diet, prebiotics, probiotics, faecal microbiome transplantation and selected drugs to treat and/or prevent metabolic disorders via microbiome modulation (Figure 1).

| The role of diet
Changes in the diet can alter energy balance, provide various nutrients and modulate the gut microbiome, which can have different effects on the development of diseases, including obesity and MetS.
F I G U R E 1 Our review summarizes the effect of diet, prebiotics, probiotics, fecal microbiome transplantation and drugs on the gut microbiome in metabolic diseases.
More than a decade ago, this was proven by Vijay-Kumar et al., 11 who provided evidence for the direct relationship between changes in the composition of the gut microbiota of mice and the development of the MetS.The role of certain diets or food ingredients on MetS in humans and the human gut microbiome remains unclear; thus, it is necessary to study it in more detail.The included studies with their clinical endpoints are summarized in Table 1 and Figure 2.

| Whole-grain diet
A whole grain is any grain that contains the endosperm, germ and bran, as opposed to refined grains that retain only the endosperm.
Whole-grain products are metabolized into short-chain fatty acids, 12 which are involved in glucose and lipid metabolism, 13,14 immune homeostasis 15 and intestinal permeability. 16As part of an overall healthy diet, consuming whole grains is associated with a lower risk of several diseases, including type 2 diabetes and cardiovascular diseases. 15,17Insufficient consumption of whole grains has been listed as a contributing factor to a higher risk of disability-adjusted life years in worldwide studies. 18Moreover, the consumption of wholegrain products, nuts and seeds may have the greatest health benefits of all the dietary factors studied, including fruit and vegetable consumption. 19randomized, controlled cross-over clinical trial, which aimed to investigate the effects of a whole-grain diet on insulin sensitivity, metabolic health and its impact on the gut microbiome compared with refined grain, did not show any significant effect of a wholegrain diet on glucose homeostasis and did not induce major changes in the diversity or richness of the gut microbiome. 20Analysing distinct species, the abundance of four distinct strains of Faecalibacterium prausnitzii and one strain of Prevotella copri increased after adhering to a whole-grain diet and decreased in a refined-grain diet, whereas Bacteroides thetaiotaomicron showed opposite tendencies.However, the whole-grain diet showed a positive impact on overall metabolic health as body weight and serum inflammatory markers were reduced in the study group. 20other randomized, double-blind, controlled crossover clinical study similarly aimed to determine how a whole-grain diet affects glucose tolerance and insulin resistance, compared with a refined grain diet.As opposed to the previously described study, these parameters improved with adherence to the whole-grain diet, suggesting its potential positive effects on diabetes. 21However, this study was conducted merely on 14 moderately obese adults at risk for diabetes, not including patients with clinically confirmed MetS.
Moreover, in this study, the authors did not analyse the diet's impact on the changes in gut microbiome composition.Although the intervention time in both studies was 8 weeks, the differences in the results could be due to the chosen diet strategies. 21rley beta-glucans, which form highly viscous solutions in the gut, thus extending the transit time of nutrients through the gut, may modify gut microbiota composition and improve overall metabolic health in MetS patients. 22,23It has been shown that a diet enriched with barley beta-glucans improved markers of MetS, as cholesterol levels were reduced in the test group.The analysis of gut microbiota composition, however, showed that bacterial diversity and richness both decreased after the dietary intervention with barley beta-glucans. 22Loss of diversity is usually related to gastrointestinal or liver diseases. 24However, greater gut microbial richness or diversity is not an absolute sign of a healthy gut, as could be observed in several diseases; 25,26 thus, further studies on the impact of barley beta-glucans on MetS are needed.

| Eggs
Eggs, being an essential component of the human diet, play an important role in human health.Egg yolks are known to contain highly available forms of antioxidants, such as choline or lutein carotenoids and zeaxanthin, which can protect against the development of metabolic diseases.This protection is thought to be due to an increased mRNA expression of antioxidant enzymes and a decreased efficiency of pro-inflammatory cytokines. 279][30] The effects of eggs on the gut microbiome remain largely unknown.
The effect of two sources of choline-whole eggs and a choline supplement (choline bitartrate)-on plasma lipids, glucose, insulin resistance and inflammatory biomarkers were evaluated.An increase in plasma choline in individuals with MetS, who consumed either three eggs per day or a choline supplement was observed.Interestingly, decreased inflammation was detected only after egg intake, which could be due to the antioxidants present in eggs. 31A similar study aimed to further explore the effects of choline provided by eggs or choline supplements on the gut microbiota in subjects with MetS.
However, although blood plasma values of choline were increased, diet intervention had no significant effects on microbiome diversity parameters or relative taxa abundances, possibly due to the short intervention period. 32Longer intervention times could be needed to obtain significant changes in the gut microbiome.Similarly, no relevant changes in the taxonomic composition of the gut microbiome were reported after a 2-week intervention period with two eggs per day. 33

| Dairy products
Dairy products are known as a source of multiple essential vitamins, minerals and high-quality protein.In addition, additional health benefits can be obtained after milk fermentation.Fermented dairy products are among the most effective sources of probiotics. 34Kefir is a fermented milk product, which has a very specific combination of bacteria and yeasts, present in grains. 35Lactobacillus, Lactococcus, Streptococcus leuconostoc and acetic acid bacteria are the most common bacteria, and Saccharomyces, Kluyveromyces and Candida are commonly found yeasts in kefir grains. 36Knowing the benefits of

TA B L E 1 (Continued)
kefir to overall health, it was aimed to study its potential effect on gut microbiota in MetS patients.Knowing the benefits of kefir to overall health, it was the aim of a clinical trial to study its potential effect on gut microbiota in MetS patients: The consumption of 180 mL of kefir per day in MetS patients resulted in a significant increase in the relative abundance of Actinobacteria.A strong positive correlation with systolic and a weak positive correlation with diastolic blood pressure and Actinobacteria abundance was observed compared with the consumption of unfermented milk in the control group. 37e benefits of fermented dairy products on changes in insulin resistance were investigated in a randomized, parallel and controlled clinical study where MetS patients with fatty liver disease consumed 220 g per day of yogurt or unfermented milk (in the control group).Yogurt had higher effects at ameliorating insulin resistance than unfermented milk and decreased the abundances of Firmicutes phylum, the Clostridiales order and Blautia and Eubacterium ventriosum group genera. 38

| Dietary polyphenols
Polyphenols are naturally found in various plant parts, such as leaves, stems and flowers. 42According to a recent study, the average polyphenol intake for the general population was estimated to be 0.9 g perday, and the main sources were not only fruits and vegetables but also beverages, such as coffee, tea and red wine. 43e concern about the impact of the consumption of red wine on the gut microbiome composition is increasing.Significant increases in the abundance of Enterococcus, Prevotella, Bacteroides, Bifidobacterium, Bacteroides uniformis, Eggerthella lenta and the Blautia coccoides-Eubacterium rectale group were observed, indicating potential positive effects of polyphenols in red wine that must be weighed against the well-known negative effects of alcohol on the liver and metabolic health. 44In a small randomized, crossover-controlled intervention study, red wine consumption was associated with a greater reduction in body weight and a significant decrease in Bacteroides as well as a significant increase in protectors of the gut mucosal barrier, such as Bifidobacterium spp.

| Specialized diets
The Mediterranean diet (MedDiet) includes a high consumption of fruits, vegetables, whole-grain foods, fish, nuts and olive oil.In addition, it is characterized by a low intake of red wine, meat and dairy products. 46MedDiet is rich in food primarily containing unsaturated fatty acids. 47It is proved that this diet has positive effects on lipid profile, glycemic control and blood pressure, 48 as it provides F I G U R E 2 Summary of the impact of diet on patients with metabolic disease focussing on changes in gut microbiome composition.
anti-inflammatory compounds. 49MedDiet can be used as a potential therapy for MetS to prevent excess adiposity and related inflammatory responses associated with obesity. 50The effects of MedDiet on insulin resistance and gut microbiome composition were studied in a crossover randomized clinical trial in MetS patients; MedDiet was associated with changes in the plasma metabolome that were associated with improvements of insulin resistance and correlated with the uncultured genus of Lachnospiraceae, Ruminococcaceae UCG002, Lachnoclostridium, and some genera from the Prevotellaceae family.
Moreover, negative correlations with changes in phosphoethanolamine and taurine were found for these bacteria. 51ile more research is needed to fully understand the mechanisms underlying MedDiet's impact on improving gut microbiome health, it is considered that the previously described polyphenols and other bioactive compounds found in red wine can have a positive impact.Red wine consumption was significantly associated with an increase in the relative abundance of Bacteroides, Ruminococcaceae, Roseburia and Prevotella.Interestingly, Parasutterella was the most prominent genus for differentiating the gut microbiota of the participants who consumed red wine. 52A previous study showed that Parasutterella supports interspecies metabolic interactions within the healthy gut ecosystem. 53A recent review investigated the potential benefits of theaflavins, found in red wine, on metabolic syndrome, with a focus on the gut microbiome.It was discussed that theaflavins have the potential to positively impact the gut microbiota by increasing the abundance of beneficial gut bacteria such as Lachnoclostridium and Bifidobacterium. 54diet rich in proteins was shown to have a negative impact on health and is associated with increased mortality. 55

| Probiotics
'Probiotics' is a collective term for living organisms (usually bacteria or fungi) that can exert health benefits on the host if consumed in sufficient amounts. 60In fermented foods, probiotic bacteria have been a part of human nutrition for a long time but have been neglected in Western-style diets, which are associated with the current obesity epidemic. 61Meta-analyses show that supplementation with probiotics has been successfully used to improve classical traits of T2D, such as fasting plasma glucose, insulin concentration, insulin resistance, glycated haemoglobin (HbA1c) [62][63][64] and improved lipid parameters. 65Furthermore, a slight improvement in body weight, BMI and fat percentage following a probiotic intervention has been observed in obese and overweight people. 66Accordingly, probiotics might be a valuable asset in the treatment of obesity, MetS and diabetes, but the available data are heterogenous in regard to study design, products, duration of intervention and outcome and is, therefore, inconclusive.Furthermore, the working mechanisms behind the observed effects and the involvement of the microbiome and microbiome modulation are still not understood.Table 2 summarizes the current body of literature focusing on probiotic microbiome modulation to ameliorate obesity, MetS or type 2 diabetes.Probiotics have been tested as single-strain or multi-strain/ multi-species formulations, sometimes with a prebiotic substance, resulting in a so-called synbiotic.

| Single-strain probiotics
Lactobacillus casei Shirota (LcS) is commonly available as a singlestrain probiotic milk drink.This milk drink given to obese children for 24 weeks as part of a diet and exercise therapy improved weight loss and increased HDL and acetic acid levels compared with an untreated control group.The intervention increased Bifidobacteria and L. casei counts in the gut microbiome of the treated children. 67These effects stand in direct contrast to the effects of LcS on Japanese adults with T2D.Although Lactobacillus abundance including L. reuteri increased in the microbiome, Bifidobacteria and HDL increased only in the control group.While patients treated with LcS showed a decreased count of bacteria in the bloodstream, organic acids decreased in the gut and hs-CRP increased. 68The increase in hs-CRP was also previously reported in a European cohort of MetS patients following a 12-week LcS intervention, while neither changes in gut permeability or serum bile acids nor changes in the microbiome other than an increase in Parabacteroides were observed. 69,70ctobacillus reuteri V3401 in a single-strain product reduced IL-6, sVCAM and insulin levels in newly diagnosed MetS patients after 12 weeks of intervention, while it increasing the abundance of Verrucomicrobia/Akkermansia muciniphila. 71L. reuteri ADR-1 decreased HbA1c and heat-killed L. reuteri ADR-3 decreased blood pressure, mean arterial pressure and IL-1b and increased the abundance of Bifidobacteria in treatment-naive T2D patients.Both groups showed an enrichment of the microbiome with the ingested probiotic but no other effects on the microbiome. 72Lactobacillus plantarum K50, a probiotic isolated from Kimchi, decreased total cholesterol and triglyceride levels in obese adults after a 12-week intervention but had no effect on body weight or composition.The intervention increased the L. plantarum, the genus Enterococcus and Enterococcus hirae abundance, a potentially beneficial bacterium 73 and decreased the abundance of Actinobacteria. 74L. plantarum HAC01 reduced 2-h postprandial glucose (2 h-PPG) and HbA1c levels in prediabetic adults without modulating the microbiome. 75L. plantarum Dad-13 led to a decrease in body weight and BMI in obese adults, while decreasing Firmicutes and increasing Bacteroidetes in the microbiome. 76An intervention with L. salivarius Ls-33 in obese adolescents showed no effect on metabolic parameters nor on the microbiome. 77fidobacterium lactis UBBLa-70 decreased serum lipids in obese women on a low-caloric diet, while this probiotic in combination with fructooligosaccharides (FOS) had a modulating effect on the metabolome but no effect on the lipid profile.Neither intervention had a measurable effect on weight, BMI or the microbiome. 78fidobacterium animalis subsp.lactis 420 in combination with a prebiotic modulated the microbiome of overweight adults, a circumstance, which was mainly driven by the prebiotic component while the probiotic without prebiotic showed only minimal effects on the microbiome composition and function. 79An intervention with either B. adolescentis IVS-1 or B. animalis BB-12 with or without galactooligosaccharides increased the abundance of the probiotic strains as well as the parent taxa in the microbiome to various degrees but had no effect on the gut permeability of overweight adults in a short term 3-week intervention. 80In combination with a very low-caloric ketogenic weight loss program, a single-strain synbiotic containing B. animalis subsp.lactis was more effective in regards to weight loss and reduction of gamma gutamyl transferase than a multistrain synbiotic given in succession with the single-strain synbiotic or a placebo.This intervention was associated with higher levels of Porphyromonadaceae, Christensenellaceae, Parabacteroides and Lachnospira. 81

| Multi-strain/multi-species probiotics
A probiotic containing B. longum BB536 and L. rhamnosus HN001 in combination with a Mediterranean diet reduced waist circumference, waist-to-hip ratio and insulin levels, while increasing alpha diversity, Eubacterium, L-Ruminococcus and decreasing Bacteroides and Butyricicoccus in the microbiome compared with a Mediterranean diet alone in obese female breast cancer survivors. 82The combination of two B. breve strains (BR03 and B632) also reduced waist circumference, the BMI standard deviation score, fasting insulin and ALT in obese children and adolescents in a weight loss program, while the changes in the microbiome were similar to the placebo group. 83L. curvatus HY7601 in combination with L. plantarum KY1032 improved body weight and composition and increased adiponectin levels in otherwise healthy overweight and obese adults.The probiotic and the placebo group underwent changes in the microbiome, including an increase in A. muciniphila and a decrease of F. prausnitzii in the probiotic group. 84L#3, one of the most researched probiotic consortia, increased body weight and adiposity in Latino adolescents without exerting changes on the microbiome 85 and increased liver fat content, indicating that the choice of probiotic strains is crucial. 86similar consortium also containing Streptococcus thermophilus, Lactobacillus spp.and Bifidobacteria spp.(for details see Table 2) increased HDL levels and probiotic bacteria in the microbiome of overweight women taking Bofutsushosan, a Japanese Kampo against obesity, but did not further influence metabolic parameters or the overall microbiome composition compared with women treated with Bofutsushosan alone. 87A Streptococcus/Lactobacilli/ Bifidobacteria consortium with Saccharomyces boulardii DBVPG 6763 showed no effect on metabolic parameters compared with placebo; microbiome changes were reported for both groups. 88crobiome changes in both groups were also observed in obese women treated with a Lactobacilli/Bifidobacteria consortium or placebo. 89Another Lactobacilli/Bifidobacteria combination modestly decreased HbA1c, fasting insulin and HOMA-IR, while increasing Lactobacillus and Bifidobacterium counts in the microbiome (no further analysis was performed). 90Postmenopausal women showed improvements in lipopolysaccharides (or lipopolysaccharide levels), waist circumference, fat mass, subcutaneous fat, uric acid, total cholesterol, triglycerides, low-density lipoprotein cholesterol, glucose, insulin and HOMA-IR after a 12-week intervention with a Lactobacilli/Bifidobacteria/Lactococci formulation, although the modulation of the microbiome structure and function was minimal. 91,92The same probiotic combined with FOS and GOS decreased hip circumference and lipoprotein (a) and improved quality of life, also with minimal impact on the microbiome structure of obese type 2 diabetic patients. 93A synbiotic consortium with considerable overlap in probiotic strains used in 91,93 combined with FOS, improved zonulin levels and decreased proteolytic bacteria in the microbiome while increasing total bacterial count in overweight adults. 94A Lactobacilli-Bifidobacteria-GOS combination in addition to a weight loss program reduced HbA1c levels in obese adults, while altering several taxa of the microbiome (for details see Table 2). 95nversely, a combination of LcS and B. breve YIT 12272 (BbrY) with GOS increased HbA1c and fasting glucose in patients with type 2 diabetes while modulating the microbiome, especially taxa taxonomically related to the ingested probiotic strains. 96Bacteroides fragilis to E. coli ratio increased and Firmicutes to Bacteroidetes ratio decreased in prediabetic patients after a 24-week intervention with a Lactobacilli/Bifidobacteria/inulin-formulation. 97 the currently largest clinical trial testing a probiotic consortium in newly diagnosed type 2 diabetes patients, neither a metabolic nor a microbiome modulating effect of a standalone probiotic intervention was observed. 98In a secondary analysis, a synergistic effect of the multi-species probiotic and berberin on postprandial cholesterols and other lipidomic parameters was found. 99Unfortunately, the trial included a study-specific 7-day run-in period with antibiotics, which influenced the microbiome composition, which is of questionable clinical usability considering the antibiotic-resistance crisis.

| Prebiotics
Prebiotics are defined as 'substrates that are selectively utilized by host microorganisms conferring a health benefit'. 100Since the recognition of their beneficial physiological effect over two decades ago, the interest in prebiotics has grown markedly.Currently, the best-known prebiotics are fructo-oligosaccharides, inulin, oligofructose, galacto-oligosaccharides and resistant starch, wich for example demonstrated the promotion of growth of Bifidobacteria and Lactobacilli. 101Since the gut microbiome in people with excessive weight shows a reduction in diversity of bacteria in general and specifically a reduction of the abundance of Bifidobacteria, using prebiotics to restore gut microbiome ecology and function has been considered an attractive therapeutic option/method. 102Inulin, resistant starches or fructo-oligosaccharide-enriched inulin may have beneficial effects on components of MetS. 102Supplementation of trans-galactooligosaccharides (GOS) improved insulin and lipid metabolism, increased the abundance of Bifidobacteria and decreased Gram-negative bacteria such as Bacteroides spp., Desulfovibrio spp.and the C. histolyticum group. 103Alpha-galacto-oligosaccharides led to a dose-dependent decrease of appetite and food intake. 104pplementation of GOS selectively increased the abundance of Bifidobacterium species, but did not lead to significant changes in peripheral insulin sensitivity, energy and substrate metabolism in prediabetic patients. 105Inulin-type fructan supplementation decreased fat mass slightly, but did not significantly change BMI, lipid or glucose homeostasis. 106Inulin also favourably influenced microbiome composition. 107,108While in adults no convincing effect on body weight could be observed, in obese children inulin and dietary fibre together with increased exercise showed a significant decrease in weight and body fat. 1094][115] Taken together, prebiotics seem to modify gut microbiome composition but the effects on metabolic syndrome markers and body weight are rare.It is still unknown, which factors, such as timing, dose, treatment duration, type of the intervention and combination with other, well-proven, interventions influence the effectiveness.The included clinical studies are summarized in Table 3.

| Faecal microbiota transplantation for metabolic diseases
FMT, the transfer of faecal matter from one individual to another with the aim to improve/restore the composition of the gut microbiome and thereby treat a disease, has gained much attention in the scientific field and in the general public.As a method that in principle dates back to China in the fourth century, its modern application took off in 2013.Until now the only routine application is the treatment of recurrent Clostridioides difficile infections.Additionally, many non-infectious diseases have been extensively studied. 118The first indication of efficacy for FMT in human metabolic diseases was published in 2012, where in a pilot study in the Netherlands insulin resistance was improved in nine male adults with metabolic syndrome who received FMT from a lean donor, whereas no changes were observed in nine controls who received an autologous FMT. 8 A potential effect on body weight was first suspected by a 'complication' of FMT for Clostridioides difficile, where an initially normalweight patient received an FMT from an obese donor and gained weight after FMT. 119While consecutive studies did not report such effects, 120 the opposite strategy-increasing body weight of cachectic cancer patients through FMT from obese donors-also failed. 121vertheless, BMI in the normal range has been included into the strict selection criteria for stool donors. 122ter these initial proof of concept trials, a number of clinical trials have been conducted, where researchers aimed to improve me-  4. It is notable, that the method of FMT may play a role in its effectiveness.
4][125][126] From a mechanistic point of view it is still not clear what the 'effective agent' in FMT is.
For the treatment of Clostridioides difficile infections, sterile filtrated faecal preparations were similarly effective as conventional faecal preparations containing living microorganisms.This indicates that bacterial components, metabolites, or bacteriophages may mediate the effects of FMT.The results of a pilot study suggests that FMT treatment, in absence of previous antibiotic treatment, significantly changes the bacteriophage community of the recipients.However, changing the bacteriophage community in this trial was not sufficient to present a clinical improvement in all the individuals. 127other study reported an influence of FMT on plasma metabolites related to lipid metabolism and DNA methylation status; however, a clear-cut pathophysiological explanation for a potential mechanism to influence glucose metabolism could not be identified. 128cent studies also explored how the selection of the donor could impact on the effect of FMT.A vegan diet is associated with reduced trimethylamine-N-oxide (TMAO) production and therefore lower cardiovascular risk; however, the FMT from vegan donors did not decrease TMAO production in patients with metabolic syndrome, which indicates that despite existing evidence of compositional changes that resemble the microbiome of the donor, the functional capacity is not easily transferred. 129FMT TA B L E 3 Clinical studies on prebiotics and the gut microbiome in patients with metabolic syndrome.

TA B L E 3 (Continued)
from patients after bariatric surgery to obese patients with MetS revealed microbiome-driven modulation of brain dopamine and serotonin transporters. 130Furthermore, the use of autologous faecal transplants with faecal material obtained at the 'weight nadir' of a successful diet was able to delay weight regain after the diet. 131,132These studies indicate that the concept of FMT most likely needs to be augmented by adequate preparatory measures like dietary changes or additional prebiotic 'fertilizers' of the transplanted microbiomes.Since studies also show considerable differences between different donors, it is essential to characterize donors and understand the interaction between the dysbiotic microbiome of the recipient and the microbiome of the donor. 133Special dietary measures, such as fibre supplementation may improve functional engraftment of FMT. 125,134A recent systematic review did not identify consistent changes in clinically relevant endpoints for metabolic diseases (such as insulin sensitivity) achieved in the recipient after FMT. 135When transferring living microorganisms to a new host, adverse events have to be considered.It is surprising that some studies do not report safety data (see Table 4).A meta-analysis on the safety of FMT across different disease entities showed no significant differences in the incidence of adverse events between FMT and the control group.
Adverse events can be related to the transplanted microbiome or to the route of administration.It seems that administration via oral capsules or endoscopically via the lower gastrointestinal tract is less prone to adverse events.Translocation and infection with transplanted bacteria can occur, the risk seems to be higher in patients with an altered intestinal barrier. 136In the studies related to metabolic diseases no events of bacteremia or sepsis have been described so far.
However, from a practical point of view, it is unlikely that FMT, which requires highly skilled personnel and is resource-intensive, will be applicable to treat the worldwide 'obesity pandemic'.
Further efforts are necessary to improve the timely and personalized diagnosis of the individual dysbiosis in obesity and to augment and retain the effect of a diet by influencing the microbiome in a personalized but also 'affordable' microbiome modulation strategy.

| Influence of drugs for the treatment of metabolic diseases on the gut microbiome
Common drugs used in the treatment of metabolic diseases include metformin, statins and GLP-1 agonists.These drugs also affect the gut microbiome composition and the gut microbiome composition in turn is thought to affect their therapeutic effects.Therefore, this review summarizes the current knowledge about the interactions of the drugs used for the treatment of metabolic diseases with the gut microbiome.
While metformin is a widely used therapy to treat type 2 diabetes mellitus, it is in a bidirectional relationship with the gut microbiome. 142In an attempt to disentangle diabetes and the metformin

Adverse events
5][146][147] The increase in Escherichia may explain some of the side-effects of metformin including an increase in virulence factors and gas metabolism genes. 143rthermore, the functional profile of the microbiome showed an increased potential for the production of butyrate and propionate, which has beneficial effects on glucose and energy. 143An increase in the production of short-chain fatty acids was also described by other authors/studies. 145,147Moreover, mucin-degrading and butyrate-producing Akkermansia muciniphila and Bifidobacterium adolescentis were described to be increased through metformin. 147,148other study by Sun et al, 2018 showed the decreased abundance of Bacteroides fragilis, which is a bile acid metabolizing bacterium, in the group of metformin treated diabetic patients, and a prominent increase in glycoursodeoxycholic acid levels. 149The link to bile acid metabolism was strengthened by the finding of increased Blautia species abundance associated with metformin treatment, with Blautia species being involved in bile acid metabolism, 150,151 and by the finding of significantly increased levels of total, primary, secondary and unconjugated plasma bile acids, which significantly correlated with lower HbA1c levels. 152Generally several studies reported an association between metformin mediated changes of gut microbiome composition and the function and effects of metformin on glycemic control. 149,153An association between adverse effects of metformin and microbiome composition was also reported. 146,154erefore, metformin seems to influence microbiome composition and function, which might explain both adverse and beneficial effects of metformin treatment.Furthermore, tolerance and response to metformin were also associated with gut microbiome composition.Thus, patients with a higher abundance of Megamonas rupellensis and Phascolarctobacterium spp., a higher activity of the amino acid biosynthesis pathways and a lower activity of sugar degradation pathways before the start of a metformin therapy, were more tolerant to a subsequent metformin therapy. 155Maintenance of optimal glycemic control with metformin therapy in type 2 diabetes mellitus patients was associated with reduced alpha diversity and a peculiar signature of microbiome composition and functional pathways. 156sponse to metformin was associated with a higher abundance of Enterococcus faecium, Lactococcus lactis, Odoribacter and Dialister before the start of metformin treatment in patients with type 2 diabetes mellitus. 1548 However, some studies showed conflicting results, for example, regarding the effect on Bacteroides abundance in patients treated with statins with some reports of increased Bacteroides 159 and some reports of decreased Bacteroides, 160 which might be attributed to, for example, different statin types predominantly used for treatment. 159Some studies show that there is no statin associated microbiome change, 161 while others describe a whole range of altered bacterial taxa, also indicating the importance of standardization of microbiome sequencing and data analysis. 162milar to metformin, statin treatment efficacy was linked to microbiome composition.Rosuvastatin was associated with differences in gut microbiome composition in patients with hyperlipidemia, with Lactobacillaceae and Bifidobacteriaceae being more abundant in patients with more pronounced effects of rosuvastatin. 163,164Another study connected more successful statin treatment to a higher abundance of Akkermansia muciniphila and Lactobacillus and a lower abundance of Holdemanella and Facecallibacterium. 152A better response to atorvastatin was associated with a higher relative abundance of Lactobacillus, Eubacterium, Faecalibacterium and Bifidobacterium, but a lower relative abundance of Clostridium. 165ucagon-like peptide-1 agonists are another group of drugs currently used in therapy of metabolic diseases.However, the data on its associations with the gut microbiome composition are so far relatively scarce.Interestingly, when comparing effects of metformin and GLP-1 agonists on the gut microbiome, GLP-1 agonists seem to have a stronger effect on increasing Akkermansia abundance than metformin. 166However, another study reported no effects of the GLP-1 agonist liraglutide on the composition of the gut microbiome, despite its metabolic effects.The reason for contrasting results could be that Wang et al. 2018 did not account for the differences in pre-treatment microbiome composition in patient groups in their analysis.8 This suggests that the composition of the gut microbiome influences the efficacy of the treatment with GLP-1 agonists.Contrasting and scarce findings on GLP-1 agonist interaction with the gut microbiome urge further investigation of this topic.More details of the studies about the influence of the above described drugs on the gut microbiome are presented in Table 5. the consumption of red wine improve some biomarkers of metabolic syndrome and change the composition of the gut microbiome.This gives hope to the hypothesis that the appropriate diet can influence the composition of the gut microbiome, and these changes can reduce the risk of the disease.However, the perfect diet has not been found yet-most likely, there will be no 'one size fits all' diet.The available literature on the use of probiotics in obese, MetS or diabetes patients is highly diverse in study designs, products, endpoints, microbiome analysis techniques and duration and dosage of interventions and therefore cannot support a sound conclusion of the efficacy of probiotics in obesity and related diseases.Prebiotics as a well-defined form of dietary intervention, showed beneficial clinical effects for GOS, inulin and resistant starch on biomarkers of metabolic syndrome and in some cases these findings were associated with changes in the composition and function of the gut microbiome.

| SUMMARY
FMT has so far not been able to show convincing clinical effects and the use of FMT as a strategy to treat obesity and metabolic diseases would also be limited by the fact that FMT is a resource-intensive treatment that also contains some procedural risks.Changes in the composition of the gut microbiome that are associated with the intake of medications used to treat metabolic syndrome such as metformin, statins and GLP-1 agonists and that can explain and modulate therapeutic and adverse effects of these drugs underpin the feasibility of managing the metabolic diseases with approaches targeting the microbiome.

| LI M ITATI O N S
The heterogeneity of metabolic diseases and the complexity of

| FUTURE DIREC TIONS
To fulfil the promise of microbially derived therapies aimed at restoring metabolic health in humans, major efforts in translational science are needed to dissect the interaction of environmental influences on host-microbe interplay.FMT clearly helped to understand the relationship between the gut microbiome and metabolic disorders and facilitated the notion that microbiome modulation can be an effective therapeutic strategy.However, unless developments such as encapsulated FMT are proven to be successful, the domain of FMT will be experimental rather than therapeutic.Furthermore, the fact that certain gut microbiome signatures in patients with metabolic syndrome are associated with the responce to these drugs highlights the urgent need for developing personalized gut microbiome targeting strategies not only to independently treat metabolic syndrome but also to ensure the efficacy of the currently used standard therapies via, for example, combination therapy.However, there are still a lot of unanswered questions regarding gut microbiome interactions with metabolic syndrome therapies, which need to be further

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors have no conflicts of interest to declare.
tabolism and reduce weight by FMT in obesity, metabolic syndrome, non-alcoholic fatty liver disease and type 2 diabetes.However, especially in adequately powered, randomized, placebo-controlled trials, mixed results with regards to improvement in metabolic parameters were reported.While most studies demonstrated changes in microbiome composition, clear effects on clinically important endpoints, such as body weight or insulin resistance are missing.The included trials with their clinical endpoints are summarized in Table the gut microbiome make it extremely difficult to deliver targeted microbiome modulation.A major shortcoming is for example the ubiquitous use of the umbrella term 'probiotics' which suggests a uniform intervention but includes vastly different bacterial species with potentially widely deviating functions.Although technically all discussed interventions are probiotics, there is no true replication and therefore validation of results.Similar patient cohorts have been treated with different products and vice versa, products used more than once have been trialled in different patient cohorts.This also limits the conclusion that can be drawn with regard to the involvement of microbiome modulation in the effects of probiotics.Some studies report clinical effects without microbiome modulation or microbiome modulation without resulting clinical effects.Furthermore, the techniques for microbiome analysis used in the discussed trials range from plating and counting over targeted qPCR to 16S sequencing, which massively influences the expectable resolution of results.Further research with evidence-based formulations in adequately powered high-quality randomized control trials is needed to improve the available data and support a sound conclusion on probiotics in obesity related diseases.Especially studies on prebiotics suffer from a small sample size as well as short and heterogeneous interventions, which limit the general applicability of these results in clinical practice.
investigated, including understanding the reasons for conflicting findings, the effects of combination therapies, and the influence of confounding variables on the results.The low predictability of the intervention warrants further research towards a better translation or transformation into clinical practice.Further efforts are necessary to improve a timely and personalized diagnosis of the individual dysbiosis in obesity and to augment and retain the effect of a diet by influencing the microbiome in a personalized but also 'affordable' microbiome modulation strategy.A clear commitment in research from all stakeholders (politics, funding bodies, health industry, researchers and the society) is necessary to move forward into the direction of developing live microbial agents, next-generation probiotics and targeted dietary interventions to let the current hype develop into a realistic hope for patients with metabolic syndrome.AUTH O R CO NTR I B UTI O N S Angela Horvath: Writing -original draft (equal); writing -review and editing (equal).Kristina Zukauskaite: Visualization (lead); Writingoriginal draft (equal); writing -review and editing (equal).Olha Hazia: Writing -original draft (equal); writing -review and editing (equal).Irina Balazs: Writing -original draft (equal); writing -review and editing (equal).Vanessa Stadlbauer: Conceptualization (lead), Writing -original draft (lead); writing -review and editing (lead).FU N D I N G I N FO R M ATI O N OH is supported by the 'Crisis support for Researchers from Ukraine', an Austrian Science Fund initiative, VS received funding from the Austrian Science Fund (KLI 741), and the project was partly conducted at the Center for Biomarker Research in Medicine (CBmed), a COMET K1 center funded by the Austrian Research Promotion Agency (Project 3.23).

of clinical trial Population/number of participants Duration Intervention Primary endpoint Result metabolic Result microbiome Adverse events
Clinical studies on diet and the gut microbiome in patients with MetS.
TA B L E 1

Study Population/number of participants Duration Intervention Primary endpoint Result metabolic Result microbiome Adverse events
TA B L E 2 Clinical studies testing probiotics as microbiome modulators in obesity, MetS and diabetes.

Study Participants Prebiotic intervention Effect of dietary intervention on metabolic outcomes Effect of dietary intervention on microbiome Adverse events
Clinical studies on faecal microbiome transplantation in patients with metabolic syndrome.
Human studies on the influence of metformin, statins and GLP-1 agonists on the gut microbiome.
Diet influences both metabolic diseases and the microbiome and changing dietary habits is a safe intervention.Clinical studies provide evidence that various dietary interventions, such as increases in whole-grain or fermented foods, the Mediterranean diet or even TA B L E 5