Bidirectional Interactions Between the Gut Microbiota and Incretin-Based Therapies

Obesity is regarded as a global epidemic, with prevalence figures exceeding 2 billion. The prevailing definition of obesity, as outlined by the World Health Organization, is a body mass index (BMI) of 30 kg/m² or above [1]. However, it is important to note that obesity is not solely defined by the BMI; rather, it is a complex condition influenced by individual factors, including genetic and epigenetic influences, and lifestyle factors such as unhealthy diet, overeating, and low physical activity [2]. This complex condition is associated with an elevated risk of several chronic conditions, including diabetes, hypercholesterolemia, cardiovascular disease (from hypertension to heart failure), and cancer [3]. Type 2 diabetes mellitus (T2DM) is a prevalent chronic metabolic disorder, characterised by hyperglycaemia resulting from a combination of insulin resistance and inadequate insulin secretion [4,5,6,7]. The diagnosis of T2DM is typically made on the basis of fasting plasma glucose values, 2 h plasma glucose values derived from a 75 g oral glucose tolerance test, or haemoglobin A1C (HbA1c) levels [5]. It is well established that carrying excess weight, or being obese, is a major risk factor for the development of T2DM [8,9]. The prevalence and incidence of both obesity and T2DM are steadily increasing and reflect each other. An unhealthy diet, characterised by a high consumption of carbohydrates and lipids, and a low intake of fibre, is a major contributing factor to the development of obesity and, potentially, T2DM. In individuals with T2DM and a BMI in the overweight or obese range, weight reduction has been shown to enhance glycaemic control and decrease reliance on glucose-lowering medications [9]. The management of T2DM necessitates a multifaceted, person-centred approach. The promotion of healthy lifestyle behaviours and diabetes self-management is to be emphasised alongside any pharmacotherapy [10].

Furthermore, obesity, insulin resistance and type 2 diabetes mellitus have been largely correlated to a reduction in bacterial load and diversity, resulting in a condition known as intestinal dysbiosis. The recent emergence of novel antidiabetic medications has been demonstrated to exert a favourable influence on the composition of the intestinal microbiota. In this review, the extant data on the bidirectional interactions between the gut microbiota and incretin-based therapies, including GLP-1R agonists, DPP-4 inhibitors and GLP-1/GIP receptor dual agonists, are summarised.

Incretin-based therapies represent a novel treatment for both T2DM and obesity, relying on the insulinotropic actions of the gut hormone glucagon-like peptide-1 (GLP-1) and, most recently, on the combined action of GLP-1 and the gastric inhibitory polypeptide (GIP) hormones. Incretins are gastrointestinal hormones that promote postprandial insulin secretion in a glucose-dependent manner. There are two types of incretins: GLP-1 and GIP. Both are secreted by intestinal cells in response to meals, with GLP-1 being secreted from L cells that are found in the large and lower bowel and GIP from K cells in the upper bowel [11]. There is a specific threshold, between 1 and 2 kcal/min, for glucose-induced GLP-1 secretion in the small intestine, while the secretion of GIP is more sensitive [12]. GLP-1 subsequently binds to GLP-1 receptors (GLP-1R), which are widely expressed in multiple organs and tissues, including the gastrointestinal tract, the endocrine pancreas, the heart, and the central nervous system, thereby exerting pleiotropic functions (Figure 1).

The biological action of GIP is promoted through binding to the GIP receptor (GIPR), a class B G-protein-coupled receptor that is similar to GLP-1R and belongs to the glucagon receptor family. The GIPR is expressed in the endocrine pancreas, adipocytes, myeloid cells, the endothelium of the heart and blood vessels, the inner layers of the adrenal cortex and the central nervous system. The role of GIP in regulating energy balance is achieved through cell surface receptor signalling in the brain and adipose tissue [13]. It is evident that both GLP-1 and GIP play a substantial role in the function of beta cells, including the promotion of increased beta-cell mass, enhanced insulin secretion, and the preservation of beta-cell survival [14]. Moreover, the GIP-dependent reduction in insulin clearance contributes to the enhancement of peripheral insulin levels, thereby ensuring the maintenance of normal blood glucose levels (Figure 2) [15].

However, GLP-1 and GIP are rapidly degraded in plasma by dipeptidyl peptidase IV (DPP-4), resulting in a very short half-life of approximately 1.5 min [16,17,18].

DPP-4 is a type II integral transmembrane glycoprotein with enzymatic activity; it also exists in a soluble form in the plasma, lacking the cytoplasmic and transmembrane domains [19,20]. The primary substrates for DPP-4's catalytic activity are incretins responsible for glucose metabolism, including GIP, GLP-1 and GLP-2 [21,22,23]. Consequently, DPP-4 inhibitors may have a pivotal role in prolonging the circulating half-life of GLP-1 and GIP, thereby preventing their cleavage [24]. Indeed, DPP-4 inhibitors represent a therapeutic approach for the management of patients with T2DM (Figure 3).

These inhibitors reduce blood glucose levels, enhance glycated haemoglobin levels, and do not induce hypoglycaemia. There is evidence that the rate of gastric emptying (GE) may influence the magnitude of effect on glucose-lowering to DPP-4 inhibition. Indeed, the ability of DPP-4 inhibitors to reduce postprandial glycaemia increases when GE is faster, partly because this leads to enhanced GLP-1 secretion, but also because higher postprandial glucose levels are necessary for GLP-1-mediated insulin secretion and glucagon suppression [25,26].

Moreover, nutritional strategies may improve the lowering of blood glucose levels induced by DPP-4 inhibitors. A promising approach to optimise DPP-4 inhibitor action involves the administration of a small quantity of a specific macronutrient at a fixed interval before a meal. The presence of nutrients in the small intestine induces the release of gut peptides, including endogenous GLP-1 and GIP; these peptides improve the glycaemic response to the subsequent meal and slow gastric emptying [27,28].

DPP-4 inhibitors have been shown to improve β-cell function and inhibit α-cell secretion, with the potential to enhance insulin sensitivity, as evidenced by increased β-cell mass in animal studies.

Furthermore, many studies demonstrated that DPP-4 gene-deficient mice exhibited enhanced postprandial glucose control and resistance to developing hyperinsulinemia and obesity. Exogenous DPP-4 inhibitors improve glucose tolerance in wild-type mice, but not in DPP-4 knockout mice [29]. Beyond its catalytic functions, DPP-4 may also regulate the immune system through the cleavage of various chemokines and cytokines, such as erythropoietin, stromal cell-derived factor-1, granulocyte colony-stimulating factor, Interleukin-3, and granulocyte-macrophage colony-stimulating factor [30]. Additionally, DPP-4 exert non-catalytic functions, including the reduction in dendritic cell/macrophage-mediated adipose tissue inflammation in obesity by affecting macrophage migration (CD11b+, CD11c+, and Ly6Chi) [31,32]. Moreover, DPP-4 enhances regulatory T cell (Treg) expansion and transforming growth factor beta levels in non-obese diabetic mice whilst concomitantly promoting T-cell activation via its interaction with adenosine deaminase [33,34].

GLP-1R agonists used in clinical settings (Figure 3) are structurally modified to exhibit relative resistance to DPP-4 cleavage, resulting in a long-circulating half-life [22,35].

The rapid cleavage of the N-terminal His–Ala residues of GLP-1 by DPP-4 expressed on surrounding tissues results in the inactivation of GLP-1 and a consequent short half-life [19]. GLP-1 analogues exert various pharmacological actions by increasing GLP-1 levels 10-fold or more. GLP-1R agonists have been shown to restore the sensitivity of β-cells to insulin, induce insulin secretion [19], and modulate body weight loss by potentially inhibiting GE [36]. In the central nervous system, GLP-1R agonists regulate glycaemic control and possess a protective effect on neuronal damage [37]. Furthermore, GLP-1 may also act on the heart and gut, directly or indirectly through the sympathetic nervous system, to regulate heart rate and gut peristalsis via the nitric oxide-mediated suppression of intestinal motility [38,39,40]. Common adverse effects include nausea, diarrhoea and constipation [41]. The relationship between intestinal norepinephrine and GLP-1 remains to be fully elucidated [11]. The acute stimulation of the β-adrenergic receptor by isoproterenol induces a significant increase in the secretion of GLP-1 and PYY. This suggests that the activation of β2-adrenergic receptors on intestinal L cells could promote the release of these hormones [42]. At the same time, it has been observed that a prolonged increase in catecholamine production may inhibit GLP-1 secretion [43].

Recently, a new pharmaceutical agent was developed, which has the capacity to simultaneously co-activate both the GIPR and the GLP-1R (Figure 3). Tirzepatide, a drug that has been hailed as a pioneering innovation in its field, was shown to improve glycaemic control by increasing insulin sensitivity and lipid metabolism and reducing body weight. The co-activation of these two receptors enhances β cell function, offering a more effective treatment for diabetes and obesity with a reduced adverse effect profile compared to selective GLP-1R agonists. Until now, clinical trials have demonstrated the remarkable effectiveness of this compound, once-weekly injected, in achieving optimal glycaemic control and substantial weight reduction [44,45]. The magnitude of the effects of tirzepatide on glycaemia and weight loss marks the commencement of a new era in diabetes therapy, with the potential to treat a significant proportion of patients according to currently established targets.

Even the SURMOUNT-3 study evaluated the efficacy and safety of tirzepatide in the treatment of obesity. This study demonstrated that the combination of tirzepatide with intensive behavioural therapy resulted in significant weight loss, with a mean reduction of 25.3% after 84 weeks of treatment, a result that is superior to the 20.9% weight loss reported in the SURMOUNT-1 study at week 72, which used tirzepatide alone without intensive behavioural therapy. The sequential therapy (intensive behavioural therapy followed by tirzepatide) may have an additive effect on weight loss, but further studies are needed to confirm this result. But the main study's results highlight the importance of combining new obesity medications with lifestyle modification interventions to achieve optimal results [46].

In the course of its development, the human gastrointestinal tract accumulates a dense, varied population of microorganisms called 'microbiota'. The bacteria populating human intestines are in the process of evolving and coevolving with their host in a system of beneficial symbiosis [47].

Until a decade ago, information available on the human microbiota was based upon traditional culture techniques. More recently, the use of genomic sequencing methods has enhanced the capabilities of studying the gut microbiota [47,48]. Numerous fragments from MetaHit and Human Microbiome Project databases have contributed towards building a better and more exhaustive understanding of the microorganisms present in the human gut [49,50,51]. In a typical adult, gut microbiota composition includes fungi, viruses, and parasites in addition to bacteria. The total number of species of microbes in the gut microbiota can reach up to 5000, with a combined weight of approximately 2000 g. This is equivalent to 10 times the amount of microbial cells found in the entire human body [52]. Studies that have been carried out have contributed to the identification and isolation of 2172 bacterial species, which have been subsequently categorised into 12 different phyla. The main species are, in order of abundance, Firmicutes (Ruminococcus, Clostridium and Eubacterium), Bacteroidetes, Actinobacteria and Prevotella (Figure 4) [52,53,54].

Features of the small intestine such as high levels of acids, oxygen, and antimicrobials and a short transit time inhibit the growth of bacteria [47,51]. Thus, in this environment, only facultative anaerobes can survive, which rapidly grow and are membranous epithelium-adhering and permeable to oxygen [47,51]. On the contrary, the shallow characteristics of the large intestine support the existence of a highly diverse microbial community, predominantly anaerobes trained on complex carbohydrates and polysaccharides not utilised in the small intestine [47,51].

The microbiota present within the gastrointestinal tract are understood to exert a pivotal influence on host physiology, engaging in interactions with both the endocrine and immune systems within the gastrointestinal tissues.

The bacteria within the gut microbiome are involved in many biological processes, including the harvesting of energy from food (degrading fibres and complex polysaccharides) and the manufacturing of neurotransmitters (e.g., serotonin, vitamins and enzymes). The production of microbial metabolites can be categorised into three distinct pathways: the first involves the fermentation of food elements by intestinal microorganisms, such as 2-oleoyl glycerol (derived from dietary fats); the second pathway entails the direct production of microbial metabolites by the intestinal microbiota, including lipopolysaccharides (LPSs); and the third pathway involves the synthesis of these microbial metabolites by the host, followed by subsequent modification by bacteria. In addition, the gut microbiota protects the body from pathogens with a competitive mechanism and through the production of antimicrobial substances. In summary, the intestinal microbiota modulate the immune system and help maintain the integrity of the intestinal barrier by strengthening tight junctions between enterocytes and stimulating mucus production [52].

Nutritional intake exerts a profound influence on the equilibrium between beneficial and opportunistic microbial composition, with dietary modifications capable of affecting this ratio [52]. In addition to nutrient availability, various factors influence fluctuations and stabilities of bacterial levels, such as osmolality, pH, temperature, hormones, neurotransmitters derived from the host, cytokines, and viruses that infect bacteria (bacteriophages) [55,56,57,58]. Advances in sequencing technologies and population-scale studies have revealed that the microbiome assists in the expansion of host genomes by facilitating the host's physiology and metabolism and associating with several diseases [59,60]. An imbalance of the microbiota called dysbiosis has been associated with numerous diseases, including obesity, diabetes, cardiovascular diseases, chronic inflammatory bowel diseases, autoimmune diseases and neurological disorders [61].

Incretin-based therapies represent a novel treatment for both T2DM and obesity, relying on the insulinotropic actions of the gut hormone GLP-1 and, most recently, on the combined action of GLP-1 and the GIP hormones. The gut microbiota is a complex community consisting of more than 500 microbial species and it is involved in many biological processes, including energy metabolism, inflammatory response, immunity and gut–brain neural circuits. Despite the numerous preclinical and clinical studies conducted on the interaction between incretin-based drugs and gut microbiota, there are still several aspects of this interaction that are not yet fully understood.

A number of studies have shown that gut microbiota metabolites, including SCFAs and/or indole, can directly stimulate the release of incretins from colonic enteroendocrine cells (EECs) (Figure 1). This influences host satiety and food intake.

SCFAs play a pivotal role in the communication between host cells and microbes, directly stimulating the release of GLP-1 from enteroendocrine cells. The induction of cell-specific signalling cascades by SCFAs is achieved through the activation of specific receptors. Two distinct receptor types, designated as free fatty acid receptor (FFAR) 3 and FFAR2 (also termed G-protein-coupled receptor (GPR) 41 and GPR43, respectively), have been identified on EECs [98,99]. The activation of these receptors enhances GLP-1 secretion through increased intracellular calcium [100].

The intestinal microbiota plays an important role in many biological processes, including gut barrier function, digestion, the modulation of the immune system and/or glucose metabolism. Obesity, insulin resistance, T2DM and metabolic syndrome have been largely correlated to a reduction in bacterial load and diversity, configuring a picture termed intestinal dysbiosis [116,117]. The ratio between the bacterial species Firmicutes and Bacteroidetes is widely regarded as playing a significant role in disease and health [60,61,118].

A multitude of metabolites produced by the gut microbiota modulate the activity of enteroendocrine cells and their hormonal secretion. Such metabolites include 5-hydroxytryptamine [119], indole [111], LPS [120], secondary BAs [99] and SCFAs [94]. These increase GLP-1 secretion, either directly or indirectly, through a variety of mechanisms [88,96,121,122,123,124]. Of particular interest is the role of SCFAs, which activate various receptors, including GPR41 (FFA3) and GPR43 (FFA2), expressed by enteroendocrine L cells. The activation of these receptors have been shown to enhance GLP-1 secretion through increased intracellular calcium [94,96,123]. Furthermore, the gut microbiota exerts a regulatory effect on GLP-1 secretion through Takeda G protein-coupled receptor 5 on intestinal L cells via secondary BAs metabolism [96,97]. Conversely, some metabolic diseases, including T2DM, are partly related to a decreased production of SCFAs [118,125], which are predominantly formed in the colon.

Moreover, a plethora of evidence suggests that the intestinal microbiota can not only influence incretin secretion, but in turn can also be influenced by incretin-based therapies and modulate the individual response to such drugs [96]. This interaction is complex and bidirectional. Numerous studies have indicated that the administration of DPP-4 inhibitors and the GLP-1R agonist could potentially modify the intestinal microbiota (Table 1).

The mechanisms by which DDP-IV inhibitors and GLP-1R agonists modify the composition of the gut microbiota are still not clear. As suggested by Zeng Y. et al. [126] and as reported in our manuscript (see Section 4.1 and Section 4.2, Preclinical Studies and Clinical Studies, respectively), DDP-IV inhibitors impact the composition and function of the gut microbiota and they correct the dysbiosis of the microbiota. It is hypothesised that DPP-4 inhibitors can normalise the faecal microbiota composition and promote a functional shift in the gut microbiome. Numerous studies have demonstrated that the administration of DPP-4 inhibitors exerts a substantial influence on the composition of the gut microbiota, as evidenced by an augmentation in the abundance of Bacteroidetes, a concomitant escalation in the production of succinate, and an amelioration of microbiota dysbiosis in obese and T2DM mice [66,67]. These changes may contribute to the observed beneficial effects. As previously reported, DPP-4 inhibitors improve glucose metabolism and increase plasma GLP-1 concentrations. However, the magnitude of GLP-1 secretion is dependent on the rate and load of nutrient entry to the small intestine, so the individual basal rate of gastric emptying may well explain the different response to DPP-4 inhibitors [27].

It is plausible that the shifts in gut microbiota could contribute to the modulation of glucose homeostasis by increasing GLP-1 levels.

Furthermore, liraglutide treatment increases the Firmicutes to Bacteroides ratio in diabetic mice with normal weight, thereby indicating that liraglutide has the capacity to modulate the gut microbiota composition in a positive manner. Collectively, the findings from animal studies imply that GLP-1R agonists and DPP-4 inhibitors may have a significant impact on the composition of the gut microbiota. Treatment with GLP-1R agonists increases the richness and diversity of the gut microbiota and changes the overall structure of the gut microbiota, especially some bacteria related to intestinal inflammation and glucolipid inflammation [76,77,91,127]. In addition, preclinical models suggest that changes in the gut microbiota induced by DPP-4 inhibitors and GLP-1R agonists could be linked to metabolic improvements, such as the attenuation of endotoxaemia and the increased production of short-chain fatty acids.

However, the paucity of human studies has yielded equivocal results to date. Additional studies and research are necessary due to the heterogenicity in human studies. Some human studies have suggested that incretin-based therapies do not result in any change to the structure of the gut microbiota using incretin-based therapies [94,95]. One potential explanation for this observation is that these medications are often administered in conjunction with other therapies, which may obscure the impact on the gut microbiota. Indeed, both DPP-4 inhibitors and GLP-1R agonists are invariably used in conjunction with metformin, and it has been observed that metformin dosage affects gut microbiota diversity [128]. It can be hypothesised that patients undergoing metformin treatment already possess a more favourable microbiota profile, thereby resulting in a diminished impact of DPP-4 inhibitors and GLP-1-R agonist therapies. Furthermore, these studies demonstrated that diet is the most significant factor in modulating the composition of the intestinal microbiota. The composition of the intestinal microbiota is influenced by the type of food consumed, which in turn affects the absorption of nutrients.

Especially, the Mediterranean diet increases Bifidobacterium, Faecalibacterium prausnitzii, Roseburia, and Lachnospiraceae (polyphenols and unsaturated fats appear to be the main drivers of the observed benefits); vegetarian diets increase microbial diversity and promote SCFA-producing bacteria like Akkermansia and Eubacterium; Ketogenic diets, in mouse models, improve insulin sensitivity and reduce inflammation, but their effects in humans are less clear; and Western diets reduce microbial diversity, increase gut permeability, and promote proinflammatory metabolites like TMAO (Trimethylamin N-Oxid) and LPS [129].

The microbiome plays a pivotal role in nutrient extraction, energy production, and low-grade inflammation, all of which have the potential to contribute to the development of obesity and T2DM [130,131].

As established, GLP-1R agonists induce a decrease in caloric intake. Consequently, there is a possibility that alterations to the gut microbiome may be observed if the diet is not standardised. Currently, few studies have directly compared different dietary profiles during incretin therapies. Most research focuses on the effect of incretin on the gut microbiota, without considering the interaction with specific dietary regimens. This lack of data makes it difficult to interpret discrepancies between clinical and preclinical trials, where dietary conditions can vary significantly.

In the context of animal experiments, the provision of a uniform diet to all subjects during the intervention period serves to mitigate the potential for confounding variables. This may provide a rationale for the observed discrepancies between animal studies and human trials. To fully understand the interaction between diet, gut microbiota and incretin-based therapies, studies are needed to compare directly different dietary profiles (e.g., the Mediterranean diet, ketogenic diet, vegetarian diet, Western diet, etc.) during treatment with incretins, evaluate changes in microbiota composition and functionality in response to specific combinations of diet and therapy, and analyse the metabolic and clinical effects resulting from these interactions.

Conversely, the increasingly frequent use of non-caloric artificial sweeteners (NASs) in diabetic and obese populations has led to their independent role in modifying the gut microbiota. Indeed, most of the NASs cross the gastrointestinal tract without being absorbed and therefore come into direct contact with the microbiota.

Suez et al. demonstrated that the consumption of NASs (firstly saccharin), both in mice and in humans, increases the risk of glucose intolerance, and changes in the composition of the microbiota in bacterial species are associated with the onset of type 2 diabetes in humans, first of all manifesting as the overexpression of Bacteroides [132].

However, recent studies have demonstrated a close association between GLP-1R agonist treatment and intestinal flora in T2DM subjects [98].

The changes in intestinal flora appear to be related to the modifications in metabolic parameters (e.g., fasting glucose levels, glycated haemoglobin levels, and BMI) induced by GLP-1R agonist therapies. Furthermore, Liang L. et al. reported that alterations in intestinal flora were time-related. Indeed, no significant changes in intestinal flora were observed after one week of GLP-1R agonist therapy in T2DM subjects; however, during long-term use, there was a significant change in the structure and overall composition of the flora and a significant decrease in the abundance of intestinal microorganisms compared to those before drug administration [97]. It is important to note that there is another relevant point of interaction between GLP-1 receptor agonists or tirzepatide and microbiota. This interaction is particularly significant in terms of their profound effects on both gastric emptying [133,134] and small intestinal transit [135]. Indeed, the inverse relationships in duodenal microbiota between the relative abundances of Streptococcus and Prevotella and the relative abundance of Veillonella spp with gastric emptying time is demonstrated. Consequently, this variation in gastric emptying and the exposure of nutrients to the small intestine warrants further investigations to delineate the underlying mechanisms and clarify its relevance to the risk of alterations in the gut microbiota [136].

Finally, several results show that alterations in the gut microbiota associated with metabolic diseases are different in men and women, and these characteristics may influence sex differences in the development and prevalence of metabolic diseases. Sex steroids, mainly oestrogen and testosterone, play a prominent role in the sexual dimorphism of gut microbiota [137]. Furthermore, there is recent evidence that the secretion of the GLP-1 may also be influenced by sex difference, probably through oestrogen signalling [138]. Therefore, in the complex interaction between the gut microbiota and incretin drugs, the interaction with sex hormones plays an important role, so the therapies may have sex-specific effects.

Despite numerous preclinical and clinical studies, the interaction between incretin drugs and the gut microbiota still shows several dark sides. Human data remain largely correlative, and correlations between improvements in metabolic parameters (e.g., reduced blood glucose levels, enhanced glycated haemoglobin levels, weight loss, improved hepatic steatosis) and specific microbial taxa have not established a causal link. Therefore, claims that DPP-4 inhibitors and GLP-1R agonists can significantly modulate the microbiota must be interpreted with caution, recognising that the observed changes may be secondary effects of metabolic improvement rather than primary therapeutic mechanisms.

To better understand the relationship between microbiota and drugs, multiomics techniques, such as metagenomics, metatranscriptomics, and metabolomics, should be applicated. These techniques enable the analysis of the composition and activity of the microbiota in a detailed manner, identifying specific patterns of gene expression and metabolic activity associated with drug response [139]. However, studies that have highlighted, through multiomics and strain-specific metabolite techniques, bidirectional interactions between the gut microbiota and incretin treatments are currently limited.

In conclusion, as demonstrated by several studies, incretin-based therapies have the capacity to influence the composition of the gut microbiota, thereby resulting in alterations to the bacterial flora. In addition, a lot of evidence suggests that the intestinal microbiota can not only influence incretin secretion, but in turn can also be influenced by incretin-based therapies and modulate the individual response to such drugs. The potential mechanisms by which the microbiota modulates drug metabolism and efficacy may involve the microbial-mediated metabolism of xenobiotics (oral drugs) and microbial metabolites (short-chain fatty acids and BAs). However, despite numerous preclinical and clinical studies, the interaction between incretin drugs and the gut microbiota still shows several dark sides. Taxonomic data are important, but gut microbiota–incretin crosstalk should be studied not only through metagenomic description (what the bacteria are) but also in the metabolomic direction (how they interact). An important limitation is the heterogeneity of clinical trials due to interindividual complexity (age, gender, and ethnicity), which includes dietary variability and polypharmacy.

Further research is necessary to determine whether targeting the gut microbiota could enhance the endogenous production of incretins.