Gut microbiota: an emerging target connecting polycystic ovarian syndrome and insulin resistance

In patients with PCOS, the efficiency of glucose uptake and utilization decreases. The body compensates by secreting excessive insulin, leading to hyperinsulinemia (HINS) to maintain blood glucose stability. Hyperandrogenism (HA) mainly originates from the theca cells of the ovaries. Elevated levels of luteinizing hormone (LH) in PCOS patients can stimulate the ovaries to produce more testosterone, inhibit ovulation, and cause ovarian cyst enlargement. In fact, HINS and HA are the most important pathophysiological mechanisms in the development of PCOS (Rosenfield and Ehrmann, 2016; Joham et al., 2022), and they influence and depend on each other. On one hand, IR can exacerbate HA: 1) Elevated insulin levels and insulin-like growth factor (IGF) within the ovaries synergize to stimulate theca cells to produce a large amount of androgens. 2) HINS directly stimulates the pituitary to secrete LH, leading to overexpression of luteinizing hormone-releasing hormone (LHRH) and LH. LH is the primary driver of excess androgens in PCOS patients. 3) IR induces dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, increasing the response to ACTH stimulation and leading to increased adrenal androgen production. 4) The biologically active testosterone in PCOS patients is free testosterone rather than sex hormone-binding globulin (SHBG)-bound testosterone. Excessive insulin can inhibit liver synthesis of SHBG, upregulating free testosterone levels and amplifying its biological effects, making HA manifestations such as hirsutism, acne, and alopecia more pronounced. On the other hand, HA can also promote the progression of IR: 1) HA increases the conversion rate of free fatty acids in peripheral muscle tissues, inducing IR. 2) HA reduces insulin-mediated glucose utilization in muscle tissues. Dysfunctional adipocytes secrete more adipokines (such as leptin and resistin), decreasing insulin sensitivity and worsening IR. Additionally, evidence shows that anti-IR treatment in IR-PCOS patients can improve HA once insulin levels return to normal. Since insulin promotes the secretion of androgens from theca cells, controlling IR can effectively improve PCOS (Figure 1).

The development of IR is accompanied by impaired energy utilization, and the direct metabolism of carbohydrates by microbes is one of the key sources of energy for the host. Once the physiological balance between commensal bacteria and the intestinal mucus, epithelial cells, and immune cells is disrupted, the opportunistic growth of Gram-negative bacteria and other pathogens will promote gut microbiota dysbiosis, thereby facilitating the development of IR. Akkermansia muciniphila is one of the microbes mediating gut barrier function and IR. It is a resident bacterium in the human gut, located in the mucus layer of the intestinal lumen and mucosal epithelium, and is an oval-shaped Gram-negative anaerobe. Under physiological conditions, Akkermansia muciniphila feeds on mucin secreted by the intestinal mucosa. Although its abundance only accounts for 0.5-5% of the gut microbiota, its consumption of mucin and the regeneration of mucin by goblet cells of the intestinal epithelium can reach a dynamic balance. Therefore, a decrease in its abundance can be seen as an indicator of dysbiosis (Chelakkot et al., 2018; Hasani et al., 2021). Studies have shown that the outer membrane protein Amuc_1100 of Akkermansia muciniphila enhances tight junctions in the intestinal epithelium through the TLR2 signaling pathway, maintaining the integrity of the gut barrier and intestinal homeostasis, and alleviating metabolic endotoxemia, thereby protecting the gut from pathogen invasion (Plovier et al., 2017). The outer membrane protein Amuc_1100 of Akkermansia can also promote the release of anti-inflammatory cytokines such as IL-10 by activating Toll-like receptors, regulating the host’s immune response and improving inflammation in visceral adipose tissue, thereby enhancing insulin sensitivity (Ottman et al., 2017). Additionally, the effector protein P9 of Akkermansia can bind to the ICAM-2 receptor on enteroendocrine cells, specifically the L cells of the intestinal epithelium, promoting the secretion of GLP-1, thus improving insulin sensitivity, lowering blood sugar, and ameliorating obesity (Yoon et al., 2021). Although current changes in the gut microbiota in PCOS, like its clinical manifestations, exhibit high heterogeneity, several PCOS-based microbiota studies have revealed that a reduction in Akkermansia muciniphila is associated with PCOS (Liu et al., 2017; Sherman et al., 2018; Huang et al., 2022; Zhang et al., 2023). The study by Liu R (Liu et al., 2017) and colleagues indicates that the decreased abundance of Akkermansia muciniphila is negatively correlated with the degree of obesity, sex hormones, particularly testosterone, and gut-brain peptides in PCOS patients. Metformin, one of the most commonly used insulin sensitizers in PCOS patients, has been shown by Huang J (Huang et al., 2022) et al. to not only mitigate microbiota dysbiosis but also increase the abundance of Akkermansia muciniphila in the gut, reduce serum IFN-γ levels, and inhibit pyroptosis of ovarian macrophages, thereby improving PCOS. These pieces of evidence suggest that gut microbiota dysbiosis may downregulate the abundance of Akkermansia muciniphila, affecting gut barrier integrity, inducing immune and metabolic imbalances, and promoting IR in PCOS patients. Supplementing the abundance of Akkermansia muciniphila may be a promising probiotic approach to enhancing insulin sensitivity. However, in-depth research on the effects of oral Akkermansia muciniphila on ovulatory dysfunction and metabolic abnormalities in PCOS is still lacking.

Probiotics are active microorganisms present in the human gut that confer beneficial effects by maintaining microbial balance or modulating host immunity, including Bifidobacterium and Lactobacillus species. Prebiotics are organic substances that selectively promote the metabolism and proliferation of probiotics in the gut, thereby improving the dynamics and metabolic functions of the gut microbiota. Increasing evidence suggests that probiotics and prebiotics have significant advantages in treating IR-PCOS (Li et al., 2022; Luo et al., 2023). At the human level, a decrease in Bifidobacterium abundance is a characteristic of dysbiosis in PCOS patients. Supplementing with Bifidobacterium lactis V9 can promote the growth of SCFA-producing microorganisms such as Faecalibacterium prausnitzii, Butyricimonas, and Akkermansia, which can improve gut health by enhancing barrier function and reducing the translocation of bacterial endotoxins across the gut wall. These endotoxins can cause inflammation and IR in the gut wall (Zhang et al., 2019). Supplementing with a multi-strain probiotic formulation (including Lactobacillus casei, Lactobacillus acidophilus, Lactobacillus paracasei, and Bifidobacterium bifidum) not only lowers fasting blood glucose, HOMA-IR, and dyslipidemia but also improves systemic inflammation levels and hyperandrogenism in patients (Cozzolino et al., 2020). At the animal level, similar effects have been observed with probiotics or prebiotics. Intake of probiotics and prebiotics increased the relative abundance of Lactobacillus, Bifidobacterium, and Akkermansia, improving reproductive dysfunction and metabolic abnormalities in PCOS phenotype mice (Li et al., 2022). Escherichia coli Nissle 1917 (EcN), a genetically controlled probiotic, increased the abundance of Adlercreutzia, Allobaculum, Escherichia-Shigella, and Ileibacterium in PCOS mice, improving metabolic disorders by enhancing amino sugar and nucleotide sugar metabolism pathways (Luo et al., 2023). Despite the promising benefits of probiotics and prebiotics observed in various clinical trials and animal studies for IR-PCOS, the diverse range of probiotic strains, dosages, and treatment durations lack standardization. Therefore, further research is needed to refine the use of probiotics and prebiotics for optimal therapeutic protocols (Figure 3).

Fecal microbiota transplantation (FMT) aims to reshape the gut microbiota by transplanting fecal microbiota from healthy donors into the patient’s gut, thereby enhancing the host’s gut mucosal immunity. FMT has shown promising results in treating diseases such as inflammatory bowel disease and recurrent Clostridium difficile colitis. However, current research primarily focuses on fecal transplantation in individuals with PCOS. Both fecal samples from PCOS patients and PCOS phenotype animals have shown strong PCO and IR-inducing abilities. Qi X et al. found that mice gavaged with fecal matter from PCOS patients exhibited IR, disrupted estrous cycles, increased cystic follicle numbers, and decreased corpora lutea, a phenomenon associated with the high abundance of Bacteroides in the PCOS gut (Qi et al., 2019). At the animal level, Han Q et al. discovered that transplanting the gut microbiota from DHEA-induced PCOS phenotype rats into germ-free recipients promoted IR and reproductive hormone imbalance (Han et al., 2021). Guo Y et al. performed FMT in PCOS phenotype rats and found that the PCOS-like rats had lower levels of Lactobacillus, Ruminococcus, and Clostridium, while Prevotella was more abundant. After administration of probiotics containing Lactobacillus and FMT, the gut microecological structure was restored, and metabolic abnormalities and hormone imbalances were ameliorated (Han et al., 2021). Additionally, in diabetes characterized by IR, FMT significantly reversed IR and enhanced the insulin-sensitizing effects of metformin (Wu et al., 2023). Therefore, modulating gut microbiota holds promise for treating IR-PCOS. In summary, while it is widely recognized that PCOS patients exhibit gut microbiota imbalance, FMT research in PCOS is still in its early stages globally. The application of FMT in IR-PCOS faces several challenges, including the anaerobic handling and preservation of donor feces, donor-recipient incompatibility, and the uniformity and effectiveness of donor feces recolonization in recipients. These challenges necessitate careful and comprehensive evaluation from both clinical and mechanistic perspectives.

Polyphenols, found in natural plants, are compounds with antioxidant and anti-inflammatory properties, including flavonoids and non-flavonoids. Polyphenols can reduce inflammation and stabilize glucose and lipid metabolism in IR-PCOS by promoting beneficial gut bacteria and reducing potential pathogens (Zhou et al., 2024). Gut microbiota can metabolize polyphenols through hydrolysis and reduction reactions, and the beneficial effects of polyphenols on PCOS may arise from aromatic metabolites produced by gut microbiota metabolism. In recent years, polyphenols involved in regulating IR-PCOS mainly include resveratrol, catechins, and anthocyanins, with resveratrol being the primary polyphenol affecting gut microbiota. Resveratrol is generally considered a scavenger of reactive oxygen species and free radicals, offering some protection to the ovaries, and its value in improving IR in PCOS has been extensively studied (Pasquariello et al., 2020; Mansour et al., 2021; Chen et al., 2022). Chen M et al. found that resveratrol improves ovarian ovulation disorders in PCOS patients by reducing granulosa cell apoptosis and oxidative stress levels (Chen et al., 2022). In addition to reducing oxidative stress, resveratrol can reverse pyruvate and lactate levels, upregulate key rate-limiting enzymes related to glycolysis pathways (such as LDHA, HK2, and PKM2), activate the SIRT2 signaling pathway, and improve IR-PCOS (Chen et al., 2022; Liang et al., 2023). To further explore the relationship between resveratrol and gut microbiota, Chinese researchers evaluated the effect of resveratrol on follicle development through FMT. Evidence shows that resveratrol regulates the SIRT1-FoxO1/P53 pathway to reduce follicle atresia, a process involving changes in gut microbiota. After transplanting the gut microbiota from resveratrol-treated donors, the Firmicutes/Bacteroidetes ratio significantly increased, along with the relative abundance of Lactobacillus murinus and Lactobacillus salivarius (Wang et al., 2022). Another study found that resveratrol significantly improved IR in high-fat diet mice by reducing endotoxemia, inflammation, and restoring gut barrier defects, while increasing the abundance of Verrucomicrobia and Akkermansia (Chen et al., 2020). These findings suggest that resveratrol may treat IR-PCOS by improving gut microbiota. However, due to the vast variety of polyphenols, there are still many limitations and challenges in verifying their mechanisms and efficacy. In conclusion, it is certain that FMT may be effective for PCOS, but it only remains at the level of clinical practice and its future use in humans remains to be considered.

Metformin is the most commonly used insulin sensitizer in clinical practice for PCOS patients. Apart from inhibiting endogenous glucose production and improving IR, it also plays a role in alleviating hyperandrogenism in PCOS patients, facilitating ovulation restoration, and reducing long-term pregnancy and metabolic risks (Furat Rencber et al., 2018). Recent findings indicate that increasing gut microbiota diversity and abundance is one of the mechanisms of metformin’s action. Following metformin treatment, PCOS patients show significantly increased abundance of beneficial bacteria such as Bacteroidetes, Proteobacteria, Hungatella, Phocaeicola, Anaerobutyricum, predominantly Clostridium, Fusobacterium, and Oxalobacter (Gan et al., 2023). Animal experiments demonstrate that metformin reduces LH, LH/FSH, and TNF-α levels in PCOS phenotype mice, improves the number of ovarian atretic follicles and Graafian follicles, and upregulates Akkermansia abundance in the gut while lowering serum IFN-γ levels. This mechanism may involve reducing gut LPS endotoxemia and enhancing inhibition of IFN-γ-induced macrophage pyroptosis (Zhou et al., 2016; Furat Rencber et al., 2018; Huang et al., 2022). Importantly, gut Akkermansia appears to be a target of metformin’s action, and its role in improving IR is widely recognized. In vitro studies show that adding metformin promotes Akkermansia growth (Ropot et al., 2020). At the animal level, metformin-treated IR mice exhibit improved glucose profiles, increased mucin layer thickness in the intestines, and higher Akkermansia abundance. Oral administration of Akkermansia to mice results in similar improvements in glucose tolerance as metformin treatment (Shin et al., 2014). These findings suggest that metformin improves IR-PCOS by increasing the abundance of beneficial gut bacteria.

Whether lean-type or obese-type PCOS, physical exercise is the most effective and economical method for treating menstrual irregularities, hormonal disturbances, and metabolic abnormalities in PCOS patients, linked to enhancing immune function, maintaining neuroendocrine balance, and reducing inflammation (Sivasankari and Usha, 2022). Studies indicate that regular physical exercise can improve menstrual irregularities and reproductive disorders in 50% of women with PCOS (Lim et al., 2019). However, there is currently a lack of evidence on whether exercise affects gut microbiota and thereby regulates IR-PCOS. During exercise, the internal environment undergoes dynamic changes, allowing beneficial bacteria to proliferate rapidly in response to the homeostatic and physiological changes induced by exercise (Mika et al., 2015). Therefore, exercise can shape the microbiota independently of diet. Compared to non-athletes, athletes show higher α-diversity of gut microbiota, and the abundance of species such as Bifidobacterium, Lactobacillus, Prevotella, and Faecalibacterium varies with different exercise intensities, indicating that exercise influences microbiota composition (O’Donovan et al., 2020; O’Brien et al., 2022). For PCOS, static exercises like yoga and meditation can improve glycemic and lipid metabolism and IR status in adolescent PCOS females, increase melatonin secretion, and effectively regulate endocrine and metabolic disorders (Harinath et al., 2004; Nidhi et al., 2012). Imbalances in gut microbiota can affect neuroendocrine networks, including the hypothalamic-pituitary-ovarian axis, thereby influencing melatonin secretion (Iesanu et al., 2022). In conclusion, regardless of the type of exercise, physical exercise may alter gut microbiota composition and affect IR-PCOS.

TCM in the treatment of PCOS, particularly in the context of gut microbiota modulation, has gained increasing attention. TCM, which includes herbal medicine, acupuncture, and dietary therapy, has been practiced for centuries in China and is recognized for its potential in addressing the complex pathophysiology of PCOS. Recent studies suggest that certain TCM formulations may exert therapeutic effects by altering gut microbiota composition. For instance, Wang et al. demonstrated that a TCM herbal formula can alleviate metabolic abnormalities in PCOS rats by targeting the intestinal LPS/TLR4 pathway (Wang et al., 2022). Moreover, individual herbs such as Astragalus have been linked to changes in gut microbiota, potentially reducing IR and oxidative stress in PCOS patients (Li et al., 2024). Acupuncture, another key component of TCM, has shown promise in early studies by Fan et al., which suggest that it may improve PCOS symptoms through gut microbiota modulation, though the mechanisms remain unclear (Fan et al., 2021). Dietary therapy, a fundamental aspect of TCM, emphasizes the consumption of foods that promote health and balance. Preliminary evidence indicates that fiber-rich diets recommended in TCM may support beneficial gut bacteria growth and alleviate PCOS symptoms (Leung et al., 2022). Although research on TCM and its effects on the gut microbiota in PCOS is still in its early stages, these findings underscore the potential of integrating TCM with modern treatment strategies for a more comprehensive approach to PCOS management.

Inositol, a naturally occurring cyclitol, exists predominantly in its active forms, myo-inositol (MI) and D-chiro-inositol (DCI), within the human body, playing a significant role in the functional metabolism of ovarian cells. Prior research has established MI as a key molecular mediator in intercellular and intracellular communication, pivotal in signaling pathways associated with reproduction, hormonal regulation, and metabolism. Although high-level evidence supporting the use of inositol for the treatment of PCOS is currently lacking from an evidence-based medicine perspective, clinical trial outcomes suggest that inositol supplementation in PCOS patients can exert insulin-sensitizing effects akin to metformin (Greff et al., 2023). The regulatory effect of inositol on the gut microbiota remains understudied. Authoritative evidence from Cell Host & Microbe demonstrates that, in obese individuals at high risk of IR, shotgun metagenomic sequencing revealed a distinct microbiota structure characterized by a Megamonas-dominated, enterotype-like cluster. In-depth analysis indicated the presence of inositol-degrading genes in these microbes, leading to reduced inositol levels, increased intestinal fatty acid absorption, and consequently, the promotion of obesity (Wu et al., 2024). While the study subjects were not women with PCOS, it is anticipated that future research will elucidate the relationship between inositol and the gut microbiota in the context of PCOS as investigations into the role of the gut microbiota in PCOS progress.