Gut Microbiota and Critical Metabolites: Potential Target in Preventing Gestational Diabetes Mellitus?

Gestational diabetes mellitus (GDM) is an intractable issue that negatively impacts the quality of pregnancy. The incidence of GDM is on the rise, becoming a major health burden for both mothers and children. However, the specific etiology and pathophysiology of GDM remain unknown. Recently, the importance of gut microbiota and related metabolic molecules has gained prominence. Studies have indicated that women with GDM have significantly distinct gut microbiota and gut metabolites than healthy pregnant women. Given that the metabolic pathways of gut flora and related metabolites have a substantial impact on inflammation, insulin signaling, glucose, and lipid metabolism, and so on, gut microbiota or its metabolites, such as short-chain fatty acids, may play a significant role in both pathogenesis and progression of GDM. Whereas the role of intestinal flora during pregnancy is still in its infancy, this review aims to summarize the effects and mechanisms of gut microbiota and related metabolic molecules involved in GDM, thus providing potential intervention targets.


Introduction
Gestational diabetes mellitus (GDM) is the most common metabolic complication of pregnancy, affecting up to 25.5% of pregnant women [1,2]. It is defined as glucose intolerance provoking a transient state of hyperglycemia early or first detected during pregnancy [3]. GDM greatly increases the odds of polyhydramnios, pre-eclampsia, macrosomia, shoulder dystocia, cesarean section, and neonatal death [1,[4][5][6][7]. Even with a successful birth, children may have a higher risk of developing obesity, diabetes, cardiometabolic disorders, and other metabolic disorders in the future [1,3]. Evidence suggests that women diagnosed with GDM in their first pregnancy have a 34.5% increased risk of developing GDM in subsequent pregnancies when compared to healthy women [8], and their prevalence of T2DM diagnosed within 5 years is approximately 20-50% [9][10][11]. Therefore, GDM has become a major health burden for both mothers and children.
Until now, the specific etiology and pathophysiology of GDM are still unknown. Profound hormonal, metabolic, and immunological changes that occur during pregnancy may be important contributors to GDM [7]. The first trimester is dominated by anabolism, storing adequate energy for fetal development. At this time, maternal insulin secretion elevates, glucose uptake by adipose tissue rises, and pregnant women begin to gain weight. As the pregnancy progresses, levels of placental and metabolic hormones, as well as Ma, 2020 [46] Case-control study 98 Women who developed GDM VS. 98    In a nutshell, despite many studies focusing on gut microbiota in GDM, the findings are inconclusive. Disparities in these findings can be attributed to sample size, geographical location, study design, and participant selection criteria (including population BMI, food habits, gestational age, and sequencing platform) [7]. Moreover, studies that dynamically investigate the changes through multi-point fecal sampling during different trimesters are rare. Still, it can be concluded that gut microbiota dysbiosis in GDM can be characterized by changes in α-diversity and β-diversity, an increase in gram-negative bacteria and part of gram-positive bacteria, alterations in SCFA-producing bacteria (including Akkermansia, Bifidobacterium bifidum, Coprococcus, Lactobacillus casei, Roseburia, and Ruminococcus), and a decrease in bacteria with probiotics properties [17][18][19]35,36,44,47,50,54].

Effect of Metabolic Overload and Chronic Inflammation on Gut Microecology
Currently, research into the underlying mechanisms involving GDM and gut microbiota dysbiosis is at the preliminary stage. Many studies have reported that changes in gut microecology were associated with decreased insulin sensitivity, higher plasma glucose concentration, and greater BMI in GDM women. These changes appear to begin in early pregnancy and last for several months after delivery, negatively affecting intestinal function.
Studies also indicated that pregnant women with GDM tend to have a significantly higher pre-pregnancy BMI than women without GDM [37,43]. Overweight and obesity before pregnancy are associated with a greater risk of developing GDM [55]. The main reason may be related to gut dysbiosis, which plays a role in the pathogenesis and progression of GDM [56]. Most studies revealed that in genetical obese ob/ob mice, diet-induced obese (DIO) mice, and obese people, the proportion of Akkermancia muciniphila, butyrate-producing bacteria, including Roseburia and Faecalibacterium prauznitzii, were reduced [57][58][59]. At the same time, the proportion of Bacteroidetes and Firmicutes was aberrant; though some studies agreed on a significant increase in Firmicutes, others reported a significant increase in Bacteroidetes and a decrease in Firmicutes [60][61][62][63][64][65][66]. Early white adipose tissue dilatation and chronic obesity can both activate the inflammatory programs, permanently skewing the immune system toward proinflammatory phenotypes [67].
GDM is usually accompanied by hyperglycemia and IR. Human metagenome-wide association studies in patients with type 2 diabetes demonstrated highly significant correlations between hyperglycemia and IR and specific intestinal bacteria, bacterial genes, and associated metabolic pathways [68]. Hyperglycemia affects goblet cell proliferation, maturation, and mucus biosynthesis, which will disrupt the intestinal barrier and intestinal mucus, allowing pathogenic bacteria and their elements, such as LPS, to pass through the epithelial barrier [69,70]. Subsequently, LPS binds to toll-like receptors on the surface of intestinal epithelial cells to recruit immune cells in the intestine [67,71]. IFN-γ, IL-1, and other proinflammatory cytokines are released by recruited immune cells, disrupting the enteral environment and increasing intestinal permeability, further eliciting an immune response and triggering chronic inflammation [70,72,73]. Hyperglycemia can also drive immune cell dysfunction via mitochondrial dysfunction [74,75]. Damage to the enteral environment and immune response will affect the value and growth of intestinal flora, leading to gut microbiota dysbiosis [67,71,[76][77][78].

Roles of Gut Microecology in GDM
Hypotheses have been put forward that changes in microecology may be related to GDM by affecting intestinal barrier function, glucose metabolism disorder, energy accumulation, and other ways.

Gut Microbiota Dysbiosis
Zonulin, which is released by the liver and gut epithelial cells, is a physiological tight junction modulator and a potential predictor of gastrointestinal permeability [7,79]. Recent research has found that the zonulin level in plasma is significantly elevated in GDM women, indicating their greater gastrointestinal permeability and dysfunction of the gut barrier [44,80]. The gut barrier consists of mucus, intercellular tight junctions, immunoglobulins, antimicrobial peptides secreted by Paneth cells, microbes, and other components [79]. The mucus barrier is one of the first lines of the gut barrier. Gut microbiota dysbiosis may result in the thinning of the mucosal layer in GDM patients [81]. For example, Prevotella and Akkermansia levels are reported to be elevated in women with GDM. They are mucin-degrading pathobionts, which can increase mucin oligosaccharide degradation beyond the normal limit, leading to gut barrier dysfunction and leaky gut [57,82]. Additionally, several studies have proved a decrease in SCFA-producing bacteria in women with GDM during the second and third trimesters [24,35]. Elhaseen et al. discovered that SCFAs could significantly upregulate the tight junction protein genes (ZO-1 and occludin) of Caco-2 cells, implying that SCFAs have the potential to strengthen the gut barrier [83]. Therefore, a decrease in SCFAs may be associated with increased permeability of the gut epithelium.
The other feature of gut microbiota dysbiosis in women with GDM is the elevated level of gram-negative bacteria, including Prevotella, Haemophilus, and Desulfovibrio in both mid and late pregnancy [18,37,38,42,52]. Rises in Gram-negative pathobionts are linked to LPS biosynthesis. Accordingly, functional analysis revealed that pathways relating to LPS biosynthesis and transport system were increased in women with GDM [17,52]. Elevated levels of Gram-negative bacteria and LPS in women with GDM may weaken the intestinal epithelial barrier [7]. Elevated LPS and pathobionts, on the other hand, can adhere to the mucosal layer, cross the epithelial layer of the gut via toll-like receptor 2/4 (TLR2/4) activation, translocate via phagocytosis and dendritic cell (DC) co-localization, ultimately entering systemic circulation and causing metabolic endotoxemia [7,79]. Furthermore, the compromised intestinal barrier can facilitate the migration of LPS and pathobionts, which enter the systemic circulation and peripheral tissues, binding to and activating TLR [70,72,73]. TLR activation triggers macrophage infiltration and inflammation pathways, such as C-Jun N-terminal kinase (JNK), inhibitory B kinase (IKK), and nuclear factor kappa-B (NF-κB), which might evoke serine phosphorylation of the insulin receptor substrate-1 +Ser307 (IRS-1 +Ser307 ), resulting in suppression of phosphatidylinositol 3-kinase (PI3-K) and protein kinase B Ser473 (Akt Ser473 ). This procedure will impair insulin signaling and reduce glucose uptake in peripheral tissues, resulting in hyperglycemia in GDM women [7,70,72,73] (Figure 1). vation, translocate via phagocytosis and dendritic cell (DC) co-localization, ultimately entering systemic circulation and causing metabolic endotoxemia [7,79]. Furthermore, the compromised intestinal barrier can facilitate the migration of LPS and pathobionts, which enter the systemic circulation and peripheral tissues, binding to and activating TLR [70,72,73]. TLR activation triggers macrophage infiltration and inflammation pathways, such as C-Jun N-terminal kinase (JNK), inhibitory B kinase (IKK), and nuclear factor kappa-B (NF-κB), which might evoke serine phosphorylation of the insulin receptor substrate-1 +Ser307 (IRS-1 +Ser307 ), resulting in suppression of phosphatidylinositol 3-kinase (PI3-K) and protein kinase B Ser473 (Akt Ser473 ). This procedure will impair insulin signaling and reduce glucose uptake in peripheral tissues, resulting in hyperglycemia in GDM women [7,70,72,73] (Figure 1).

Roles of Critical Metabolites
Gut-related metabolites include SCFAs, bile acids (BAs), amino acids, and so on. Metabolomics analysis has revealed that these metabolites were significantly altered in women with GDM, especially SCFAs.
SCFAs are derived from microbiota-accessible carbohydrates (MACs) fermented in the colon from dietary fibers and resistant starch fermentation [84,85]. They are extensively involved in processing undigested diets for additional energy, lipid metabolism, glucose metabolism, and inflammation, in addition to their potential role in gut barrier maintenance [52,83,86]. SCFAs promote adipogenesis in adipose tissue by increasing the expression of the peroxisome proliferator-activated receptor (PPAR) while decreasing serum FFA levels by suppressing lipolysis through activating GPR43 [87,88]. Propionate is an essential substrate for gluconeogenesis in the liver and skeletal muscle, which can also inhibit lipogenesis by suppressing fatty acid synthase [88]. Butyrate and acetate have been linked to lipogenesis in the liver [89]. SCFAs also endorse glycogen storage while inhibiting glycolysis in the liver and skeletal muscles [90]. In the gut, SCFAs can stimulate enteroendocrine L cells to release glucagon-like peptide-1 (GLP-1) and intestinal hormone peptide YY (PYY) [91][92][93]. They both are essential brain-gut peptides that help to control appetite, promote insulin secretion, and suppress the release of pancreatic glucagon [93,94]. On the other hand, butyrate constitutes a major energy source for intestinal epithelial cells and metabolic responses, while acetate regulates intestinal PH value and intestinal flora [54,95]. They are essential for enhancing epithelial barrier function and intestinal microecology. Moreover, SCFAs influence hematopoietic progenitors in the murine bone marrow and maintain the balance between anti-inflammatory and proinflammatory cells in mice by affecting peripheral DC cells and T cells, implying that they are important for the development of the innate and adaptive immune system [96,97].
As mentioned above, the amount and types of SCFAs are largely determined by the number of SCFA-producing bacteria. The Firmicutes phylum, particularly Faecalibacterium, Roseburia, and Bifidobacterium, produce butyrate, whereas Bacteroidetes produce acetate and propionate [98]. The majority of butyrate will be absorbed as an energy source by colonocytes (7), whereas the majority of acetate will be metabolized by the liver, adipose, muscle, heart, and kidney tissues [7,99].
Alterations in the abundance of SCFA-producing bacteria have been reported in women with GDM. The compositions of SCFA-producing genus Faecalibacterium, Bifidobacterium, Ruminococcus, Roseburia, Coprococcus, Akkermansia, Phascolarctobacterium, and Eubacterium were deficient in women with GDM and were inversely correlated with glucose tolerance [17][18][19]37,48,51,53]. As a result, acetate, butanoate, and propanoate production were revealed to be reduced during the second and third trimesters [95,100]. Inadequate SCFAs reduce adipose tissue lipid storage capacity, inhibit fatty acid oxidation, and increase lipolysis, which then raises serum FFA levels and increases lipid storage in the liver and muscle [7,48]. Simultaneously, insufficient SCFAs may be unable to maintain the balance of anti-inflammatory and proinflammatory cells, leading to low-grade inflammation. Elevated serum FFA levels, increased lipid storage in the liver and muscle, and low-grade inflammation may all contribute to insulin resistance and hyperglycemia in women with GDM [7,48]. At the same time, insufficient SCFAs may be unable to maintain the gut barrier and intestinal PH value, evoking gut leakage and gut dysbiosis and forming a vicious cycle [54]. On the other hand, elevations of SCFAs-producing bacteria such as Firmicutes, Phascolarctobacterium, Faecalibacterium, and Bacteroidetes also occur in women with GDM, which lead to excessive butyrate, isobutyrate, isovalerate, acetic and propionic production [52,101]. Excessive SCFAs production may lead to extra energy harvesting capacity and FFA overflow, increasing lipid storage in the liver and skeletal muscle and leading to obesity [102]. Additionally, it may upregulate gluconeogenesis pathways and suppress glycolysis pathways and insulin signaling in peripheral tissues, resulting in hyperglycemia and IR [7,102]. Both isobutyric and isovaleric acids are branched-chain SCFAs, which may be involved in the release of proinflammatory cytokines [101] (Figure 2). chain SCFAs, which may be involved in the release of proinflammatory cytokines [101] ( Figure 2).  Figure legend: Inadequate SCFAs may be associated with increased permeability of gut epithelium. Inadequate SCFAs also reduce adipose tissue lipid storage capacity, inhibit fatty acid oxidation, and increase lipolysis, which then raises serum FFA levels and increases lipid storage in the liver and muscle. Simultaneously, insufficient SCFAs may be unable to maintain the balance of anti-inflammatory and proinflammatory cells, leading to low-grade inflammation. Elevated serum FFA levels, increased lipid storage in the liver and muscle, and low-grade inflammation may all contribute to insulin resistance and hyperglycemia in women with GDM. On the other hand, excessive SCFA production may lead to extra energy harvesting capacity and FFA overflow, increasing lipid storage in the liver and skeletal muscle and leading to obesity. Excessive SCFAs may upregulate gluconeogenesis pathways and suppress glycolysis pathways and insulin signaling in peripheral tissues, resulting in hyperglycemia and insulin resistance. The figure was created with BioRender.com. URL: https://biorender.com (accessed on 24 June 2023).
Apart from the effect of SCFA on energy storage, changes in the gut microbiota of women with GDM may also have a direct impact on energy storage. It was reported that bacteria associated with energy storage promotion, such as Akkermansia, Bifidobacterium, and Firmicutes, were augmented during the third trimester in healthy pregnant women. However, bacterium levels appeared to be lower in the second trimester [35,36,47], whereas Akkermansia levels were higher in the third trimester in women with GDM [18,51]. Hence, women with GDM may experience energy storage disorders, eliciting energy metabolism disturbance, which may be associated with glucose and lipid metabolism disorders [58,62,86]. On the other hand, Bacteroidetes was reported to be elevated in GDM women and was positively correlated with blood glucose. Bacteroidetes is considered a gram-negative bacterium that can produce a proinflammatory marker-LPS [75]. It may be implicated in the pathogenesis and progression of metabolic disturbance via inflammation [17,38,103].
Moreover, aromatic amino acids (AAA)-degrading bacteria such as Clostridium, Fusobacterium, and Eubacterium were decreased in GDM women compared to healthy pregnant women [17,37]. That might be the reason for the notably delayed and blunted decrease in AAA in GDM women [104]. Since AAA catabolism by the gut microbiome yields numerous bioactive molecules that regulate murine immune, metabolic, and neuronal responses  Figure legend: Inadequate SCFAs may be associated with increased permeability of gut epithelium. Inadequate SCFAs also reduce adipose tissue lipid storage capacity, inhibit fatty acid oxidation, and increase lipolysis, which then raises serum FFA levels and increases lipid storage in the liver and muscle. Simultaneously, insufficient SCFAs may be unable to maintain the balance of anti-inflammatory and proinflammatory cells, leading to low-grade inflammation. Elevated serum FFA levels, increased lipid storage in the liver and muscle, and low-grade inflammation may all contribute to insulin resistance and hyperglycemia in women with GDM. On the other hand, excessive SCFA production may lead to extra energy harvesting capacity and FFA overflow, increasing lipid storage in the liver and skeletal muscle and leading to obesity. Excessive SCFAs may upregulate gluconeogenesis pathways and suppress glycolysis pathways and insulin signaling in peripheral tissues, resulting in hyperglycemia and insulin resistance. The figure was created with BioRender.com. URL: https://biorender.com (accessed on 24 June 2023).
Apart from the effect of SCFA on energy storage, changes in the gut microbiota of women with GDM may also have a direct impact on energy storage. It was reported that bacteria associated with energy storage promotion, such as Akkermansia, Bifidobacterium, and Firmicutes, were augmented during the third trimester in healthy pregnant women. However, bacterium levels appeared to be lower in the second trimester [35,36,47], whereas Akkermansia levels were higher in the third trimester in women with GDM [18,51]. Hence, women with GDM may experience energy storage disorders, eliciting energy metabolism disturbance, which may be associated with glucose and lipid metabolism disorders [58,62,86]. On the other hand, Bacteroidetes was reported to be elevated in GDM women and was positively correlated with blood glucose. Bacteroidetes is considered a gram-negative bacterium that can produce a proinflammatory marker-LPS [75]. It may be implicated in the pathogenesis and progression of metabolic disturbance via inflammation [17,38,103].
Moreover, aromatic amino acids (AAA)-degrading bacteria such as Clostridium, Fusobacterium, and Eubacterium were decreased in GDM women compared to healthy pregnant women [17,37]. That might be the reason for the notably delayed and blunted decrease in AAA in GDM women [104]. Since AAA catabolism by the gut microbiome yields numerous bioactive molecules that regulate murine immune, metabolic, and neuronal responses in the gut and distant organs, decreased AAA catabolism may disturb host-microbe metabolic axes [105]. This needs to be further clarified in human trials. Metabolomics analysis of meconium and serum showed that metabolic pathways, including taurine and hypotaurine metabolism, pyrimidine metabolism, beta-alanine metabolism, and BAs biosynthesis, were altered in GDM subjects [106]. However, the correlation between these changes in metabolites and intestinal flora and GDM still remains unclear.

Prospect and Implication
As GDM has become a major health burden for both mothers and children, it is critical to propose effective prevention and treatment strategies. Nonetheless, pregnancy is a unique time, and treatment must take the safety of the fetus into account. In the United States, insulin is currently the first-line agent recommended for the treatment of GDM, whereas metformin and glyburide are not because they can cross the placenta to the fetus and cause neonatal hypoglycemia [107]. Hence, the current options for pharmacologic therapy are extremely limited. Given the importance of gut microecology in GDM, regulating intestinal flora may be a potential strategy for GDM ( Figure 3).
Microorganisms 2023, 11, x FOR PEER REVIEW 12 of 18 in the gut and distant organs, decreased AAA catabolism may disturb host-microbe metabolic axes [105]. This needs to be further clarified in human trials. Metabolomics analysis of meconium and serum showed that metabolic pathways, including taurine and hypotaurine metabolism, pyrimidine metabolism, beta-alanine metabolism, and BAs biosynthesis, were altered in GDM subjects [106]. However, the correlation between these changes in metabolites and intestinal flora and GDM still remains unclear.

Prospect and Implication
As GDM has become a major health burden for both mothers and children, it is critical to propose effective prevention and treatment strategies. Nonetheless, pregnancy is a unique time, and treatment must take the safety of the fetus into account. In the United States, insulin is currently the first-line agent recommended for the treatment of GDM, whereas metformin and glyburide are not because they can cross the placenta to the fetus and cause neonatal hypoglycemia [107]. Hence, the current options for pharmacologic therapy are extremely limited. Given the importance of gut microecology in GDM, regulating intestinal flora may be a potential strategy for GDM ( Figure 3). Lifestyle behavior change, including dietary restriction and exercise, is the first line of GDM management [107], which are well-established to have the greatest impact on gut microbiota [108]. Dietary carbohydrates (CHO) are an important source of energy for both the mother and the fetus, which can impact gut microbiota and blood glucose levels. Nondigestible CHO components (fiber) can serve as an energy source for colonic microbiota, modulating gut microbiota composition [109]. Lower fiber intake has been linked to decreased gut microbiota diversity and richness [110], increased abundance of Collinsella (a genus closely linked to higher blood glucose) and Sutterella (a Proteobacteria linked to Lifestyle behavior change, including dietary restriction and exercise, is the first line of GDM management [107], which are well-established to have the greatest impact on gut microbiota [108]. Dietary carbohydrates (CHO) are an important source of energy for both the mother and the fetus, which can impact gut microbiota and blood glucose levels. Nondigestible CHO components (fiber) can serve as an energy source for colonic microbiota, modulating gut microbiota composition [109]. Lower fiber intake has been linked to decreased gut microbiota diversity and richness [110], increased abundance of Collinsella (a genus closely linked to higher blood glucose) and Sutterella (a Proteobacteria linked to inflammatory status), and higher serum zonulin levels [111][112][113][114]. Furthermore, monounsaturated fatty acids (MUFAs) have been associated with an increase in the abundance of Firmicutes, Proteobacteria, and Bacteroidetes [113]. In pregnant women, fat-soluble vitamins also appear to modulate gut microbiota. Mandal et al. conducted the cohort study and discovered that higher vitamin D intake was associated with decreased microbial α-diversity and a relative decrease in the abundance of proinflammatory Pro-teobacteria phylum [113]. Higher vitamin E intake was also coupled with a decrease in the abundance of Proteobacteria [113]. To control maternal fasting and postprandial glucose during GDM, current dietary guidelines recommend limiting CHO intake or replacing high glycemic/rapidly digesting CHO with those that are more slowly digesting, as well as taking enough vitamins [107].
Exercise, on the other hand, has been shown to strengthen gut health by increasing microbiome diversity and balancing beneficial and pathogenic bacterial communities [115][116][117]. Specifically, exercise increases butyrate-producing bacteria such as Roseburia hominis, which raises butyrate concentrations in both humans and mice [116,118]. It can also boost key antioxidant enzymes and anti-inflammatory cytokines in intestinal lymphocytes, assisting in the reduction of intestinal inflammation [116,118].
Probiotics and prebiotics are live microorganisms that benefit the host when administered in adequate amounts [119]. As previously stated, women with GDM had a decrease in bacteria with probiotic and prebiotic properties. Therefore, probiotic and prebiotic supplementation may be advantageous. Parallel, double-blind, randomized control trials (RCTs) were published by Dolatkhah et al. They enrolled women with GDM at 24 to 30 weeks of gestation and randomly assigned them to receive probiotics or placebo in capsule form for 6 to 8 weeks. Participants who received probiotics experienced a decrease in fasting blood sugar (FBS) and a significant decrease in the homeostatic model assessment of IR (HOMA-IR) after treatment, implying the potential roles of probiotics on glucose regulation [120,121]. In another RCT, women with GDM were randomly assigned to receive either Probiotic Mixture (VSL#3 probiotics) in a capsule or a placebo for 8 weeks. Hs-CRP, TNF-α, and IL-6 levels were significantly lower in participants who received probiotics, disclosing the potential effect of probiotics on the immune response [122]. However, no effect of probiotics on glycemic control was found in women with GDM receiving a singlestrain probiotic capsule delivering 1 billion CFU of Lactobacillus salivarius UCC118 [123]. Differences between studies could be attributed to differences in sample size, probiotic dosage, and duration of probiotic/fermented food/antibiotic consumption prior to intervention [123,124]. Therefore, more RCTs are needed to determine which probiotic or prebiotic strains are optimal and how to utilize them for treating GDM. On the other hand, consuming probiotics and prebiotics requires constant monitoring to avoid adverse effects such as systemic infections and mild gastrointestinal upset [125].
The scope of this review is limited because research into the gut and GDM is still in its early stages. Moreover, the results may be affected by the different study designs and participant selection criteria (diet type, geographical environment, lifestyle, gestational age, and drug factors). We believe it is necessary to further investigate the changes in intestinal flora before the occurrence of hyperglycemia in GDM to provide new approaches for GDM prevention.

Conclusions
Although the abnormalities of the intestinal tract may not be the initial factor of GDM, it has been considered to participate in the pathological process of GDM greatly. Women with GDM have significantly distinct gut microbiota and gut metabolites than healthy pregnant women. In turn, the alerted gut microecology and related metabolites substantially impact intestinal barrier function, inflammation, insulin signaling, glucose metabolism, lipid metabolism, and energy accumulation, playing a significant role in both the pathogenesis and progression of GDM. Dietary restrictions, exercise, and probiotics (prebiotics), which can reverse the altered gut microbiota and metabolites, may help alleviate symptoms of GDM.