Preventing gestational diabetes mellitus (GDM), rather than treating it, can have a number of benefits, both health and economic. Homayouni et al. suggest that probiotics are a relatively new intervention that can lower glucose levels, prevent GDM, and reduce maternal and fetal complications resulting from it ^[1][102]. Several studies have shown that the gut microflora is significantly altered in women with GDM and is similar to that of adults with T2DM ^[2][3][4][103,104,105]. An increased number of gram-negative bacteria, such as Parabacteroides, Prevotella, Haemophilus, and Desulfovibrio, has been found in the intestines of GDM patients ^[3][5][6][7][104,106,107,108]. However, a study by Mokkala et al. showed that the presence or absence of a specific bacterial species or function did not predict the onset of GDM, nor did it differ depending on the severity of GDM ^[8][109], although in the group of women with GDM, a higher number of Ruminococcus obeum was found in late pregnancy ^[8][109].

The roles of probiotics in modulating the composition of the intestinal microbiota and reducing the adherence of pathobionts, regulating the permeability of the intestinal epithelium and reducing the inflammatory process has been observed ^[9][77]. A randomized study of probiotics (Lactobacillus rhamnosus GG and Bifidobacterium lactis) showed a reduction of more than 60% in GDM, with an incidence of 13% in the probiotic group compared with 34% in the control group ^[10][110]. However, some studies found no significant differences between the probiotic and placebo groups in terms of glycemic control and antioxidant capability ^[11][12][111,112]. Nevertheless, the study by Sahhaf Ebrahimi et al. found that the use of probiotic yogurt improved blood glucose levels as well as glycated hemoglobin (HbA1c) levels ^[12][112]. Probiotic supplementation may improve blood glucose control during the third trimester, according to a study conducted in healthy pregnant women ^[13][113]. Supplementing with probiotics reduced fasting plasma glucose, serum insulin levels, and IR while increasing insulin sensitivity ^[14][15][16][7,114,115]. However, other studies have shown no benefit of taking probiotics in reducing the risk of GDM ^{[17][18][19][20][21]}[116,117,118,119,120] or improving glucose metabolism in overweight and obese women ^[8][20][21][109,119,120]. Studies in women with GDM found no significant differences in fasting glucose levels between a group receiving supplementation of Lactobacillus salivarius and a placebo group ^[22][121]. In contrast, a reduction in insulin resistance (HOMA-IR) and β-cell function (HOMA-B) was observed after probiotic administration ^{[14][16][23][24][25]}[7,115,122,123,124]. Other studies have shown that taking probiotics containing Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium bifidum in GDM patients had a beneficial effect on glycemic control, TG levels and very low-density lipoprotein (VLDL) ^[15][23][114,122].

The potential positive effects of using probiotics may also result from other mechanisms. Reduction of oxidative stress and increased secretion of incretin are considered as potential mechanisms by which probiotics improve glucose metabolism ^{[26][27][28][29]}[125,126,127,128]. Bifidobacterium and Lactobacillus are among the most common, non-pathogenic living microorganisms used as probiotics ^[30][129]. They have been identified as probiotics that reduce systemic inflammation, regulate immune function, improve intestinal mucosa permeability, and ultimately reduce IR ^{[31][32][33][34]}[130,131,132,133]. Bifidobacteria are commensals of humans and animals, they belong to the phylum of Actinobacteria ^[35][4]. Lactic acid bacteria are gram-positive bacteria that are traditionally used in the production of yogurt and other fermented dairy products ^[35][4]. Similar results were presented by Badehnoosh et al., who showed that taking a probiotic capsule containing Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidum (2 × 109 CFU/g each) for 6 weeks improved the glycemic response and markers of inflammation but did not affect pregnancy outcomes ^[36][134].

In addition, recent studies indicate that plasma vitamin C levels negatively correlate with the development of MS. It has been suggested that vitamin C supplementation may help reduce oxidative stress and postpone the chronic inflammation associated with the development of MS.

The relative abundance of short-chain fatty acids (SCFA) producing bacteria from the genera Faecalibacterium, Ruminococcus, Roseburia, Coprococcus, Akkermansia, Phascolarctobacterium, and Eubacterium was found to be lower in GDM women, obese and T2DM ^{[3][4][5][7][37][38][39][40][41][42][43]}[104,105,106,108,135,136,137,138,139,140,141]. SCFA affect the activity of cells of the immune system, as well as their migration to the site of inflammation, showing a significant anti-inflammatory potential ^[44][142]. The results of many animal studies show that supplementation with the probiotic Lactobacillus spp. induces the production of SCFA by modulating the intestinal microbiome ^{[43][45][46][47][48]}[141,143,144,145,146]. Manipulating the composition of the gut microbiome, and thus the level of SCFA, may prove to be a promising method in the treatment of inflammatory diseases ^[44][142].

In the peripheral blood of patients with GDM, probiotic intake increases the expression of peroxisome proliferator-activated receptor gamma (PPAR-γ), transform growth factor beta (TGF-β), and vascular endothelial growth factor (VEGF) and decreases the expression of tumor necrosis factor alpha (TNF-α) ^[15][114]. This suggests that probiotics alleviate IR and chronic inflammation through the PPAR pathway ^[49][147].

The supplementation of probiotics also results in a considerable decrease in plasma malondialdehyde (MDA) and a significant increase in plasma nitric oxide (NO) and total antioxidant capacity ^[15][114]. MDA has cytotoxic, mutagenic and carcinogenic properties ^[50][148]. It can also inhibit enzymes related to the cell’s defence against oxidative stress ^[50][148]. NO is the main factor in endothelial cells responsible for maintaining vascular homeostasis ^[51][149].

PE is a complicated disorder in pregnancy which occurs after 20 weeks of gestation and affects 2–8% of all pregnancies in the world ^[52][53][54][152,153,154]. PE is a vascular pregnancy complication with high fetal and maternal mortality and morbidity rates ^[55][56][155,156]. The presence of PE in the past increases the risk of hypertension, ischemic heart disease, venous thromboembolism, kidney disease and CVD, including myocardial infarction, stroke, and heart failure ^{[52][57][58][59][60][61]}[152,157,158,159,160,161]. PE is associated with a 4-fold increased risk of future stroke ^[62][162]. PE is associated with a 2–7 times higher risk of CVD, especially if PE occurs before 34 weeks of gestation ^{[57][63][64][65][66]}[157,163,164,165,166]. Research has shown a significant relationship between the occurrence of PE and MS later in life ^{[67][68][69][70][71][72]}[167,168,169,170,171,172]. PE is connected to MS risk factors, which are modifiable CVD risk factors ^[64][73][74][164,173,174]. According to research by Heidema et al., obesity, IR, and arterial hypertension are all common within the first year following a pregnancy affected by PE ^[59][159]. Moreover, Hooijschuur et al. showed that the incidence of MS was higher in the subgroup of women with PE and small-for-gestational-age (SGA) neonate than in women without SGA neonate ^[64][164]. It is important to look for new interventions that can reduce the risk of developing PE and consequently, complications for the mother and offspring.

The influence of obesity on the development of the MS is unquestionable, and the prevention of excessive weight gain may reduce the risk of complications in the future. Diets high in saturated fat and fructose affect the composition of the gut flora ^[75][181]. The resulting dysbiosis leads to a cascade of events including increased intestinal barrier permeability, bacterial translocation, and activation of hepatic receptor-induced inflammation ^[76][182]. One proposed mechanism pertains to the production of endogenous alcohol and acetaldehyde ^[77][78][79][183,184,185].

Probiotics containing Lactobacillus paracasei can impact adipose tissue mass by modulating the activity of angiopoietin-like protein 4, a circulating lipoprotein lipase (LPL) inhibitor that controls TG deposition into adipocytes, which can help prevent obesity and metabolic disorders ^[80][186].

The beneficial effect of weight reduction was obtained during Lactobacillus gasseri (SBT2055) supplementation in overweight and obese people ^[81][82][187,188]. Daily consumption of 200 g of yogurt containing Lactobacillus gasseri (108 CFU/g) for 12 weeks significantly reduced abdominal obesity. BMI, waist and hip circumferences, and body fat mass were also significantly decreased from the baseline, but discontinuation of the probiotic for 4 weeks weakened these effects ^[81][187]. Additionally, in studies conducted by Ilmonen et al., supplementation with probiotics Lactobacillus rhamnosus GG and Bifidobacterium lactis showed a reduction in maternal central adiposity at 6 months postpartum ^[83][189].

Lactobacillus plantarum (PL62) as a probiotic has been shown in two mouse trials to produce conjugated linoleic acid (CLA), which has been linked to weight loss ^[84][85][190,191]. In a study on mice by Bagarolli et al. the effects of probiotics (Lactobacillus rhamnosus, Lactobacillus acidophilus and Bifidobacterium bifidum) on the intestinal microflora, changes in insulin permeability, sensitivity and signaling in a high-fat diet (HFD) were investigated ^[86][87][192,193]. Probiotic-treated mice receiving a HFD gained significantly less weight and had reduced food consumption and also reduced fasting blood glucose and serum insulin compared with animals that did not receive probiotics ^[88][28]. The administration of probiotics in mice fed HFD improved leptin sensitivity ^[86][89][90][192,194,195]. The study showed that the administration of probiotics to obese animals was able to reduce Toll-like receptor 4 (TLR4) activation, downstream c-Jun N-terminal kinases (JNK) phosphorylation and the subsequent insulin receptor substrate-1 (IRS1Ser307) phosphorylation ^[86][192]. The probiotic administration had no effect on TLR4 signalling in mice with a normal body weight ^[86][192]. TLR4 is a LPS receptor that plays an important role in the regulation of immunological responses to infection ^[91][196]. As one of the components of the outer membrane of Gram-negative bacteria, LPS is considered an endotoxin that may contribute to the development of inflammation and IR ^[92][93][197,198]. The relative levels of TNF-α and Interleukin 6 (IL-6) transcripts in the liver, muscle and blood of probiotic-treated mice were significantly lower than in the control group ^[86][192]. When compared to untreated mice, treatment with probiotics boosted the predominance of Firmicutes and Actinobacteria while decreasing the presence of Bacteroidetes ^[86][192]. Obese animals also had a higher frequency of Bacteroidetes and a lower number of Firmicutes and Actinobacteria ^[86][192]. In obese animals, probiotic therapy resulted in the continuing presence of Bacteroidetes, an increase in the prevalence of Actinobacteria, and a decrease in Firmicutes compared with the control group ^[86][192]. An increased number of bacteria from the taxonomic family Lachnospiraceae has been associated with the development of diabetes in obese mice ^[94][95][18,199]. Probiotic administration has beneficial effects on the host, including increased adipose tissue lipolysis. Anorexigenic peptides, such as glucagon-like peptide-1 (GLP-1) and peptide YY (PYY), are secreted as a result, leading to an enhanced glucose tolerance and greater energy utilization ^[96][97][98][200,201,202]. In addition, treatment with probiotics decreased the expression of the main molecules involved in intestinal microflora inflammation and bacterial translocation, nucleotide-binding oligomerization domain-containing protein 1 (NOD-1) and CD-14 ^[86][192]. Treatment with probiotics significantly reduced the adipocyte surface area in obese mice, but the values were still outside the norm ^[86][192].

Numerous studies have shown that dysbiosis, small intestinal bacterial overgrowth and increased intestinal permeability have a role in the development of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH), both of which are closely linked to MS ^{[99][100][101][102]}[207,208,209,210]. Dysbiosis was manifested in these disorders by an increase in Enterobacteriaceae and Proteobacteria and a decrease in Bacteroidetes ^[100][208]. The specific composition of gut microbiota may play a role in both the inflammatory and fibrosis responses in patients with NAFLD ^[103][211]. In patients with NASH, an increase in the numbers of Bacteroides has been shown, and a higher degree of fibrosis has been observed in patients with an increased amount of Ruminococcus ^[104][105][212,213].

A meta-analysis showed that treatment with Lactobacillus acidophilus, a mixture of Lactobacillus acidophilus and Bifidobacterium lactis, and Lactobacillus plantarum for 3 to 12 weeks lowered total and low-density lipoprotein (LDL) cholesterol concentrations compared with a placebo ^{[106][107][108]}[214,215,216]. On the other hand, no effect of Lactobacillus helveticus and Enterococcus faecium on cholesterol concentrations was demonstrated ^[106][214]. Studies found no significant differences in LDL cholesterol in women with GDM between a group with supplementation of Lactobacillus salivarius and a placebo group ^[21][120]. In a research review by Okesene-Gafa et al. it was observed that taking probiotics may be associated with a slight reduction in TG and total cholesterol ^[16][115]. In a study by Babadi et al., probiotic capsules containing Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium bifidum and Lactobacillus fermentum reduced the concentration of TG, VLDL cholesterol, and the total/HDL cholesterol ratio while increasing HDL cholesterol levels ^[15][114].

NAFLD is strongly associated with obesity and thus closely related to the elements of MS (abdominal fat distribution, IR, diabetes, dyslipidemia, and hypertension) ^{[109][110][111][112][113][114]}[217,218,219,220,221,222]. Preventing the development of NAFLD and NASH may contribute to reducing the risk of developing MS in the future.

The table below summarizes the studies conducted in pregnant women on the effects of probiotics on metabolic disorders (Table 12).

The studies suggest that the intestinal microbiota is a key player in the development of a chronic low-grade inflammatory state associated with MS ^[118][206]. The relationship between gut microbiota and the onset of metabolic inflammation related to obesity, IR and T2DM has been demonstrated ^[119][120][223,224]. Metabolic endotoxemia, which is caused primarily by the Gram-negative bacterial membrane component—LPS, is a critical event in the development of these conditions ^[120][121][224,225]. Through metabolic pathways, LPS leads to pro-inflammatory changes (increases in TNF-α, IL-1β and IL-6, leptin and resistin, plasminogen activator inhibitor-1 and C-reactive protein) and induces IR ^[122][226].

It has been noted that specific probiotics may, apart from their immunomodulatory and metabolic effects, modulate the intestinal microflora ^[123][12]. For these reasons, probiotics may play an important role in immunomodulation to help prevent the low-grade chronic inflammation associated with MS ^{[35][122][124]}[4,94,226].

Inflammatory reactions in the gut can occur through activation of the TLR pathway, degradation of the IĸB kinase, and release of nuclear-kappa B factor (NF-ĸB), which activates the pro-inflammatory cascade ^[125][126][227,228]. Several probiotic strains such as Lactobacillus rhamnosus or Lactobacillus casei have been shown to be effective in preventing IĸB breakdown and thus reducing the release of pro-inflammatory molecules ^[127][128][229,230].

SCFAs such as acetate, propionate, and butyrate can be catabolized by probiotics from complex polysaccharides from the diet ^[129][231]. These substances are thought to help with metabolic disorders associated with MS. SCFAs show significant anti-inflammatory potential by reducing IR, and increasing the secretion of the protective GLP-1, which stimulates insulin release and improves β-cell function ^[44][142]. A study by Yadav et al. showed that acetate can suppress insulin signaling in adipocytes, inhibiting fat accumulation in adipose tissue ^[27][126].

It is important not only to take probiotics, but also other substances which affect probiotics bioavailability. Therefore, further work should pay attention to additional aspects including plasma concentrations of substances such as beta glucans and curcuminoids in pregnant women, which may affect probiotic absorption and increase the well-being of the gut microflora ^[130][131][232,233]. Vitamin C taken with probiotics may multiply their beneficial effect by reducing oxidative stress and postponing the chronic inflammation associated with the development of chronic diseases, including MS ^[132][234]. In addition, a proper diet rich in whole grains should be beneficial and could impact the microbiota profile in these patients ^[130][232].