1. Introduction

Gestational diabetes mellitus (GDM) is the most common pregnancy complication, and its prevalence is continually rising worldwide due to increased obesity and the average age of pregnant women [1], from 4.5% to 20.3% in the Western Pacific [2] and 14.8% in China [3]. Overweight and obesity can contribute to half of GDM’s prevalence [4]. Meanwhile, obesity occurs in up to 30% of women [5], and obese women have a higher risk for GDM in comparison with normal-weight women [6]. Both GDM and obesity induce metabolic traits, including hyperglycemia, hyperinsulinemia, and insulin resistance [7, 8], as well as imposing a huge economic burden [1, 9, 10], especially in developing countries. GDM and obesity also cause ongoing maternal and neonatal health problems [2, 7, 8, 11–13]. Women with GDM are more likely to develop diabetes at rates of 20%–60% in five to ten years after pregnancy, and the incidence of metabolic diseases in their offspring also significantly increases [13]. Therefore, the prevention of obesity may be directly associated with a lower risk of GDM, and the prevention and treatment of hyperglycemia and insulin resistance among pregnant women have become a global concern.

Lifestyle intervention is the major method to control maternal hyperglycemia and insulin resistance, including medical nutrition therapy, exercise intervention, and self-monitoring of blood glucose [14]. Women with GDM whose blood sugar cannot be controlled at an ideal level by diet or exercise should be accepted for pharmacological therapy [14]. Although insulin therapy is the most common and safest pharmacological therapy, it is very labor-intensive and time-consuming for nurses and creates a financial burden on women with GDM [9]. Oral medications, such as metformin and sulfonylureas, are associated with higher risks for adverse pregnancy outcomes, such as large-for-gestational age, neonatal hypoglycemia, and birth injury. Pregnant women may also face many barriers during clinical implementation [15–17]. Owing to the poor management of lifestyle intervention and the limitations of pharmacotherapy, seeking a better way to improve hyperglycemia and insulin resistance is essential.

Probiotic is defined as beneficial live microorganisms in the host when they reach an adequate dose [18], and it plays an important role in improving the intestinal microenvironment, modulating the immune system, and preventing systemic disease and inflammation [19]. Previous studies have demonstrated that gut microbiota promotes the digestion of complex polysaccharides to produce monosaccharides and short-chain fatty acids (SCFAs) [20], which have positive relations with metabolism [21]. The composition and function of obese-diabetic microbiota change in comparison with healthy gut microbiota, which leads to metabolic disorders such as overweight/obesity, elevated blood glucose, insulin resistance, and inflammation [22, 23]. Accordingly, probiotics may be seen as a potential, economic, and practical approach to improve blood glucose and insulin resistance/sensitivity among pregnant women.

Even though previous studies indicated the effects of probiotics in preventing and treating GDM, some conflicting conclusions were still drawn from other studies. Zhang et al. (2019) [24] showed that probiotics could reduce the blood glucose level of women with GDM. On the contrary, several system reviews indicated that probiotic supplements did not reduce the blood glucose of women with GDM in comparison with the placebo group [25–27]. Moreover, in another two system reviews [28, 29], such probiotic supplements had a positive effect on women without a GDM diagnosis. Less is also known about the effects of probiotics on different pregnancy status as well as the duration, dosages, and type of probiotic interventions.

This study aimed to synthesize more high-quality randomized controlled trials (RCTs) to provide evidence of the effects of probiotic supplements on glycemic control, insulin resistance/sensitivity, and prevention of GDM among pregnant women.

2. Method

This review completely followed the PRISMA guidelines [30].

3. Results

A total of 4,644 articles were identified by searching 11 electronic databases. Fifty-two studies were kept after irrelevant articles (n = 4474), duplication (n = 98), and reviews (n = 20) were removed through screening titles and abstracts. Of the remaining studies retrieved, ten studies were protocols, two studies were reviews, and eight studies were duplicated publications. Among the rest of the studies, two studies did not utilize probiotics to intervene, eight studies reported irrelevant outcomes, and one study was a case report. The lone Chinese literature just reported the incident of GDM without any other relevant index. Hence, we finally included 20 RCTs based on a careful checking of the full text according to the selection criteria (Figure 1).

3.2. Quality Assessments of the Literature

Figure 2 shows the methodological quality and risk for the bias of all included studies. Random sequence generation and allocation concealment were conducted in 18 included studies. Blinding of participants and personnel was reported in 19 studies, of which 5 studies reported triple blinding. A total of 7 studies used intention-to-treat analysis, 8 studies were considered “low risk for bias,” and only one study was evaluated as “high risk” for selective reporting. There was only one study (5%) with a small sample size, which was less than or equal to 50 participants.

3.6. Sensitivity Analysis and Publication Risk Bias

After the subgroup analysis, a sensitivity analysis was performed to remove several articles (Callaway et al. [33]; Pellonperä et al. [36]; Jafarnejad et al. [37]; Dolatkhah et al. [43]) that mainly contributed to substantial heterogeneity. This review largely produced similar results but better precision with less heterogeneity, and most pooled results showed relative stability (Table 3). Except for the sensitivity analyses of the FPG in the overweight/obese women subgroup and long-term intervention subgroup, the FPG and HOMA-IR in long-term intervention subgroup, all of them, had opposite results. The funnel plots of the FPG level, insulin, and HOMA-IR were also visually symmetric, which showed no significant publication risk bias. Most of the included studies focused on the top of the funnel, which further demonstrated that the sample size was large and this review is credible (Figure 3).

Table 3. Sensitivity analysis on effects of probiotic supplement on glucose metabolism.

Outcome	Before sensitivity analysis			Remove study	After sensitivity analysis
	Effect estimate	P	I2 (%)		Effect estimate	P	I2 (%)
FPG
FPG level	−0.11 [−0.17, −0.04]	0.0007	72	Callaway et al., 2019; Dolatkhah et al., 2015	−0.10 [−0.15, −0.05]	<0.00001	46
Different pregnancy status for FPG
Overweight/obesity	−0.09 [−0.20, 0.02]	0.12	78	Callaway et al., 2019	−0.13 [−0.21, −0.06]	0.0006	34
GDM	−0.12 [−0.24, −0.01]	0.04	76	Jafarnejad et al., 2016; Lindsay et al., 2015; Nabhani et al., 2018	−0.22 [−0.32, −0.13]	<0.00001	35
Different intervention durations for FPG
Short term	−0.13 [−0.22, −0.04]	0.006	74	Dolatkhah et al., 2015; Jafarnejad et al., 2016	−0.12 [−0.20, −0.03]	0.007	48
Long term	−0.08 [−0.17, 0.00]	0.06	72	Callaway et al., 2019	−0.11 [−0.18, −0.05]	0.0005	35
Different intervention forms for FPG
Capsule	−0.10 [−0.17, −0.03]	0.008	77	Callaway et al., 2019; Dolatkhah et al., 2015; Pellonperä et al., 2019B	−0.08 [−0.13, −0.02]	0.008	38
Different probiotic bacteria doses for FPG
Large dose	−0.10 [−0.17, −0.03]	0.004	75	Callaway et al., 2019; Dolatkhah et al., 2015; Jafarnejad et al., 2016	−0.12 [−0.17, −0.06]	<0.0001	41
2 h OGTT
Different pregnancy status for 2 h OGTT
Overweight/obesity	0.01 [−0.15, 0.18]	0.89	52	Asgharian et al., 2020	0.10 [−0.07, 0.27]	0.26	0
Insulin
Insulin	−1.68 [−2.44, −0.92]	<0.00001	76	Jafarnejad et al., 2016; Karamali et al., 2016; Pellonperä et al., 2019B	−1.52 [−2.03, −1.02]	<0.00001	44
Different pregnancy status for insulin
Overweight/obesity	0.94 [−3.36, 5.24]	0.67	90	Pellonperä et al., 2019A	−1.41 [−2.93, 0.12]	0.07	0
GDM	−2.40 [−3.70, −1.09]	0.0003	70	Ahmadi et al., 2016; Jafarnejad et al., 2016; Karamali, 2016	−0.99 [−1.18, −0.80]	<0.00001	0
Different intervention durations for insulin
Short term	−2.27 [−3.13, −1.41]	<0.00001	72	Ahmadi et al., 2016; Jafarnejad et al., 2016; Karamali, 2016	−1.55 [−2.17, −0.93]	<0.00001	49
Long term	0.42 [−2.34, 3.18]	0.77	87	Pellonperä et al., 2019B	−1.32 [−2.02, −0.63]	0.0002	0
Different types of intervention for insulin
Capsule	−1.66 [−2.59, −0.73]	0.0005	76	Jafarnejad et al., 2016; Pellonperä et al., 2019B	−1.52 [−2.14, −0.91]	<0.00001	38
Different probiotic bacteria doses for insulin
Large dose	−1.04 [−1.22, −0.85]	<0.00001	76	Jafarnejad et al., 2016; Pellonperä et al., 2019B	−1.04 [−1.23, −0.86]	<0.00001	38
HOMA-IR
HOMA-IR	−0.36 [−0.53, −0.20]	<0.00001	74	Pellonperä et al., 2019B; Taghizadeh et al., 2014	−0.39 [−0.52, −0.27]	<0.00001	49
Different pregnancy status for HOMA-IR
Overweight/obesity	0.07 [−0.55, 0.70]	0.81	89	Pellonperä et al., 2019B	−0.23 [−0.47, 0.02]	0.07	0
GDM	−0.59 [−0.88, −0.30]	<0.00001	64	Dolatkhah et al., 2015; Jafarnejad et al., 2016	−0.59 [−0.93, −0.25]	0.0007	44
Different intervention durations for HOMA-IR
Short term	−0.45 [−0.62, −0.29]	<0.00001	57	Karamali et al., 2016	−0.42 [−0.57, −0.28]	<0.00001	49
Long term	−0.15 [−0.61, 0.32]	0.53	85	Pellonperä et al., 2019B	−0.27 [−0.47, −0.08]	0.006	0
Different types of intervention for HOMA-IR
Capsule	−0.40 [−0.63, −0.18]	0.0004	77	Jafarnejad et al., 2016; Pellonperä et al., 2019B	−0.39 [−0.54, −0.24]	<0.00001	37
Different probiotic bacteria doses for HOMA-IR
Large dose	−0.29 [−0.34, −0.24]	<0.00001	77	Karamali, 2016; Pellonperä et al., 2019B	−0.30 [−0.35, −0.26]	<0.00001	43
QUIICKI
Different types of intervention for QUIICKI
Capsule	0.01 [0.00, 0.01]	0.002	55	Babadi et al., 2019	0.01 [0.01, 0.02]	<0.00001	0
Different intervention durations for the incident of GDM
Long term	1.05 [0.81, 1.36]	0.69	52	Wickens et al., 2017	1.18 [0.89, 1.57]	0.24	36

4. Discussion

The pooled analyses indicated that probiotic supplements had positive effects on improving FPG level, insulin level, insulin resistance, and insulin sensitivity, especially in GDM and healthy pregnant women. After subgroup analysis, the effects of the probiotic capsule were better than those of probiotic yogurt, short-term intervention (≤12 weeks) seemed to be more effective in glucose metabolism, and a large dose of probiotics (≥10⁹ CFU) played a role in decreasing FPG. However, probiotic supplements neither improved the level of HbA1c and 1 h and 2 h OGTT nor reduced the incidence of GDM.

Compared with previous systematic reviews, this present review included more RCTs and conducted several subgroups to control clinical heterogeneity, including different pregnancy status, duration of probiotic intervention, intervention types, and dosages, except for when grouping studies by probiotic species, because each combination of probiotic chains was only utilized in one or two studies. Sensitivity analyses were also performed to confirm the improved credibility of the results. Furthermore, this review carefully evaluated the characteristics and qualities of each included study and concluded cautiously.

Probiotic supplements had a certain function in improving the level of FPG in this review, though it was discrepant in different pregnant women. Based on this outcome, there was no meta-analysis discussing the effects of probiotics on the improvement of blood glucose in obese pregnant women. The meta-analysis results of this study illustrated that probiotics seem to only affect blood glucose in women with GDM or healthy women. This was consistent with Barengolts et al. [52], who pointed out no evidence to prove that probiotics could help control glucose in obese participants. The probable reason for this finding could be the varied composition of the gut microbiome according to the states of gestation [53, 54]. Mokkala et al. pointed out that the inflexibility of gut microbiota may influence the probiotics to regulate glucose metabolism [55]. The gut microbiota in early pregnancy is comparable to that in nonpregnancy women. Women with GDM have lower biodiversity of the intestinal microbiota in the first trimester. The diversity of gut microbiota among prepregnancy obese pregnant women is the lowest in the early and third trimesters of pregnancy [53]. However, the result of the overweight/obese women subgroup changed obviously after sensitivity analysis. This might be contributed by one dubious study [33] that had contrary findings. Therefore, it is still unclear whether probiotics can improve blood sugar in overweight/obese pregnant women. Of particular interest is that pregnancy decreased insulin sensitivity and increased insulin, insulin resistance, and HbA1c compared with those in nonpregnancy women. Thus, the effect of probiotics could be limited among pregnant women. Additionally, the subgroup analysis of different duration of probiotic intervention found that short-term (≤12 weeks) probiotic had more positive effects on improving FPG than did long-term intervention (>12 weeks). Similarly, short-term intervention could affect FPI and HOMA-IR, which is opposite to the meta-analysis results of Han et al. [56]. With the advance of gestation, the gut microbiota associated with inflammatory states is increased substantially in healthy pregnant women. Therefore, probiotic supplements might be more helpful to control blood glucose in the short term. The available studies only performed subgroup analyses of different intervention forms in FPG. Probiotic capsules were a better choice than probiotic yogurt in the level of FPG because only a certain dose of probiotics can confer a benefit, and probiotic capsules have a more accurate dose compared with probiotic yogurt. The traditional storage method of adding probiotics to dairy products or food limits the survival of probiotics [57]. The advanced technology of making capsules ensures probiotic survival, including drying technology, microencapsulation of probiotic bacteria, improved encapsulating material, capsule size, and structure [57–59]. Intestinal flora plays an important role in energy metabolism (including diabetes and obesity) [22, 23]. There is no evidence indicating that the best dosage of probiotics can regulate blood glucose, although it is universally accepted that an adequate dose of probiotics could make a difference. The results of this meta-analysis suggested that probiotic bacteria equal to and more than 10⁹ CFU per day is significantly more effective than a lower dosage. While the results in this study did not show a remarkable effect on controlling glucose, the positive performance of probiotics cannot be denied. Thus, probiotics are expected to be an assistant treatment strategy for diabetes.

The specific mechanism of probiotics remains elusive. An increasing number of researches demonstrate that microbes and metabolites work together with metabolic function and human health. Probiotics can improve metabolism by regulating the gut microbiota that will produce numerous organic compound metabolites, such as SCFAs [49] and bile acids [60]. They are the key point in the development of metabolic diseases and the improvement of insulin sensitivity, energy metabolism, and appetite suppression [21]. The imbalance of the intestinal flora may contribute to the abnormal absorption of lipopolysaccharides and increase aberrant circulating levels of SCFAs and bile acids [21, 61]. Therefore, dysbacteriosis is a vital factor of metabolic diseases like obesity, GDM, and insulin resistance [13]. Probiotics can work through the following three mechanisms: firstly, probiotics interact with gut flora and consequently produce metabolites like SCFAs; secondly, probiotics improve the intestinal epithelial barrier; and thirdly, probiotics regulate the secretion of proinflammatory mediators like tumor necrosis factor-α, interleukin-6, and intestinal glucagon-like peptide 1. The reduction of them can improve glycemic control and insulin sensitivity [19, 56, 62]. Therefore, probiotic supplements are expected to be a promising approach for glucose metabolism.

Probiotic supplements can improve insulin level, β-cell function, insulin resistance, and insulin sensitivity, especially in healthy pregnant women or those with GDM in this study. These results resembled those of several previous systematic reviews [24–29]. In particular, we found that probiotic supplements had no positive effects on insulin and HOMA-IR among overweight/obese women. Therefore, it is necessary to conduct more research to confirm this result. Meanwhile, this review did not conduct a meta-analysis of HOMA-B and QUICKI for overweight/obese women owing to a lack of related data. In addition, the probiotic capsule, which was the most applied probiotic form in this review, had effective improvement on insulin, HOMA-IR, HOMA-B, and QUICKI. Correspondingly, both small dose and large dose had positive effects. Moreover, a short-term intervention (≤12 weeks) could make more of a difference instead of a long-term intervention.

Supplementing probiotics did not have positive effects on reducing the incidence of GDM in this study, no matter the status of the pregnant women and how much probiotics they took. In the previous conclusion, the effects of improving blood glucose were mild in healthy women and barely reduced FPG among overweight/obese pregnant women. Hence, probiotics may not be an ideal preventive strategy.

This review has several limitations. Firstly, even though both English and Chinese literature were searched, only English literature was finally included. Secondly, most of the included studies came from Iran, but some full texts from Iran could not be accessed, which might contribute to risk bias in the results and affect the generality of this study. Thirdly, there was high-level heterogeneity in the meta-analysis process which came from participants in different pregnancy status as well as the difference in duration, forms, species, and doses of the probiotic interventions. Meanwhile, given the limitations of the original article, even though participants were divided into overweight/obese women, women with GDM, and healthy pregnant women, there were still women who were concurrent with overweight/obese and GDM, which contributed to some heterogeneity for results. Moreover, the beginning and termination of intervention of the included studies differed, which might, to a certain extent, affect the results. Fourthly, outcome measurements only included glucose metabolism, insulin sensitivity, and the incidence of GDM; other maternal and adverse outcomes were not included in this review, which might lead to a limitation. Finally, while this current meta-analysis was not registered and may have small deviations, we still strictly followed the process of the systematic review.

In conclusion, probiotics could modulate blood sugar and improve insulin level within a certain range in healthy and GDM women instead of overweight/obese pregnant women. There were no related meta-analyses discussing the effects of probiotics among obese/overweight pregnant women. The specific mechanism that causes this difference among pregnant women in normal and pathological pregnancies remains unknown. One of the possibilities may be that the richness, diversity, and sensitivity of intestinal flora interfere with the role of probiotics, thereby calling for more high-quality trials with a larger number of participants to explore the effects of probiotics in obese pregnant women. A significant reduction was likewise observed for metabolism parameters in taking short-term probiotics (≤12 weeks). A dosage of 10⁹ CFU per day or more could be considered an effective dose to modulate FPG. It is suggested that the regulatory effects of probiotics are short-lived. Furthermore, probiotic capsules might be more effective than probiotic yogurt in terms of FPG level. However, probiotic supplements neither improve the levels of HbA1c and 1 h and 2 h OGTT nor reduced the incidence of GDM. The optimal probiotic dose in this study is also still unclear. Further high-quality, multicentre, and large-scale RCTs are needed to ensure the safety of probiotics, probe into the positive effects on pregnant women and infants, and confirm the dosage and duration of probiotic supplements. Additional research can be conducted to further determine the effects of probiotics on insulin resistance and insulin sensitivity among overweight/obese women.

Disclosure

The funder had no role in study design, data collection, and analysis, decision to publish, or preparation of the manuscript. Yu-Qing Pan and Qing-Xiang Zheng are the co-first authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Authors’ Contributions

Yu-Qing Pan was responsible for manuscript drafting and revision, data collection, data extraction, and data analysis; Qing-Xiang Zheng contributed to study design, manuscript drafting and revision, data analysis, and data interpretation; Xiu-Min Jiang was responsible for study design, data interpretation, and manuscript revision; Xiao-Qian Chen, Jing-Ling Wu, and Xiao-Yun Zhang contributed to data analysis and data interpretation.