The protocol design of this study followed the Cochrane Handbook (Huang et al., 2020d) and was registered in the PROSPERO database. We reported this overview in accordance with the PRIOR statement (Gates et al., 2022).

We performed a systematic search of the publications using Web of Science (1900–May 2023), EMBASE (1947–May 2023), PubMed (1946–May 2023), The Cochrane Library (1993–May 2023). Search terms used for FMT were “fecal microbiota transplant” or “faecal microbiota transplant” or “stool transplant” or “fecal transfusion” or “fecal bacteriotherapy.” The results were combined with key words related to IBS. Table 1 shows the search strategy using in PubMed.

The titles and abstracts of all citations found through the literature search were independently examined by two reviewers. The selection criteria were then applied after retrieving studies that might be pertinent and reviewing and include any relevant references. The PICOS criteria of the present overview were as follows: (a) population: patients diagnosed with IBS by Rome III (Drossman, 2006), or Rome IV (Drossman, 2016); (b) intervention: FMT by any route of administration and any dosage; (c) comparison: placebo (autologous transfer, excipients with no microbiota); (d) outcome: reported improvement in global IBS symptoms, IBS symptom severity scale (IBS-SSS), QoL, also the adverse events of the intervention; (e) design: SRs/MAs only included randomized controlled trials (RCTs).

We excluded repeated publications and conference abstracts.

All of the data were independently extracted into a Microsoft Excel spreadsheet by two reviewers. The publication year, authors, country of origin, sample size, IBS criteria, subtypes, preparation of fecal microbiota and placebo, FMT route and frequency, follow-up, methods for quality assessment, primary outcomes, and main results were all collected from all eligible studies.

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist provides specifications for standardized reporting of SRs/MAs (Page et al., 2021). In the present study, PRISMA checklist was applied by two authors independently to evaluate reporting quality. The PRISMA checklist includes 27 items, each of which is a standardized question with results that can be responded to as “yes,” “partially yes” and “no”.

The Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) system (Pollock et al., 2016) was applied by two authors independently to evaluate certainty of the evidence. The GRADE method evaluates limitations, inconsistency of results, indirectness of evidence, imprecision, and reporting bias. The certainty of the evidence can be qualified as very low quality, low quality, moderate quality, and high quality.

Our initial searches yielded 169 records, and 30 duplicates were removed. Following that, 116 records were excluded based on their titles and abstracts, and 8 records were excluded owing to inconsistency with inclusion criteria. Finally, we included seven SRs in this overview (Ianiro et al., 2019; Xu et al., 2019; Abdelghafar et al., 2022; Samuthpongtorn et al., 2022; Wu et al., 2022; Zhao et al., 2022; Rodrigues et al., 2023). Figure 1 presents the PRISMA flow diagram.

We included seven SRs/MAs published between 2019 and 2023 (Table 2). Of the included studies, two were conducted in China, one in Italy, one in United States, one in Egypt, one in Thailand, and one in Portugal. Overall, the number of RCTs included varied from four to eight, and the number of participants was from 254 to 505. All studies report quantitative analyses, yet their results are contradictory. Furthermore, we observed very high overlap between the included reviews (Figure 2).

The response rate was assessed by all SRs/MAs, and their results consistently indicated no statistical difference between the FMT group and the placebo group (Ianiro et al., 2019; Xu et al., 2019; Abdelghafar et al., 2022; Samuthpongtorn et al., 2022; Wu et al., 2022; Zhao et al., 2022; Rodrigues et al., 2023). As assessed by three SRs/MAs, one review reported that FMT significantly improved IBS-SSS compared to placebo (Samuthpongtorn et al., 2022), while the other two reviews indicated no statistical difference between the two treatments (Abdelghafar et al., 2022; Zhao et al., 2022). As assessed by four SRs/MAs, three reviews reported that FMT significantly improved QoL compared to placebo (Abdelghafar et al., 2022; Samuthpongtorn et al., 2022; Wu et al., 2022), while the other review indicated no statistical difference between the two treatments (Zhao et al., 2022). The rate of adverse events was assessed by five SRs/MAs, and their results consistently indicated no statistical difference between the FMT group and the placebo group (Ianiro et al., 2019; Xu et al., 2019; Abdelghafar et al., 2022; Wu et al., 2022; Rodrigues et al., 2023).

There is insufficient evidence to judge whether FMT is effective in treating IBS. Although there was a very high overlap of RCTs contained in the included SRs/MAs, the findings of these reviews were inconsistent. The IBS-SSS was evaluated by the three SRs/MAs included, yet their results are contradictory. Similarly, QoL was evaluated by the four SRs/MAs included, and they have conflicting results. Although response rate was reported consistently by all included SRs/MAs, they all suggested that FMT did not have an improving effect on IBS. Furthermore, the differences in the definition of effective rate among the included SRs/MAs limit the reference value of this outcome. Fortunately, the included SRs/MAs consistently reported that FMT is safe for the treatment of IBS, which seems to build confidence for further studies in the future. The results of the methodological quality evaluation suggested that six of the seven included SRs/MAs were rated as low or very low owing to the deficiencies in items 2 (structured), 4 (objectives), and 7 (sources of information). In addition, almost all included studies were not reported items of 5 (Protocol and registration), 8 (Search), and 27 (Funding) in accordance with recommendations of the PRISMA statement. The GRADE evaluation results suggested low to moderate quality of evidence for outcome and that small sample sizes are to blame for the lack of convincing evidence. Therefore, with the unsatisfactory methodological quality, reporting quality, and evidence quality of SRs/MAs, we do not have sufficient confidence that their results are reliable. No definitive conclusion can be drawn, caution is required when recommending FMT as a complementary treatment for IBS.

The findings of this study suggest that there is considerable scope for addressing methodological and reporting quality issues in the process of SRs/MAs. The availability and credibility of evidence from SRs/MAs can be limited by methodological flaws, and underreporting of SRs/MAs may overstate the effectiveness of the intervention or report no adverse effects, and ultimately reduce the value of the research. Common areas for improvement were evident in the included SRs/MAs. With AMSTAR-2, the lack of a pre-designed protocol, the lack of a detailed search strategy available for replication, the lack of a list of excluded publications, and the failure to report funding sources were common deficiencies in the included SRs/MAs that we identified. According to PRISMA results, the included SRs/MAs have obvious reporting deficiencies, particularly regarding the registration of protocol, retrieval process and source of funding. Furthermore, the results of GRADE evaluation on the quality of evidence suggest that imprecision due to small sample size is the main factor leading to evidence degradation. Therefore, there is considerable scope for improvement in SRs/MAs that are strictly implemented in accordance with AMSTAR-2 and PRISMA and RCTs that are rigorously designed and implemented.

Based on the above identified deficiencies, researchers should conduct SRs/MAs and report fully in accordance with AMSTAR 2 and PRISMA requirements. In particular, researchers should register or publish research protocols in advance to demonstrate the transparent process of SRs/MAs. The specific search strategy used to implement the literature search needs to be provided, not only to help evaluate its scientific validity, but also to ensure its reproducibility. A list of excluded publications should be provided with an explanation, as this is what a rigorous SR must require. In addition, any potential conflict of interest or funding source issues should be declared. Finally, the sample size of the original RCTs on FMT for IBS should be expanded, which is fundamental to improve the quality of the evidence. Meta-regression or subgroup analysis should be performed if there is significant heterogeneity. When significant heterogeneity is observed, meta-regression or subgroup analysis should be conducted to explore the source of heterogeneity. Future new and updated SRs/MAs should focus on improving methodological quality and reporting quality and avoid multiple overlapping reviews.

The effectiveness and safety of FMT for IBS can be affected by many variables. These variables mainly include materials of FMT, route of FMT, and stool dose and frequency of FMT (Körner and Lorentz, 2023). The diversity of protocols for FMT also limits the overall applicability of the evidence for IBS. Stools and capsules are the two most common styles of materials of FMT. Of the included studies, the vast majority used stools FMT. Interestingly, both stools and capsules were found to be effective in patients with IBS (Ianiro et al., 2019; Xu et al., 2019; Abdelghafar et al., 2022; Samuthpongtorn et al., 2022; Wu et al., 2022; Zhao et al., 2022; Rodrigues et al., 2023). Unfortunately, the lack of sufficient data for extraction prevented a comparison examination of these two FMT materials. There are multiple routes of FMT administration, including oral capsules, enemas, endoscopy, and nasojejunal tube (Ianiro et al., 2019; Xu et al., 2019; Abdelghafar et al., 2022; Samuthpongtorn et al., 2022; Wu et al., 2022; Zhao et al., 2022; Rodrigues et al., 2023). The routes of FMT were then associated with the IBS-SSS score after 3 months of treatment, according to the pooled results (Ianiro et al., 2019; Samuthpongtorn et al., 2022; Zhao et al., 2022). While FMT employing endoscopy has been found to be more effective than enemas and nasojejunal tubes, FMT utilizing endoscopes and capsules has been found to be equally effective (Ramai et al., 2021). Single fecal doses for the studies included varied from 30 g to 80 g for the fecal FMT group and from 9.5 g to 50 g for the capsule FMT group. Neither the total stool dose of FMT nor the single stool dosage of FMT were related to the IBS-SSS score, according to the pooled results (Ianiro et al., 2019; Xu et al., 2019; Abdelghafar et al., 2022; Samuthpongtorn et al., 2022; Wu et al., 2022; Zhao et al., 2022; Rodrigues et al., 2023). Additionally, the frequency of FMT varied between the included studies, which may have contributed to the variations in outcomes. Therefore, the establishment of a standardized and unified FMT protocol is more conducive to the promotion and application of FMT in IBS. In addition, follow-up and further exploration to analyze the long-term effects of FMT are also key points that need to be clarified.

The modifiable factor that induces the development of IBS is diet. Gluten-free foods and low fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAP) have been recognized as the two major dietary plans for inducing remission of IBS (Dionne et al., 2018). As short-chain carbohydrates that are poorly absorbed in the intestinal lumen, FODMAPs are highly permeable and therefore tend to induce abdominal pain and bloating (Marsh et al., 2016). In addition, by interacting with the intestinal microbiota, FODMAPs can lead to gas production and aggravate bloating (Staudacher et al., 2012). Patients with IBS have reported a 68% reduction in symptoms and a significant improvement in quality of life after receiving a low FODMAP diet (Staudacher et al., 2012). Patients who consume gluten without celiac disease develop symptoms of irritable bowel syndrome, a phenomenon defined as “non-celiac gluten sensitivity” (Marchioni Beery and Birk, 2015). However, among human leukocyte antigen (HLA)-DQ2/8-positive patients, a gluten-free diet was effective primarily for the diarrhea subtype and dramatically reduced diarrhea symptoms (Vazquez-Roque et al., 2013). Therefore, FMT should also be fully integrated with dietary modification strategies to better improve symptoms in patients with IBS.

To the best of our knowledge, this is the first overview to evaluate the evidence of FMT for IBS. By systematically collecting, evaluating, and synthesizing the evidence for FMT for IBS, the findings of this study will aid in evidence-based decision making. However, the evaluation of the quality of included reviews is a subjective process and different researchers may have their own opinions on each factor, although our overview was assessed by two independent researchers.