Efficacy and Safety of Probiotics in Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis

Background Irritable bowel syndrome is a functional gastrointestinal disease. Evidence has suggested that probiotics may benefit IBS symptoms. However, clinical trials remain conflicting. Aims To implement a systematic review and meta-analysis of clinical trials regarding the efficacy and safety of probiotics for IBS patients. Methods We searched for relevant trials in Medline(1966 to Jan 2019), Embase(1974 to Jan 2019), the Cochrane Central Register of Controlled Trials(up to Jan 2019), the ClinicalTrials.gov trials register(up to Jan 2019), and Chinese Biomedical Literature Database(1978 to Jan 2019). Risk ratio (RR) and a 95% confidence interval (CI) were calculated for dichotomous outcomes. Standardized mean difference (SMD) and 95% CI were calculated for continuous outcomes. Results A total of 59 studies, including 6,761 patients, were obtained. The RR of the improvement or response with probiotics versus placebo was 1.52 (95% CI 1.32–1.76), with significant heterogeneity (I2 = 71%, P < 0.001). The SMD of Probiotics in improving global IBS symptoms vs. Placebo was -1.8(95% CI -0.30 to -0.06), with significant heterogeneity (I2 = 65%, P < 0.001). It was impossible to draw a determinate conclusion. However, there were differences in subgroup analyses of probiotics type, dose, treatment duration, and geographic position. Probiotics seem to be safe by the analysis of adverse events(RR = 1.07; 95% CI 0.92–1.24; I2 = 0, P = 0.83). Conclusion Probiotics are effective and safe for IBS patients. Single probiotics with a higher dose (daily dose of probiotics ≥1010) and shorter duration (< 8 weeks) seem to be a better choice, but it still needs more trials to prove it.


INTRODUCTION
Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder associated with abdominal pain, bloating and altered bowel habits (Drossman et al., 2002). It affects 11% of the world-wide population (Lovell and Ford, 2012). IBS reduces health-related quality of life (HRQOL) (Gralnek et al., 2000;Wang et al., 2012) and leads to a significant economic healthcare burden.
Although the exact etiology and pathogenesis underlying IBS are still incompletely understood, studies show that IBS was associated with the gastrointestinal (GI) microbiota, chronic low-grade mucosal inflammation, altered regulation of the gutbrain axis, immune function, visceral hypersensitivity, and psychosocial factors (Parkes et al., 2008;Dupont, 2014;Hayes et al., 2014). Since there is no effective cure for IBS, the treatment focuses on alleviating the particular symptoms. New therapeutic options for IBS include tricyclic antidepressants (Rahimi et al., 2009), spasmolytics (Tack et al., 2016, selective serotonin reuptake inhibitors (Bundeff and Woodis, 2014), lubiprostone (Chang et al., 2016) and linaclotide (Chey et al., 2011), and 5hydroxytryptamine type-3 antagonists such as ramosetron and alosetron (Andresen et al., 2008). However, current treatments are not very useful or may cause adverse reactions (Trinkley and Nahata, 2014).
Evidence (Durban et al., 2013;Jalanka-Tuovinen et al., 2014) has suggested that intestinal microorganisms play an important role in IBS, as numerous studies have indicated that an irregular composition or metabolic activity of intestinal flora in patients with IBS Spiller et al., 2016;Thijssen et al., 2016;Hod et al., 2017;Shin et al., 2018). Therefore, the regulation of the gut microbiota by probiotics is a promising treatment for IBS (Hyland et al., 2014). Probiotics can improve intestinal flora and limit colonization of pathogenic bacteria (Guarner et al., 2012). Investigators have performed numerous clinical trials to assess the efficacy of probiotics for IBS. However, the conclusions have been controversial. Some trials have suggested that probiotics can improve global IBS symptoms . Others have demonstrated no effect . Several articles have not found an apparent effect of probiotics on global IBS symptoms, but have found improvement of individual IBS symptoms . Therefore, we conducted this meta-analysis to examine the efficacy of global IBS symptoms improvement, global symptoms scores, and individual symptom scores, such as abdominal pain and bloating. Additionally, this study evaluated the safety of probiotics.

Search Strategy and Selection Criteria
We included all eligible randomized placebo-controlled, trials (RCTs) of probiotics treatment in adult IBS. We searched Medline(1966 to Jan 2019), Embase(1974 to Jan 2019), the Cochrane Central Register of Controlled Trials(up to Jan 2019), the ClinicalTrials.gov trials register(up to Jan 2019), and Chinese Biomedical Literature Database(CBM) (1978 to Jan 2019) for relevant trials. We used the terms "probiotics" and "irritable bowel syndrome" both as medical subject heading (Mesh) and free text terms. The exact search strategy in Medline was ("probiotics"[MeSH Terms] OR "probiotics"[Title/Abstract]) AND ("irritable bowel syndrome"[MeSH Terms] OR "irritable bowel syndrome"[Title/Abstract]) AND ("randomized controlled trial" [pt] OR "randomized controlled trial" [tiab]).
We used the following eligibility criteria: (1) the studies were randomized controlled trials (RCTs) comparing probiotics with placebo; (2) diagnostic criteria included but were not limited to the Manning criteria, and Rome I, Rome II, or Rome III criteria. We did not exclude trials in which patients were stated to be diagnosed with IBS but no diagnostic criteria were described; (3) the age of participants were ≥ 18 years; (4) minimum treatment duration was 7 days. Studies were excluded if they met: (1) studies with inadequate information; (2) probiotics along with other drugs; (3) control group was not placebo; (4) data were not available after contacting the authors. There were no language limitation. Articles in foreign language were translated as needed.

Outcome Assessment
The primary outcomes were the efficacy of probiotics on global IBS symptoms improvement or response to therapy. Secondary outcomes involved the effect on global symptoms scores and individual symptom scores, such as abdominal pain and bloating. The safety of probiotics was also evaluated.

Data Extraction
Two reviewers extracted data from included trials independently. All data was inspected by a third reviewer. Any divergence was solved by consensus. Following data were extracted:author publication year, country, type of IBS(%), diagnostic criteria for IBS, recruitment, sample size, number of male/female, age, probiotic, dosage, duration of therapy, criteria to define symptom improvement or response, and outcomes.

Assessment of Risk of Bias
Two reviewers performed the assessment of study quality independently. Disagreements were solved by discussion. The risk of bias were evaluated according to the Cochrane handbook (Higgins and Green, 2011). Random sequence generation and allocation concealment(selection bias), blinding of participants and personnel(performance bias), blinding of outcome assessment(detection bias), incomplete outcome data(attrition bias), selective reporting(reporting bias), and other biases were assessed.

Statistical Analyses
Random effects model was used (Dersimonian and Laird, 1986) to get a conservative estimation for the effect. As dichotomous outcomes, the efficacy on global IBS symptoms improvement or overall symptom response and the safety of probiotics were evaluated by RR(risk ratio) and 95% CIs(confidence intervals). As continuous outcomes, global symptoms scores, and individual symptoms scores were assessed using standardised mean difference (SMD) and corresponding 95% CIs. A negative SMD was defined to indicate beneficial effects of probiotics compared with placebo for outcomes. Subgroup analyses based on probiotic type, dosage, and treatment duration were conducted.
Heterogeneity was tested by I 2 statistic and the Cochran Qtest. I 2 ≥ 50 and P < 0.10 were considered as a significant heterogeneity (Higgins et al., 2003). When there was significant heterogeneity, sensitivity analyses were conducted to give possible explanation. Review Manager version 5.3.5 (the Nordic Cochrane Center, Copenhagen, Denmark) was used to obtain forest plots of RRs and SMDs Egger test (Egger et al., 1997) (P < 0.10 defined existence of possible publication bias) and funnel plots was calculated by Stata Statistical Software: Release 13 (StataCorp LP; College Station, TX).

DISCUSSION
Alterations of the intestinal microbiome could be relevant to IBS. Symptoms in IBS often developed after an infection, which was known as post-infectious IBS (Marshall et al., 2006;Marshall et al., 2007). Gut bacterial overgrowth may cause symptoms of IBS indistinguishable (Lin, 2004). Studies suggest that compared with the healthy group the colonic microbiome changes in IBS (Durban et al., 2013;Jalanka-Tuovinen et al., 2014). Despite there were many drugs and treatments for IBS, probiotics have shown beneficial Mozaffari et al., 2014). Probiotics may regulate immunity in IBS to protect the intestine (Major and Spiller, 2014). Probiotics also modify the gut microbiota, which improves some IBS symptoms, such as flatulence, bloating, and altered bowel habits (Jeffery et al., 2012;Tap et al., 2017).

Summary of Main Results
Many pieces of evidence have suggested that probiotics may benefit IBS symptoms Stevenson et al., 2014;Yoon et al., 2014). However, the results of clinical trials have been conflicting. Our meta-analysis has indicated that probiotics may be beneficial and safe to improve symptoms of IBS compared with placebo. However, it was difficult to draw a precise conclusion as a result of the existence of significant heterogeneity and possible publication bias. We found that a shorter treatment duration (< 8 weeks) could reduce global IBS symptoms scores and bloating scores Guyonnet et al., 2007;Drouault-Holowacz et al., 2008). As a chronic and recurrent disease , the improvement of IBS symptoms seems to be detected after a long time by taking probiotics continuously. However, according to current research shorter treatment duration seemed to be more beneficial. But due to many dropouts in the longer duration group, there may have an impact on research results, manifesting as greater improvement in the shorter duration group . Although the use of single probiotics tended to have a beneficial effect on improving the bloating scores Spiller et al., 2016;Kim et al., 2018;Sun et al., 2018), it was unknown which strain or species was more beneficial than others. Using a high dose of probiotics may reduce abdominal pain scores and bloating scores Yoon et al., 2015;Kim et al., 2018). However, Lyra et al. tested two different doses (10 10 CFU/D, and 10 9 CFU/D) of Lactobacillus acidophilus NCFM and reported that none of the outcomes showed a dose-response effect . Small differences of dosage may contribute to no effect of dose. Probiotics could benefit overall IBS symptoms improvement in North America, West Europe, Northern Europe, and East Asian.We also found that probiotics could reduce global IBS symptoms scores in North America, East Asian, and South Asian. More pieces of evidence are needed. Probiotics seemed safe for patients with irritable bowel syndrome Whorwell et al., 2006;Majeed et al., 2016;Spiller et al., 2016), but more long-term trials are required to prove it.

Strengths and Weaknesses
Our meta-analysis is the first to assemble the efficacy and safety of probiotics for IBS patients with all diagnostic criteria by subgroup analyses of probiotic type, dose, treatment duration, and geographic position. We conducted this metaanalysis and systematic review using a rigorous and reproducible methodology. Two reviewers assessed eligibility and extracted data independently. The random-effects model was used to minimize the possibility of overestimating treatment results. We also tried to contact researchers of possibly eligible trials to get data. These comprehensive approaches included more than 3,300 IBS patients receiving probiotics treatment. Finally, subgroup analyses of probiotics type, dose, treatment duration, and geographic position were performed to evaluate the efficacy of treatment. Our study has certain limitations. Bias risk of many studies was unknown, and the analysis shows considerable evidence of heterogeneity between trials. However, considering only studies with low bias risk, the positive effects remained. The number of studies on subgroup analyses of probiotics type, doses, and treatment duration was limited. It was not enough to detect significant differences in the efficacy of probiotics. In some studies, significant placebo effects have been found which can affect the results.

CONCLUSIONS
In summary, this meta-analysis has demonstrated moderate evidence for the use and safety of probiotics in IBS. A shorter treatment duration (< 8 weeks) and a single probiotic may be more beneficial. Probiotics seem to be safe for patients with irritable bowel syndrome. There is still a need for more clinical trials. Finally, probiotics may be a beneficial therapy for IBS patients.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/ Supplementary Material.

AUTHOR CONTRIBUTIONS
LZ and BL conceived and designed this study. BL and LL searched and selected studies. HD and JG extracted essential information. BL and HS assessed the risk of bias. BL and HD performed statistical analyses. BL and HS interpreted the pooled results. BL, LL, and LZ drafted the manuscript. All authors approved the final manuscript.

FUNDING
This research was supported by the Youth Foundation of 960th Hospital of the PLA with a unique identifier of 2017QN03.