Effects of Lactobacillus plantarum P9 Probiotics on Defecation and Quality of Life of Individuals with Chronic Constipation: Protocol for a Randomized, Double-Blind, Placebo-Controlled Clinical Trial

Background Although probiotics have been shown to improve constipation-related symptoms, a clear consensus on the use of probiotics as a constipation-relieving agent has not been reached, which is attributed to the limited available evidence and inconsistent protocols used in existing studies. Method A randomized, double-blind, placebo-controlled clinical trial is designed to study the efficiency and possible mechanism of action of probiotics for chronic constipation, in which 200 eligible volunteers with chronic constipation will be randomly assigned to a probiotic group (oral Lactobacillus plantarum P9 probiotic powder, 100 billion colony-forming units (CFUs)/day) or a placebo group. Volunteers, treatment distributors, data collectors, and data analysts will be blinded. The primary outcome is the weekly mean frequency of complete spontaneous bowel movements (CSBMs), and secondary outcomes include weekly mean frequency of CSBMs ≥3, weekly mean frequency of spontaneous bowel movements (SBMs), weekly mean stool appearance score, weekly mean difficulty of passing stool score, weekly percentage of volunteers who use auxiliary measures to assist with defecation (WPUAMA), quality-of-life (QOL) score, emotional status score, gut microbiome, and faecal metabolome. Each outcome measure will be assessed at the time points of preadministration (day 0), administration (day 14 and/or 28), and postadministration (day 42) to identify inter- and intragroup differences. Adverse events will be recorded to evaluate the safety of L. plantarum P9. Discussion. The protocol will provide methodological guidance for other similar studies, avoiding methodological bias and ultimately facilitating the formulation of consensus on the use of probiotics as a constipation-relieving agent. In addition, the results are more comprehensive than those of existing studies and may objectively and scientifically reflect the effectiveness of L. plantarum P9 on constipation. If the expected study findings are obtained, L. plantarum P9, taken as a probiotic, may become a complementary choice for chronically constipated patients. This trial is registered with Chinese Clinical Trial Registry (ChiCTR) (no. ChiCTR2000038396) registered on November 22, 2020, https://www.chictr.org.cn/showproj.aspx?proj=54024.


Background
Constipation is a common diagnosis made by gastroenterologists based on the assessment of infrequent bowel movements (<3 per week) and difficult stool passage, while patients may report multiple symptoms, including a sense of incomplete defecation, abdominal pain, bloating, excessive straining accompanied by a sensation of anorectal blockage during stool passage, and requiring manual assistance to release the stool. Acute (or nonchronic) constipation either results in blockage of the intestinal tract that requires surgery [1] or tends to be ignored by patients. Academically, constipation always refers to chronic constipation, which is categorized into primary constipation and secondary constipation (attributed to other diseases or factors). Based on the Rome IV criteria [2], the classifications of primary constipation are functional constipation, irritable bowel syndrome with constipation, and defecatory disorders. From the aspect of the transiting rate of stool movement through colonic and contraction of muscle tissue during defecation, primary constipation is also divided into slow transit constipation, normal transit constipation, and defecatory disorders [3]. e global prevalence of constipation ranges from 10% to 30% [4][5][6][7]. e variance in prevalence may be attributed to the individuals assessed in different surveys, which may be selfreported or use different Rome criteria (I, II, III or IV) to identify participants [5,[8][9][10], thus lacking a concise definition of constipation. Overall, chronic constipation is more frequent in the population with the following characteristics: elderly, female, nonwhite race, medication intake, low income and education level, physical inactivity, and depression [8,[11][12][13][14][15][16][17][18][19][20][21][22][23]. Because patients are constantly suffering from physical symptoms and psychological distress, chronic constipation potentially disturbs people's lives, studies, and work [24,25] due to dyspareunia, sexual dysfunction, urine retention [26], reduced mental health and social function [24,27], school absenteeism, a high number of lost work days, and the cost of medical care [28].
Only one-fifth of patients with constipation seek medical advice [19], and laxatives are the most frequently prescribed agents [29,30]. Laxatives, physical exercise, fibre intake, and dietary management are the traditional treatments. However, approximately 50% of constipated individuals are dissatisfied with their current treatments [31,32]. As many as 74% of nursing home residents have been reported to use laxatives daily [33][34][35], of which only 28%-57% are estimated to be satisfied with the treatment according to the results of two Internet-based surveys [36,37]. Undesirable side effects are a main reason for suboptimal satisfaction. New treatments must be developed for unsatisfied patients, and daily intake of probiotics holds great promise since various probiotics have shown benefits for constipation treatment [38][39][40][41][42][43].
However, although many studies have reported that probiotics are potentially beneficial for constipated individuals [39][40][41][42][43], a clear consensus on the use of probiotics as a constipation-relieving agent has not been reached [44]. is lack of consensus is mainly attributed to the shortage of study methodology in terms of incorrect statistical analysis, inconsistent definitions of constipation and outcomes of the intervention, and an unvalidated assessment technique [44][45][46]. erefore, high-quality randomized controlled trials (RCTs) are needed to reach a more confident consensus [47]. Moreover, RCTs examining the effectiveness of probiotics as a constipation-relieving agent may elucidate a clear mechanism of action for a more confident conclusion. Generally, a balanced intestinal flora provides health benefits to human hosts; conversely, an imbalanced flora may promote the development of constipation [48]. Consistently, probiotics may positively modify the intestinal flora [49,50] and even provide potentially beneficial microorganisms to the host.
us, we designed this protocol to study the effectiveness of L. plantarum P9 on chronic constipation and to guide other studies assessing the effectiveness of probiotics. e correlation between the clinical improvement of symptoms and changes in intestinal flora was also analysed.

Design.
is study is a randomized, double-blind, placebocontrolled clinical trial conducted in Nanchang, China, and volunteers may receive the intervention (oral probiotics or placebo) in their own homes. e protocol was prospectively registered at the Chinese Clinical Trial Registry (ChiCTR) (NO. ChiCTR2000038396) (Appendix 1) and approved by the Ethics Committee of the First Affiliated Hospital of Nanchang University (Approval Number: IIT [2020], Clinical Ethics Review NO. 004) (Appendix 2). is manuscript is prepared according to the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) [51]. e flowchart of the trial process is provided in Figure 1.

Inclusion Criteria.
Eligible patients should fully meet the Rome IV criteria [2]: (1) Onset of the following symptoms for at least 6 months before enrolment and symptoms within the past 3 months that meet the following criteria [2,52]: (1) Two or more of the following symptoms: (a.) difficulty passing stool, at least 25% of defecations; (b.) lumpy or hard stool, at least 25% of defecations (Bristol Stool Form Scale (BSFS) types 1 or 2 (Appendix 3) [53]); (c.) incomplete defecation in at least 25% of defecations; (d.) sense of anorectal obstruction in at least 25% of defecations; (e.) need for manual assistance for defecation (such as using fingers to assist with defecation or pelvic floor support) in at least 25% of defecations; and (f.) fewer than 3 spontaneous bowel movements (SBMs) per week. (2) Loose stool rarely occurs without the use of laxatives.
(3) Insufficient stools are rarely present without the use of laxatives.
(2) Willing to sign the informed consent form (Appendix 4) (3) e volunteers involved in this study will be patients with chronic constipation aged 18-65 years. For patients aged from 18 (exclusive) to 50 (inclusive) years, the result of stool tests (including occult blood) conducted during the screening period must 2 Evidence-Based Complementary and Alternative Medicine be normal or abnormal but determined to be clinically irrelevant by the investigators. For patients aged 50 (exclusive) to 65 (inclusive) years, the result of a colonoscopy performed at a tertiary or higherlevel hospital within the past 6 months must be normal or abnormal but determined clinically irrelevant by the investigators.

Exclusion Criteria.
Volunteers with any of the following conditions will be excluded: (1) Personal or family history of colon cancer, celiac disease, or inflammatory bowel disease (2) Intestinal organic diseases confirmed on a previous colonoscopy (3) Plans to become pregnant or father a child in the next 3 months, or pregnant, or breastfeeding (4) Allergies to samples or ingredients (5) Use of antibiotics or probiotics within the past two weeks (6) Use of antianxiety, antidepressant, or other psychotropic drugs within the past month (7) Need for long-term use of medications for constipation (8) History of severe diseases, such as myocardial infarction, cerebral infarction, and malignant tumour, judged by the investigators as disqualifying conditions (9) Major mental illnesses, inability to control one's actions, or inability to cooperate (10) Illiteracy, inability to understand the informed consent form, or inability to independently sign the informed consent form 2.4. Volunteer Recruitment. e investigators will recruit volunteers from the public through in-person communication, posters, and WeChat promotions. Volunteers can scan a WeChat two-dimensional (QR) code to register for enrolment, submit their personal information, and answer questions related to inclusion and exclusion criteria. Volunteers will undergo a procedure that includes three rounds of screening. First, the investigators will collect and collate the volunteers' registration information and, according to the information provided by registrants, invite potentially eligible candidates to participate in a central consultation with clinical specialists for further Data collection (during the administration period (day 0, 14, and 28) and postadministration observation period (day 42)) Primary outcome: weekly mean frequency of CSBMs Secondary outcomes: weekly mean frequency of CSBMs ≥3, weekly mean frequency of SBMs, weekly mean stool appearance score, weekly mean difficulty of passing stool score, WPUAMA, quality-of-life score, emotional status score, gut microbiome and faecal metabolome, and safety assessment.

Signed informed consent form
Volunteer registration Face-to-face consultation Complete diary daily for 14 days Inclusion and exclusion Evidence-Based Complementary and Alternative Medicine screening and selection of eligible volunteers. Second, during the central consultation, trained study implementers will introduce the trial to volunteers in the form of a video and information sheets describing the main aspects of the trial and discuss the information provided in the video and information sheets. en, volunteers will meet with the clinical specialist to confirm their registration information, especially regarding inclusion and exclusion criteria. ird, volunteers will sign an informed consent form in the presence of a member of the data management team (DMT). en, volunteers will undergo a 14-day screening period when they will not be allowed to take any medicines or health products to improve their constipation symptoms. Each volunteer will be asked to collect one stool sample and complete an online diary entry daily (Figure 2). At the end of the screening period, the diary and stool sample results will be reviewed and used to select eligible volunteers. Subsequent steps will include the formal intervention and follow-up visits ( Figure 1).

Randomization and Blinding
. Eligible volunteers will be assigned a unique serial number (e.g., 001, 002, 003, 004, 005. . .. . .). e unique number will be used as the volunteers' ID throughout the study period to guarantee anonymity and confidentiality. For each of these unique numbers, a random sequence will be generated by the computer software R 4.1.0 and used to randomly assign the unique number (participant) to the probiotic group or the placebo group. During the study, the volunteer's treatment distributors, data collectors, and data analysts will be blinded to the randomization sequence. e randomization sequence will be maintained by an independent project administrator and will only be unblinded in the case of major safety issues or when performing the interim and final data analyses. Moreover, an independent project administrator will label the probiotics or placebo packages with unique numbers corresponding to random sequences in advance to achieve allocation concealment. During the period of administration, the distributor will distribute the treatment packages according to the unique number that corresponds to each participant.

Study
Interventions. e L. plantarum P9 and placebo powders will be manufactured in parallel and independently by Research and Development Department, Jiangzhong Pharmaceutical Company Limited (Nanchang, China). Briefly, the preparation procedure consists of three steps: weighing, mixing, and packaging. L. plantarum P9 powder is composed of 20% maltodextrin, 20% orange powder, 20% maltitol, and 40% L. plantarum P9, while placebo powder consists of 60% maltodextrin, 20% orange powder, and 20% maltitol. Raw materials for L. plantarum P9 and placebo powders are mixed using a Hopper mixer (hit-400) and sealed into 2 g packages. e temperature is controlled at 25 ± 2°C, and the relative humidity is less than 65% during the three steps. In the study, when the product of L. plantarum P9 powder is manufactured, a randomized sample is sent to Jinhua Yinhe Biotechnology Co., Ltd. (https://yinhewdy168.foodmate.net/) for quality inspection. e activity of probiotics is greater than 67 billion colony-forming units (CFUs)/g. e study will include three phases (    Evidence-Based Complementary and Alternative Medicine administration), an observation period during administration, and an observation period after administration. e interventions in each phase will be administered as follows: (1) Period of screening (preadministration observation period) (days 14 to 0): in this phase, volunteers will not receive the intended interventions, i.e., probiotics and placebo. Eligibility screening will be performed as described during volunteer recruitment (Table 1). (2) Observation period during administration (days 0 to 28): (1) Probiotics group: volunteers will take 1 package of L. plantarum P9 powder directly or with warm water (below 40°C) on a full stomach at a dose of 100 billion CFUs per day; if antibiotics must be taken, probiotics should be taken 2 hours later. (2) Placebo group: volunteers will take the placebo in the same manner as the probiotics group. e placebo contains no probiotics but has the same appearance, packaging, and taste as the L. plantarum P9 powder. All remaining probiotics or placebo (unused) and empty (used) packages will be collected at the end of this period to monitor compliance. During the study, both probiotics and placebo will be stored in a cool, dry place away from direct sunlight. (3) Postadministration observation period (days 29 to 42): no probiotics or placebo will be taken.

Prohibited Confounding
Interventions. e following treatments are prohibited during the study: (1) probiotics, prebiotics, and foods containing probiotics (such as yoghurt) other than the Lactobacillus plantarum used in this study; (2) antianxiety, antidepressants, and other psychotropic drugs; and (3) other substances designed to improve intestinal symptoms.
In addition, antibiotics will be monitored during the study, and normal dietary habits will be recommended. All concomitantly used substances/drugs related to defecation should be recorded daily in the online defecation diary (Figure 2), and explanations are needed.

Primary Outcome.
A defecation diary developed with reference to the literature [32,[53][54][55][56] will be completed daily online by the participants (Figure 2). Based on the diary, the changes in primary and some of the secondary outcomes from day 0 (week 0) to day 42 (week 6) will be evaluated.
A CSBM is defined as the ability to achieve a complete bowel movement without the use of any drugs or other auxiliary measures during the previous 24 hours. Based on the diary (Figure 2), the weekly mean frequency of CSBMs from day 0 (week 0) to day 14 (week 2), day 28 (week 4), and day 42 (week 6) will be evaluated [32,54,55] (Table 1).

Secondary Outcomes
(1) Based on the diary (Figure 2), the changes in the percentage of volunteers with a weekly mean frequency of CSBMs ≥3 [32,54,55], changes in the weekly mean frequency of spontaneous bowl movements (SBMs) [32,[54][55][56], weekly mean stool appearance score [53], weekly mean difficulty of passing stool score, and weekly percentage of volunteers who use auxiliary measures to assist with defecation (WPUAMA) will be evaluated each week from day 0 (week 0) to day 14 (week 2), day 28 (week 4), and day 42 (week 6). An SBM is defined as a bowel movement achieved without the use of any drugs or other auxiliary measures during the previous 24 hours [32,54,55] (Table 1). (2) e Patient Assessment of Constipation Quality-of-Life (PAC-QOL) questionnaire (Appendix 5 [56,57]) will be used to evaluate the QOL score on days 0, 14, 28, and 42.  (Table 1). (4) For the indicator of the gut microbiome, DNA will be extracted from the stool samples with the QIAamp Fast DNA Stool Mini Kit (Qiagen, Hilden, Germany) according to the manufacturer's instructions, and the quality of DNA will be examined using agarose gel electrophoresis and a NanoDrop spectrophotometer. Shotgun metagenomic sequencing will be performed on all samples using an Illumina HiSeq 2500 instrument. Libraries will be constructed from DNA fragments with a length of ∼300 bp; paired-end reads will be generated by sequencing 150 bp in the forward and reverse directions. Meanwhile, the metagenomic analysis will include several parts: the analysis of alpha diversity and beta diversity in each group to understand whether the differences in the microbiota compositions of groups are significant and a comparison of the taxonomic characteristics of the study group and the control group at the level of phylum, genus, and species to identify specific genes related to the individual differences of constipation. Metagenomic biological pathway analysis will be used to evaluate the effect of probiotics on the function of gut metagenomics in patients with constipation and to explore the metagenomic biological pathways contributing to the mechanism underlying the effect of probiotics on the treatment of constipation. (5) For the indicator of the faecal metabolome, stool samples will be extracted using the protein precipitation method, and the supernatant will be transferred to sample vials for LC-MS/MS analysis. e original data will be subjected to peak alignment, retention time correction, and peak area extraction using the XCMS-Plus program. e structure of metabolites will be identified by accurate mass matching (<5 ppm) and two-level spectrum matching, and the METLIN database will be retrieved. We will then delete data with missing values > 50% in the group, normalize the data, and conduct 6 Evidence-Based Complementary and Alternative Medicine multidimensional statistical analysis, including unsupervised principal component analysis (PCA), supervised partial least squares discriminant analysis (PLS-DA), and potential differentially abundant metabolite analysis. e metabolomic analysis might further identify the potential differentially abundant metabolites of probiotics in the treatment of constipation, and a correlation analysis between the gut microbiota and metabolites will be performed.

Safety Evaluation.
All related clinical trials have reported that probiotics are safe and do not induce significant adverse events compared with the placebo group. However, four types of adverse reactions related to probiotics should be considered, including symptoms that may be attributed to systemic infections, deleterious metabolic activities, excessive immune stimulation, and gastrointestinal side effects [60], which will be collected as possible adverse events during the study. Severe adverse events are those adverse events leading to study withdrawal, e.g., hospitalization, disability, mortal danger, or death. All adverse events, including symptoms, time of onset, duration, causal relationship to interventions, and measures taken, will be accurately recorded during the study. Any serious adverse events will be recorded along with the corresponding criteria and emergency measures and will be promptly reported to the Medical Ethics Committee of the First Affiliated Hospital of Nanchang University within 24 hours; a separate study report will be prepared. Based on the adverse events observed during the study, the safety of the probiotics will be evaluated as excellent (safe, without any adverse events), good (relatively safe with moderate adverse events that resolve on their own without any specific treatment and do not result in study withdrawal), conditional (adverse events that resolve after certain measures are taken and allow continued participation in the study), or unsafe (adverse events that result in study withdrawal) [61].
2.11. Additional Assessment. Demographic profiles, special situations related to defecation during the study, including changes in dietary habits (e.g., eating spicy or oily foods or drinking), taking antibiotics, and any other reported situation will be recorded and assessed as needed.

Sample Size.
is study is a clinical trial during which volunteers will consume a sample medication or placebo for 28 days and will be followed up for 42 days. e primary outcome measure is the weekly frequency of CSBMs. e expected weekly frequencies of CSBMs on day 28 are 1.1 in the control group and 4.1 in the probiotic group, with a between-group difference of 3. Given a standard deviation S � 6, α � 0.05, and β � 0.20, the sample size should be at least 63 volunteers per group, as calculated with the formula for statistically significant effectiveness. After considering the drop-out rate (20% or lower), the sample size should be 76 or more participants per group. e final sample size will be 100 participants per group for a total of 200 volunteers.

Compliance Monitoring and Withdrawal.
e probiotics and placebo will be supplied as packages distributed in boxes. At the start of the administration period, the appropriate number of probiotic and placebo packages will be distributed to the participants. All used (empty) and unused packaging will be kept by volunteers. At the end of the administration period, these packages will be collected to verify the number of remaining packages and calculate the dosing rate. A dosing rate ≥80% indicates good compliance. A WeChat group will be established for the participants to improve compliance. e study personnel will post messages in the WeChat group to remind participants to take the samples as scheduled and complete the questionnaire. In addition, each participant will receive a reward of 300 RMB once they complete the follow-up.
Volunteers will be withdrawn from the trial for the following reasons: (1) continuous adverse reactions involving systemic infections that cannot be resolved after certain measures are taken, deleterious metabolic activities, excessive immune stimulation, gene transfer, and gastrointestinal side effects, which may be attributed to probiotics [60]; and (2) patients can actively withdraw from the trial at any time for any reason, and subsequent treatment will not be affected.

Data Collection and Collation.
e defecation diary ( Figure 2) completed by the volunteers will be collected online daily, and the DASS-21 (Appendix 6) will be collected online as scheduled (see Table 1). One stool sample for the gut microbiome analysis and one for the metabolomics test will be collected from each volunteer on days 0 (preadministration period), 28 (administration period), and 42 (postadministration period); these samples will be stored separately and sent for tests at a temperature of −80°C in a special stool storage kit from Guangdong Longsee Company (https://www.longseemed.com).
An independent data management team (DMT) will be established to maintain and monitor study quality and safety. e data of individual characteristics, defecation diary entries (Figure 2), and DASS-21 data (Appendix 6) completed online by volunteers will be downloaded as Excel spreadsheets for further analysis. e gut microbiome and metabolomic data from stool samples will be examined by the Key Laboratory of Dairy Biotechnology and Engineering, Ministry of Education. All data will be submitted to the DMT for management and monitoring. e DMT will promptly analyse the data and contact collectors to resolve any uncertainty that occurs. All records that contain names or other personal information that could identify a participant, such as ID number or consent forms, will be stored separately from the study records. e database will be password protected by the DMT. e team will also have the authority to conduct an interim analysis or terminate the study's next step if the following situations occur: (1) if the probiotic does not affect the gut microbiome, then the study team will not conduct tests on the faecal metabolomes; and (2) if any severe adverse events related to the probiotics occur, the DMT will suggest terminating the study.      e intention-to-treat (ITT) analysis will be conducted between the two groups. en, the following four data sets will be used for the per-protocol (PP) analysis: (1) the data from all participants who completed the follow-up and (2) a subgroup analysis regarding levels of compliance (≥80%, 60-80%, and <60%) to validate the robustness of the results. e hypothesis will be tested using the methods described below during the ITT and PP analyses.

Evidence-Based Complementary and Alternative Medicine
First, the Wilcoxon signed-rank sum test will be used for the outcome measures, including the weekly mean frequency of CSBMs, weekly mean stool appearance score, weekly mean difficulty of passing stool score, weekly mean frequency of SBMs, QOL score, and emotional status score, at each observation point to analyse the intergroup differences in each outcome measure. e chi-square (χ 2 ) test will be used for the outcome measures, namely, the percentage of volunteers with a weekly mean frequency of CSBMs ≥3 and WPUAMA, at each observation point. In the analyses, the P value indicating statistical significance will be set to 0.05. en, the Kruskal-Wallis rank sum test will be used for multiple comparisons of outcome measures, including the weekly mean frequency of CSBMs, weekly mean stool appearance score, weekly mean difficulty of passing stool score, weekly mean frequency of SBMs, QOL score, and emotional status score, and the χ 2 test will be used for multiple comparisons of outcome measures, including percentage of volunteers with a weekly mean frequency of CSBMs ≥3 and WPUAMA, at each observation point to identify the intragroup changing trends in consecutive phases, namely, pre-, during, and postprobiotic administration. In the analyses, the P value indicating statistical significance will be adjusted to 0.05/[k (k −1)/2], where k is the number of sample groups included in the comparison.
In addition, the statistical analyses of metagenomic and metabolomic data will be performed using R software (v.4.0.2) and Adobe Illustrator. PCA and PLS-DA will be performed and visualized using the R packages vegan, ggplot, and ggpubr, and the adonis P value will be generated based on 999 permutations. e t-test, Wilcoxon test, and Kruskal-Wallis test will be used to evaluate differences in variables between and within groups; P values will be corrected for multiple testing using the Benjamini-Hochberg procedure. Meanwhile, Pearson's and Spearman's correlation coefficients will be calculated to analyse the correlations between different indicators in clinical, metagenomic, and metabolomic data.

Discussion
Interest in using probiotics as a constipation-relieving agent is increasing, as approximately half of patients have been disappointed with their current treatments. Probiotics hold great promise as a complementary treatment. However, a clear consensus on its recommendation for a patient with constipation is still unavailable because the existing evidence is limited and the methodology used by the existing studies lacks consistency, precluding a comparison of the results. In this context, we developed the protocol for this study.
e key parameters of the methodology used in the previous RCTs and the present RCT are shown and compared in Table 2. e RCTs cited in Table 2 were obtained from recently published systematic reviews or meta-analyses [39-42, 62, 63] that aim to study the effect of probiotics on constipation. Seventy-six RCTs were initially obtained from these systematic reviews or meta-analyses, while 34 RCTs remained after removing 35 duplicate papers and 7 papers that only focused on the effects of probiotics on intestinal flora in healthy adults for the extraction and comparison of key parameters of the methodology ( Table 2). As shown in Table 2, the parameters of the methodology are quite inconsistent. For example, the sample size varies substantially from 8 [64] to 453 [65], and the daily dosage ranges from 10 8 [66] to 10 10 [67] CFUs; importantly, the diagnostic criteria for constipation, study stage and duration, and outcomes are also obviously different. Under these circumstances, a high-quality RCT protocol is needed to promote the consistency of methodology in future research. Based on the optimized parameters used by the 34 RCTs (Table 2) and our understanding of methodology, we designed the present protocol, of which the parameters are listed in the last row in Table 2.
We optimize the methodology from the following aspects: ① e general principle for a high-quality RCT is strictly obeyed through the participation of third-party independent companies or the DMT, including randomization, parallel control, blinding, allocation concealment, combination of ITT and PP analyses, consideration of withdrawals or dropouts, replication (enough observed study subjects), and other factors. ② e sample size (100 for each group) is obtained based on the expected difference in weekly frequency of CSBMs between the treatment group and control group, which is sufficient to ensure the reliability and accuracy of the results. e sample sizes in 33 of the 35 previous RCTs are dozens or even several participants, which is probably not sufficient to obtain an accurate result. ③ e latest Rome criteria (Rome IV for the present RCT) are suggested for the diagnosis of chronic constipation to avoid any other self-definition by researchers. ④ Multidimensional outcomes have been observed in the present RCT, including improvements in clinical symptoms, QOL score, emotional status score, and modifications in the gut microbiome and faecal metabolome, to produce a chain of evidence from causal improvements in metagenomic and metabolomic data to clinical symptoms and then to the emotional state and QOL. e outcomes are measured using accepted methods, e.g., BSFS [53], PAC-QOL [57], DASS-21 [58], next-generation sequencing, and LC-MS/MS analysis. us, the effects of probiotics on chronic constipation in the present study are more reliable and explainable. ⑤ Other key parameters, such as the study stage and duration, are also suggested.
In addition, the protocol is conducted to prove its feasibility and scientificity by studying the effects of L. plantarum P9 on chronic constipation. e study is articulated with a clear testable hypothesis stating that L. plantarum P9 improves the clinical symptoms and patient QOL by modifying the intestinal flora. Additionally, the safety of L. plantarum P9 will be assessed. us, we believe the study will be a high-quality RCT. e results will be more comprehensive than those of existing studies and will objectively and scientifically reflect the effectiveness of L. plantarum P9 on constipation. In addition, the protocol will provide methodological guidance for similar studies, avoiding methodological bias and ultimately facilitating the formulation of a consensus on the use of probiotics as a constipation-relieving agent.

Trial Status.
is study started recruiting patients on October 1, 2020. To date, 130 patients have been enrolled. Enrolment is expected to be completed by May 31, 2022, and all follow-up visits are expected to be completed by July 31, 2022. e protocol is version 2.0, and the date of the edition is 10 September 2020.

Data Availability
Data sharing is not applicable to this trial as no database is generated or analysed for the current study. And when the study is completed, the study results will be released to the public, volunteers, and the general medical community via publishing a journal article, with all related data being available.

Ethical Approval
is study was approved on November 14, 2020, by the Ethics Committee of the First Affiliated Hospital of Nanchang University (no. IIT [2020] Clinical Ethics Review NO. 002).

Consent
All participants will sign informed consent forms for the initial assessment and participation in this study. Disclosure e funders played no role in the design of the study, data collection and analysis, or preparation of the manuscript.

Conflicts of Interest
e authors have no conflicts of interest to declare.

Authors' Contributions
WJL, N-HL, and HPZ conceived and designed this study and supervised the research team. TLX, YML, YX, and LJZ are the main individuals responsible for performing the study, and WFZ, QPX, NY, KXZ, and QNW helped with implementation. TLX drafted the manuscript and participated in the design of the study. YML and XZ assisted in designing the study and drafting the manuscript. All the authors have read and agreed to the final version of the manuscript. Wenjun Liu and Nong-Hua Lu contributed equally to the study.