Randomized trial to assess FMT efficacy in improving bowel function for major LARS patients. 40 subjects, blinded, randomized to FMT or probiotics. Pre-post 16S sequencing, 4-week follow-up for bowel symptoms, 8-week microbiota analysis.

This study is a rigorous single-center, prospective, double-blinded, randomized controlled trial, aiming to evaluate the effectiveness of fecal microbiota transplantation (FMT) in improving bowel function for patients with major low anterior resection syndrome (LARS). A total of 40 subjects are planned to be recruited and randomly assigned to the experimental group and the control group at a 1:1 ratio. Prior to treatment, all subjects will undergo 16S sequencing of intestinal microbiota to assess their intestinal microecological status. During the treatment, a colonoscopic colonic catheter will be placed via the anus, with the experimental group receiving FMT solution matched to the patient's microbiota for intra-intestinal administration, while the control group will receive a standardized probiotic formula. To ensure the impartiality of the study, both patients and researchers administering the medication will be blinded to the trial drugs. The study will continuously observe and record bowel symptoms after 4 weeks of treatment as well as 16S sequencing results of intestinal microbiota after 8 weeks. Upon the completion of the entire study, the blinding will be revealed, and the results will be analyzed.

Inclusion Criteria:

- Age ≥ 18 years old, regardless of gender; Patients with rectal cancer who have undergone total mesorectal excision (TME) and sphincter-preserving surgery, and the postoperative period is more than 3 months; Patients with a LARS score of 21 to 42; Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1; Expected survival period ≥ 1 year; Voluntarily sign a written informed consent form.

Exclusion Criteria:

- Patients with pre-operative symptoms of fecal and urinary incontinence; Patients who have undergone pelvic surgery or LAR due to non-cancerous reasons; Patients who have undergone pelvic radiotherapy; Patients with prophylactic ostomy; Patients who have used any antibiotic medication, probiotic products, or prebiotic products in the past 4 weeks; Patients who have regularly used laxatives in the past two weeks; Pregnant or lactating women; Human immunodeficiency virus (HIV) positive; Known history of active pulmonary tuberculosis (TB). Subjects suspected of having active TB require chest X-ray, sputum examination, and exclusion based on clinical symptoms and signs; Untreated patients with chronic hepatitis B or HBV carriers with HBV DNA exceeding 500 IU/mL, or patients with active hepatitis C should be excluded. Non-active HBsAg carriers, treated and stable hepatitis B patients (HBV DNA < 500 IU/mL), and cured hepatitis C patients can be enrolled. For subjects with positive HCV antibodies, only those with negative HCV RNA test results are eligible to participate in the study; Known history of psychiatric drug abuse, alcoholism, and drug abuse; Patients with cognitive impairment; Any situation where the investigator believes the participant should be excluded from the study.