This prospective, single-center, single-arm, open-label clinical study aims to evaluate whether autologous fecal microbiota transplantation can improve bowel and anal function after ileostomy reversal in patients with rectal cancer who underwent radical rectal cancer surgery with protective loop ileostomy. Eligible patients will receive autologous fecal microbiota transplantation before ileostomy reversal. The primary outcome is the Low Anterior Resection Syndrome score at 3 months after ileostomy reversal. Secondary outcomes include fecal incontinence score, colonoscopic findings before ileostomy reversal, gut microbiota profiles, inflammatory markers, anorectal manometry, pelvic floor electromyography, and safety outcomes.

Patients with rectal cancer who undergo radical resection with protective loop ileostomy may experience diversion-related changes in the distal bowel, gut microbiota dysbiosis, diversion colitis, and impaired bowel function after ileostomy reversal. Autologous fecal microbiota transplantation may help restore the intestinal microbial environment in the defunctioned bowel and improve postoperative bowel function.

In this study, eligible patients will undergo standard radical rectal cancer surgery with protective loop ileostomy. At approximately 8 weeks after surgery, patients without severe postoperative complications such as anastomotic leakage will undergo anal swab sampling for 16S rRNA gene sequencing. Autologous fecal microbiota transplantation will be performed once weekly at postoperative weeks 8, 9, 10, and 11. At approximately 12 weeks after surgery, colonoscopy will be performed to evaluate the distal bowel mucosa, and intestinal secretion samples will be collected for 16S rRNA gene sequencing. Ileostomy reversal will then be performed according to routine clinical practice. Bowel function will be followed for 3 months after ileostomy reversal, with the primary endpoint being the LARS score at 3 months after reversal.

Inclusion Criteria:

1. Age 18 to 75 years.
2. Histologically or clinically confirmed rectal cancer with the lower tumor margin less than 12 cm from the anal verge.
3. Undergoing radical rectal cancer surgery with protective loop ileostomy.
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
5. Able to understand the study procedures and willing to provide written informed consent.

Exclusion Criteria:

1. History of inflammatory bowel disease, bacterial colitis, or other preoperative colonic inflammatory disease.
2. Requirement for emergency surgery due to intestinal obstruction, perforation, bleeding, or other emergency conditions.
3. Pregnant or lactating women.
4. Presence of clinically significant gut microbiota dysbiosis before enrollment, as judged by the investigator.
5. Planned postoperative radiotherapy, chemotherapy, or chemoradiotherapy.
6. Poor compliance, inability to tolerate the intervention, or judged by the investigator to be unsuitable for participation in this clinical study.
7. Severe postoperative complications such as anastomotic leakage.

Individual participant data will not be shared. The study is a single-center clinical study with a relatively small sample size and includes sensitive clinical information, surgical details, functional outcomes, and microbiome sequencing data. Even after de-identification, there may remain a potential risk of participant re-identification. In addition, the informed consent form did not include specific permission for public sharing of individual-level participant data. Aggregate study results will be reported in publications or presentations.