The goal of this observational study is to evaluate the effectiveness and safety of exclusive enteral nutrition (EEN) in adults with active Crohn's disease (CD), particularly in patients with complicated disease such as stricturing disease, enteric fistula, and intra-abdominal abscess. The main questions it aims to answer are:

• What is the clinical remission rate at Week 12 in adults with active CD treated with EEN?
• How does EEN affect clinical response, endoscopic outcomes, inflammatory markers, nutritional status, BMI, and safety during follow-up?

Participants will:

• start EEN at baseline and be followed through Week 12;
• receive EEN as the main treatment approach during the study period;
• complete clinical, laboratory, nutritional, and safety assessments at prespecified follow-up visits;
• undergo endoscopic assessment when endoscopy is performed as part of routine care; and
• if clinically indicated, some participants with large intra-abdominal abscesses may receive percutaneous drainage and necessary antibiotic treatment.

This is a single-center, prospective observational cohort study in adults with active Crohn's disease (CD) who are treated with exclusive enteral nutrition (EEN) during the study period. The study is designed to evaluate the effectiveness and safety of EEN in adult patients with active CD, with a focus on patients with complicated disease, including stricturing disease, enteric fistula, and intra-abdominal abscess.

Eligible participants will be enrolled at the time EEN is initiated (baseline) and followed through Week 12. EEN will serve as the main treatment approach during the study period. In general, no other therapeutic medications for CD will be used. However, for some participants with large intra-abdominal abscesses, percutaneous drainage and necessary antibiotic treatment may be provided when clinically indicated. EEN-related management, including formula type, administration route, caloric targets, and duration, will be recorded.

The primary observation time point is Week 12. The primary endpoint is the clinical remission rate at Week 12. Secondary endpoints include the clinical response rate, endoscopic remission rate, endoscopic response rate, mucosal healing rate, proportion of participants achieving normalization of inflammatory markers, and improvement in body mass index (BMI). Endoscopic outcomes will be assessed when endoscopy is performed as part of routine care. Clinical, laboratory, nutritional, and safety assessments will be collected at prespecified follow-up visits.

Exploratory endpoints include longitudinal changes in the gut microbiome, metabolomic profiles, transcriptomic features, and candidate molecular biomarkers, as well as their associations with clinical and nutritional improvement after EEN treatment.

Inclusion Criteria:

1. Age ≥18 years.
2. Diagnosis of Crohn's disease established on the basis of overall clinical assessment, including compatible clinical history and standard endoscopic, histologic, and/or radiologic findings, as determined by the treating physician. Histologic confirmation at baseline is not required if endoscopy or biopsy is not feasible or clinically inappropriate because of severe disease, poor nutritional status, or intra-abdominal abscess/sepsis.
3. Active Crohn's disease at baseline, as determined by the treating physician.
4. Willingness to initiate and receive exclusive enteral nutrition (EEN) as the sole induction therapy as part of physician-directed routine care.
5. Presence of malnutrition or nutritional risk and clinical indication for EEN.
6. Patients with intestinal complications, including enteric fistula, intestinal stricture, and/or intra-abdominal abscess, are eligible if considered appropriate for EEN-based management by the treating physician.
7. Ability and willingness to provide written informed consent and to comply with study assessments and follow-up for 12 weeks.

Optional clarifying note:

In participants without histologic confirmation at baseline, the diagnosis may be further confirmed during follow-up when clinically feasible, including by endoscopic biopsy or surgical pathology.

Exclusion Criteria

1. Any absolute contraindication to enteral nutrition, including but not limited to gastrointestinal perforation, uncontrolled gastrointestinal bleeding, severe hemodynamic instability/shock, or other conditions where enteral feeding is not clinically appropriate.
2. Immediate need for emergency surgery at baseline.
3. Inability or unwillingness to receive EEN as the sole induction therapy at baseline.
4. Any condition that, in the investigator's opinion, would make participation unsafe or would substantially interfere with study assessments or follow-up.

Adult patients (≥18 years) with active Crohn's disease treated at a single center who initiate exclusive enteral nutrition (EEN) as the sole induction therapy and are followed for 12 weeks. The study population primarily includes patients with malnutrition or nutritional risk, and many have complicated disease with intestinal fistula, stricture, and/or intra-abdominal abscess.

Individual participant data (IPD) will not be shared because this is a single-center study involving detailed clinical, imaging, endoscopic, surgical, and pathology data from patients with complicated Crohn's disease, and there is a potential risk of participant re-identification despite de-identification. Data sharing is also subject to local ethics approval, institutional policy, and participant consent.