The aim of the study is to evaluate the gut microbiome (i.e. bacteria, viruses, and fungi that reside in the gut) of people undergoing abdominal surgery, evaluate whether specific diets can change the gut microbiome, and, if so, whether those changes translate into better surgical outcomes.

Standard of care: patients undergoing major abdominal colorectal surgery are sometimes advised to eat healthfully prior to surgery, but without more specific recommendations regarding their diet.

Research Activities:

1. Screening: eligible patients will be identified through screening the clinic schedules.

2. Enrollment: in person or remotely, after the patient is determined to require major abdominal surgery, they will be invited to participate in the study. If they consent to participate, they will be randomized to receive either low-fat/high-fiber diet, high-fermented diet, or continue with their normal diet for 10-14 days prior to the surgery.

3. Intervention: at baseline, data will be collected regarding clinical information of the patients (age, body mass index, comorbidities, medications or supplements, physical activity levels, antibiotic or probiotic use in the past six months), and baseline dietary pattern, which will be evaluated through application of a validated Food Frequency Questionnaire (DHQ III). Patients in the intervention arms (low- fat/high-fiber diet or high-fermented diet) will receive meal samples during the 10-14 days preceding surgery. Patients in the control arm will receive a $25 grocery voucher. Adherence to the diet will be assessed through regular check ins with the patients.

   Stool samples will be collected in five timepoints: at the beginning of the study, after the dietary intervention, during surgery, in the first week after the surgery, and between 21-30 days after surgery. The stool samples will be sent for analysis of the microbiome.

   A fragment of the resected bowel will also be sent for analysis of the mucosal-associated microbiome.

4. Closeout: After surgery, all patients will receive the same standard dietary protocol and care, and will be followed for 30 days without any further intervention.

Inclusion Criteria:

• undergoing major abdominal colorectal surgery with intestinal resection in 2 or more weeks

Exclusion Criteria:

• patients who do not speak English or Spanish
• houseless patients
• decisionally impaired patients
• presence of ileostomy prior to the surgical procedure
• surgery without intestinal resection