Neoadjuvant Immune Checkpoint Blockade + a Prebiotic Food-enriched Dietary Intervention to Optimize Immune Response in Melanoma: NEO-PreFED

Inclusion Criteria: 1. Age ≥ 18 years old. 2. Body mass index (BMI) 18.5-45 kg/m2 3. Able to self-complete study assessments monitoring diet and gastrointestinal symptom burden 4. ECOG performance status of 0 or 1 5. Histologically confirmed resectable stage IIIB-D or oligometastatic Stage IV cutaneous melanoma or locally invasive/advanced mucosal melanoma as determined by multidisciplinary review 6. Planned initiation of standard-of-care neoadjuvant ipilimumab +/- nivolumab or nivolumab +/- relatlimab 7. Measurable disease per RECIST 1.1 8. WOCP must have negative UPT within 1 week of beginning dietary intervention. 9. Self-reported willingness to eat the provided foods (with some tailoring to their food preferences) 10. Self-reported willingness to comply with scheduled visits, undergo venipuncture, provide stool samples. 11. Archival tissue specimen or planned to undergo tumor biopsy as part of standard of care treatment. Exclusion Criteria: 1. Uveal melanoma 2. History of inflammatory bowel disease, total colectomy, or bariatric surgery 3. Currently taking steroids \> prednisone 10 mg/day or equivalent 4. Medical contraindications to the intervention diet as determined by the treating physician 5. Self-reported major dietary restrictions, including but not limited to relevant food allergies, celiac disease, or diets such as vegan, ketogenic, extended fasting 6. Insulin-dependent diabetes or conditions requiring bile-acid sequestrants 7. Unable or unwilling to undergo study procedures 8. Intravenous (IV) antibiotic \>1 dose in the past month or 1 dose IV/oral antibiotic use in past 2 weeks 9. Regularly taking supplements containing prebiotics, fiber and/or probiotics, and unable/unwilling to discontinue for the purpose of the study 10. Current smoker or heavy drinker (defined as \>14 drinks per week) or current illicit drug use 11. Currently pregnant, planning to become pregnant, or lactating 12. Concurrent malignancy requiring systemic therapy other than hormonal therapy 13. Cognitively impaired adults