Prebiotic Food-enriched Diet (PreFED) to Enhance the Microbiome and Response to First-line Immunotherapy in Unresectable Melanoma

Inclusion Criteria: * Age ≥18 years old * English-speaking * Body mass index (BMI) 18.5-45 kg/m2 * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Histologically confirmed stage III/IV, unresectable cutaneous, uveal, acral or mucosal melanoma. Asymptomatic brain metastases are allowed. * Planned initiation of 1st line standard-of-care approved immune checkpoint blockade (anti-PD1 +/- anti-CTLA4 or anti-LAG3 inhibitors or talimogene laherparepvec T-VEC) in the metastatic setting. Prior targeted therapy or ICB in the adjuvant setting is allowed. * Returning to MD Anderson for restaging and follow-up (ICB treatment may occur locally) * Measurable disease per RECIST 1.1 or RANO criteria. Up to 10 patients without measurable can be enrolled. * WOCP must have negative UPT within 1 week of beginning dietary intervention. * Self-reported willingness to eat the provided foods (with some tailoring to their food preferences) * Self-reported willingness to comply with scheduled visits, undergo venipuncture, provide stool samples. Exclusion Criteria: * Previous ICB treatment in the metastatic setting * History of inflammatory bowel disease, total colectomy, or bariatric surgery. * Currently taking steroids \> Prednisone 10 mg/day or equivalent * Medical contraindications to the Intervention Diet as determined by the treating physician. * Self-reported major dietary restrictions, including but not limited to relevant food allergies, celiac disease, or diets such as vegan, ketogenic, extended fasting. * Insulin-dependent diabetes or condition requiring bile acid sequestrants * Unable or unwilling to undergo study procedures. * IV antibiotic use in the past month or oral antibiotic use in past 2 weeks. * Regularly taking supplements containing prebiotics, fiber and/or probiotics, and unable/unwilling to discontinue for the purpose of the study. * Current smoker or heavy drinker (defined as \>14 drinks per week) or current illicit drug use. * Currently pregnant, planning to become pregnant, or lactating. * Concurrent malignancy requiring systemic therapy other than hormonal therapy. * Cognitively impaired adults