Diet and Immune Effects Trial: DIET- A Randomized Double Blinded Dietary Intervention Study in Patients With Metastatic Melanoma Receiving Immunotherapy

Inclusion Criteria: 1. Age ≥ 18 years old. 2. Body mass index (BMI) 18.5-40 kg/m\^2 3. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 4. English-speaking 5. Self-reported willingness to exclusively eat the provided diets 6. Self-reported willingness to comply with scheduled visits, undergo venipuncture, and provide stool samples 7. Cohort-specific: * 1 Adjuvant Melanoma: i. Resected Stage II-IV melanoma with planned initiation of adjuvant anti-PD1 +/- anti-CTLA4 or anti-LAG3 * 2 Unresectable Melanoma: i. Histologically confirmed unresectable stage III or Stage IV melanoma with planned initiation of standard of care anti-PD1 +/- CTLA4 or anti-PD1 +/- LAG3 immunotherapy and no prior immunotherapy in the metastatic setting. * 3 Neoadjuvant Melanoma: i. Histologically confirmed stage III/IV melanoma with planned initiation of neoadjuvant anti-PD1 +/- anti-CTLA4 or anti-LAG3 1. Participants must have archival tissue block available or be willing to undergo a newly-obtained core needle or incisional biopsy at baseline. Fine needle aspiration is not acceptable. * 4 Unresectable RCC: i. Unresectable clear-cell renal cell carcinoma with planned initiation of standard of care anti-PD1 +/- anti-CTLA4 immunotherapy Exclusion Criteria: 1. History of \>= grade II colitis or diarrhea on immunotherapy or any ongoing colitis or diarrhea of any grade 2. Unresolved \>= grade III immune-related adverse event on immunotherapy (other than endocrinopathy requiring hormone replacement) 3. History of active inflammatory bowel disease or major gastrointestinal surgery (not including appendectomy or cholecystectomy) within 3 months of enrollment or any history of total colectomy, or bariatric surgery (bariatric surgery which does not disrupt the gastrointestinal lumen, i.e. restrictive procedures such as banding, are permitted). 4. Medical contraindications to intervention diet as determined by the treating physician 5. Self-reported major dietary restrictions related to the intervention 6. Diagnosis of diabetes mellitus type I or type II that requires medical treatment or random glucose \> 200 mg/dL 7. Antibiotic use within 21 days of planned start of equilibration diet (self-reported and/or noted by the treating physician) 8. Has a condition requiring systemic treatment with either corticosteroids (\> 10 mg daily prednisone equivalents) or other immunosuppressive medications within 14 days of study intervention administration. Inhaled or topical steroids, and adrenal replacement doses \> 10 mg daily prednisone equivalents are permitted in the absence of active autoimmune disease 9. Regularly taking probiotics, fiber supplements, or any other medication or supplement that could affect study outcome as determined by the principal investigator and unable/unwilling to discontinue for the purpose of the study. These agents must be discontinued at least 14 days prior to start of diet 10. Currently consuming an average estimated daily fiber intake exceeding 20 grams based on the results of the preliminary dietary assessment; vegetarian or vegan 11. Current smoker or heavy drinker (defined as \> 14 drinks per week) or current self reported illicit drug use 12. Uncontrolled concurrent illness or infection or psychiatric illness/social situations that would limit compliance with study requirements 13. Unable or unwilling to undergo study procedures 14. Plan for travel during the study that would preclude adherence to prescribed diets 15. Cognitively impaired adults