Ketogenic Dietary Intervention to Improve Response to Immunotherapy in Patients With Metastatic Melanoma and Metastatic Kidney Cancer

Inclusion Criteria: * Males and females, age \>= 18 years * Clinical site A will include patients with confirmed diagnosis of metastatic melanoma (including those with brain metastases) receiving first line treatment with combination nivolumab and ipilimumab or single agent ipilimumab, nivolumab, pembrolizumab * Clinical site B will include patients with confirmed diagnosis of metastatic renal cell carcinoma (RCC) (including those with brain metastases) receiving first line treatment with combination nivolumab and ipilimumab or single agent PD-1 inhibitor (e.g., nivolumab, pembrolizumab) * Scheduled for imaging every 6 to 12 weeks for metastatic melanoma (MM) and mRCC as is standard of care per National Comprehensive Cancer Network (NCCN) guidelines * Able to read, understand, and provide written informed consent * Willing to provide stool specimen for research studies as outlined in the timeline * Willing to participate in a ketogenic diet (KD) Exclusion Criteria: * Individuals \< 18 years of age * Unable or unwilling to provide consent * Patients with type 1 diabetes mellitus or type 2 diabetes using insulin * Patients who are clinically underweight (body mass index \[BMI\] \< 18.5) at the start of treatment * Other active malignancy (other than adequately treated and cured basal or squamous cell skin cancer, curatively treated in situ disease, or any other cancer from which the patient has been disease free \>=5 years) * Currently consuming a low-carbohydrate (\< 130 g/day) or ketogenic diet or done so in the last 6-months * Women who are known to be pregnant are excluded. However no additional pregnancy testing out of what would be recommended prior to initiating anti-cancer therapy will be performed solely for this study * Patients currently participating in an interventional or therapeutic clinical trial involving the use of active anti-cancer therapy