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

This study is testing whether eating a diet rich in prebiotics (foods that feed beneficial gut bacteria) can improve how well immunotherapy drugs work against advanced melanoma (a serious skin cancer) by boosting the gut microbiome. **You may be eligible if...** - You are 18 or older - You have been diagnosed with advanced stage III or IV melanoma that cannot be surgically removed - You are about to start first-line immunotherapy (anti-PD1 or similar drug) - Your BMI is between 18.5 and 45 - You speak English and can return to MD Anderson for follow-up visits **You may NOT be eligible if...** - You have previously received immunotherapy in the metastatic setting (prior adjuvant use may be allowed) - You have symptomatic brain metastases Talk to your doctor to see if this trial is right for you.