Cancer Nutrition Study - A Study of Dietary Interventions in Cancer Patients Treated With Immune Checkpoint Inhibitors

The goal of this clinical trial is to learn whether changing the diet can help improve gut health and support recovery from immune-related side effects caused by immune checkpoint inhibitor (ICI) cancer treatment in adults with solid tumors who develop moderate to severe immune-related side effects that require steroid treatment. The main questions it aims to answer are: Does a high-fermented food diet or a high-fiber resistant starch supplement change the gut microbiome (the bacteria and other microorganisms that live in the digestive tract) during treatment? Does either dietary approach improve gut microbiome diversity and function by the end of the 7-week intervention period? Researchers will compare participants assigned to a high-fermented food diet with participants assigned to high-fiber resistant starch supplementation to see if one approach leads to greater improvements in the gut microbiome and related biological measures. Participants will: Be randomly assigned to 1 of 2 dietary intervention groups: High-fermented food diet High-fiber resistant starch supplement Follow their assigned diet for 7 weeks, starting when their ICI treatment is restarted. Provide stool samples before the intervention begins and regularly throughout the study. Have blood samples collected at the start of the study and approximately every 3 to 4 weeks during participation. Provide skin and mouth swab samples at the beginning of the study and again at Week 7. Complete food diaries and symptom assessments during the study. Participate in a 2-week follow-up period after completing the dietary intervention. Researchers will also evaluate the safety of the dietary interventions, whether immune-related side effects return, how the immune system responds, and how closely participants follow their assigned diet.