Dietary Fiber and Melanoma: Exploring Microbiome-Driven Immune Modulation

Background/Objectives: The gut microbiome influences melanoma biology and response to immune checkpoint inhibitors. Dietary fiber is a key modifiable factor that shapes the microbial composition and metabolite production. This review summarizes mechanistic, preclinical, and clinical evidence describing how fiber and fiber-responsive taxa may affect melanoma immunity and treatment outcomes. Methods: A literature search of MEDLINE, Embase, and Scopus identified studies published within the past five years examining dietary fiber, gut microbiome interactions, immune modulation, or melanoma outcomes. After screening 491 unique records, 49 peer-reviewed mechanistic, preclinical, observational, and interventional studies were synthesized qualitatively in this narrative review. Results: Fiber fermentation produces short-chain fatty acids that regulate dendritic cell activation, T-cell priming, and cytokine signaling. Preclinical melanoma models show that fibers such as inulin and β-glucan enhance IFN-γ-driven antitumor immunity, increase CD8⁺ infiltration, and improve checkpoint blockade efficacy in a microbiota-dependent manner. In humans, fiber-rich diets and enrichment of taxa such as Bifidobacterium, Faecalibacterium, and Akkermansia are associated with improved PD-1 inhibitor responses, longer progression-free survival, and possible reductions in ICI-related colitis. Although epidemiologic studies suggest no clear association between fiber intake and melanoma incidence, dietary fiber intake appears to correlate strongly with treatment-related outcomes. Conclusions: Dietary fiber represents a potentially safe and plausible adjunct to melanoma immunotherapy. However, study variability and emerging counterevidence highlight the need for controlled trials to clarify causality and define optimal fiber-based interventions.