High-Fiber Diet Helps Immunotherapy

A controlled MD Anderson trial found that melanoma patients on a high-fiber diet showed stronger signs of benefiting from immunotherapy. (mdanderson.org) The Phase 2 DIET study randomized patients 2-to-1 to a high-fiber diet or a healthy control diet while they started immune checkpoint blockade, a drug treatment that takes the brakes off immune cells so they can attack tumors. The trial enrolled 45 patients and targeted 30 to 50 grams of fiber a day in the high-fiber arm versus 20 grams in the control arm. (clinicaltrials.gov; aacrjournals.org) By weeks 5 to 8, blood tests showed fewer circulating classical monocytes in the high-fiber group, with a reported p-value of.0059, and more effector-memory T cells, the immune cells that remember and rapidly attack targets they have seen before. The American Association for Cancer Research abstract also reported lower macrophage and neutrophil abundance scores and higher T-cell activation signaling in the high-fiber arm. (aacrjournals.org) MD Anderson said those monocytes were immunosuppressive, meaning they can help blunt an anti-cancer immune response during treatment. The center said patients who began or were early in immunotherapy and ate the high-fiber diet had this lower-monocyte pattern alongside improved treatment response. (mdanderson.org) The idea behind the trial is that food changes the gut microbiome, the mix of bacteria and other microbes in the digestive tract, and those microbes can shape how the immune system behaves. The DIET protocol paper calls gut microbiome modulation a strategy for improving response to immune checkpoint blockade in melanoma. (pmc.ncbi.nlm.nih.gov) This line of research did not start with the new trial. In a 2021 Science-linked analysis led by National Cancer Institute and MD Anderson researchers, melanoma patients with sufficient fiber intake had longer progression-free survival than patients with insufficient fiber intake, and every 5-gram increase in daily fiber was associated with a 30% lower risk of progression or death. (nih.gov; mdanderson.org) The newer DIET study went further by controlling the meals instead of relying only on food questionnaires. According to the protocol, MD Anderson’s Bionutrition Core prepared isocaloric, macronutrient-controlled meals and snacks for up to 11 weeks, so the main planned difference between groups was fiber intake. (pmc.ncbi.nlm.nih.gov; clinicaltrials.gov) Researchers also reported a separate AACR 2026 analysis showing the high-fiber diet changed bile acid metabolism in blood samples, with reductions in several secondary bile acids that may affect anti-tumor immunity. That result adds another possible mechanism linking diet, gut microbes, and immunotherapy response. (aacrjournals.org) The study is still a small Phase 2 trial, and ClinicalTrials.gov lists it as active but not recruiting as of January 30, 2026. The result puts a concrete number on a simple question cancer doctors have been asking for years: whether changing what patients eat can change how well immunotherapy works. (clinicaltrials.gov; pmc.ncbi.nlm.nih.gov)