Effects of High-Fiber Diet on Gut Microbiota, Metabolism, and Immune Microenvironment in Solid Tumor Patients: A Clinical Study

Cancer remains a major global public-health challenge and a central focus of medical research. According to the International Agency for Research on Cancer (IARC, 2020), 19.29 million new malignant tumors and 9.96 million cancer deaths occurred worldwide, \>90 % being solid cancers. Lung cancer alone accounted for 2.2 million new cases and 1.8 million deaths; \>75 % of patients were already at an advanced stage at diagnosis. Current options for late-stage solid tumors are limited: surgery is often impossible because of metastasis; cytotoxic chemotherapy produces dose-limiting toxicities (grade Ⅲ-Ⅳ myelosuppression 15-40 %, mucositis 50-80 %); radiotherapy risks pneumonitis (5-15 %) or enteritis (5-20 %) when tumors abut vital organs; targeted agents succumb to acquired resistance after a median 9-13 months; and immune-checkpoint inhibitors achieve \<40 % objective response with 7-15 % grade 3-4 immune-related adverse events. Dietary intervention is therefore emerging as a promising adjunct. Dietary fibre protects against cardiovascular and metabolic diseases, yet intake is universally low. WHO and the Chinese Nutrition Society recommend 25-30 g total fibre per day (≈15-21 g insoluble), whereas Chinese adults consume only \~11 g insoluble fibre. High-fibre diets reshape gut microbiota, augment short-chain fatty acid (SCFA) production, strengthen intestinal barrier function, activate CD8⁺ T cells and dampen regulatory T cells, thereby enhancing anti-tumour immunity. A melanoma cohort showed improved progression-free survival under immunotherapy when fibre intake was high. Similar microbiota-immune axes may operate in colorectal and other solid cancers, but clinical data are scarce. We therefore propose a study to examine whether a high-insoluble-fibre diet (\>21 g/day) modulates gut-microbiota composition, metabolite profiles and peripheral-blood immune subsets in solid-tumour patients, and to evaluate consequent effects on treatment response and quality of life. The findings will clarify whether fibre-driven microbiota-immune crosstalk can be harnessed as a personalised nutritional strategy to improve cancer outcomes.