Advances in Exercise-Mediated Regulation of the Gut Microbiota via the Muscle–Gut Axis: Implications for Tumor Immunity and Treatment Responses

The gut microbiota can influence tumor immunity and treatment responses through microbial metabolism, intestinal barrier regulation, and immune–inflammatory signaling, but whether exercise engages these mechanisms through the muscle–gut axis remains uncertain. This review integrates evidence from exercise physiology, microbial metabolism, and tumor immunology to examine how exercise-associated host signals may reshape the intestinal ecological niche and microbial function. Three candidate muscle–gut axes are proposed: the myokine–enteroendocrine–substrate delivery–short-chain fatty acid (SCFA) axis, the exercise-associated lactate–microbial cross-feeding–propionate axis, and the muscle-derived endocrine signaling–intestinal epithelial repair–hypoxic niche axis. Separate studies support exercise-associated IL-6/GLP-1/PYY regulation and gastrointestinal transit, lactate entry into the intestinal lumen and lactate-associated microbial remodeling, Veillonella atypica-mediated propionate production, and irisin/apelin-related epithelial repair. Together, these pathways may influence microbial metabolism, barrier homeostasis, and immune–tumor interactions. However, tumor-related links remain incomplete, and none has been validated as a complete causal chain in a single tumor-bearing exercise model. Relatively complete preclinical evidence comes from mouse melanoma, where endurance exercise enhanced microbial folate-dependent one-carbon metabolism and formate output, with microbiota-derived formate promoting CD8+ T-cell antitumor activity and immune checkpoint inhibitor efficacy. Regular exercise with an appropriate load and adequate recovery may support microbial and intestinal barrier homeostasis, whereas excessive or prolonged exercise with inadequate recovery may impair barrier integrity. Human evidence remains limited and largely associative and does not establish microbiota-mediated improvements in tumor immunity or treatment responses. These candidate axes therefore require causal validation in tumor-bearing exercise models and prospective human studies.