Exercise and the Gut Microbiome: From Mechanisms to Clinical Applications

<b>Background/Objectives:</b> The gut microbiome is a critical regulator of host metabolism, immunity, and the gut-brain axis. Exercise is a promising non-pharmacological modulator of microbial ecology, yet human evidence remains heterogeneous and the translational gap persists. This narrative review synthesizes mechanisms, human and animal evidence, and future directions for the exercise-gut microbiome axis. <b>Methods:</b> PubMed, Scopus, Web of Science, and SID were searched for articles published between January 2000 and February 2025. Keywords included exercise, physical activity, gut microbiome, gut microbiota, short-chain fatty acids, and gut-muscle axis. From 218 initial records, 89 original studies (47 human, 42 animal) met inclusion criteria and were critically appraised. <b>Results:</b> Exercise modulates the gut microbiome via splanchnic hypoperfusion, hyperthermia, altered transit time, and immune-mediated barrier regulation. Moderate-intensity continuous training consistently increases alpha diversity and enriches butyrate-producing taxa (<i>Faecalibacterium prausnitzii</i>, <i>Roseburia hominis</i>) and mucin-degrading <i>Akkermansia muciniphila</i>. High-intensity interval training transiently increases intestinal permeability in untrained individuals but, following adaptation, stimulates butyrate production via lactate cross-feeding metabolism-a recent breakthrough. Effects are transient and reversible upon detraining. Animal models establish causality through fecal microbiota transplantation; human randomized controlled trials demonstrate modest, intensity-dependent, and highly individualistic responses. Emerging evidence supports the gut-muscle axis in sarcopenia and personalized exercise prescription guided by microbiome profiling. <b>Conclusion:</b> Exercise shows promise as a low-cost modulator of the gut microbiome for enriching health-associated taxa and improving metabolic outcomes. Definitive evidence linking exercise-induced microbial shifts to enhanced athletic performance in humans remains lacking. Future research requires diet-controlled randomized controlled trials with ≥12-week interventions, shotgun metagenomics, and mechanistic validation of the gut-muscle axis in humans.