Exercise-induced gut microbiome modulation, mucosal immunity, and potential implications for infection resistance

This narrative review elucidates the contemporary evidence concerning the interplay between exercise, defined here as planned and structured physical activity, and the gut microbiota, with a particular focus on microbial diversity, compositional alterations, and functional outcomes pertinent to host physiology. Evidence from human observational and exercise-intervention studies suggests that structured exercise training may elicit modest and sometimes transient modifications in gut microbial composition and metabolic activity. Such alterations are frequently manifested in functional pathways associated with energy metabolism, microbial motility, and fermentation capacity, rather than substantial and enduring shifts in individual taxa. In numerous instances, enhancements in advantageous metabolites, including short-chain fatty acids, have been documented; however, responses exhibit considerable variability among lean, obese, and metabolically compromised populations. Notably, certain studies propose that microbial modifications may wane or partially revert following the discontinuation of exercise, thereby underscoring the adaptability of the gut ecosystem. Evidence derived from preclinical models provides additional support for the hypothesis that physical activity and exercise training may play significant roles in modulating gut microbial communities as well as intestinal barrier functionality; however, the outcomes exhibit variability contingent upon the intensity of exercise and the specific experimental conditions employed. Moderate exercise has been associated in some studies with improved gut barrier-related measures and shifts toward a more favorable microbial profile, while excessive or high-intensity exercise may temporarily impair intestinal permeability and provoke stress-induced alterations in the microbial composition. Importantly, evidence linking exercise-induced microbiome alterations directly to reduced incidence or severity of clinical infection in humans remains limited. Available evidence more strongly supports associations with mucosal immune and intestinal barrier-related phenotypes and, in some experimental models, pathogen colonization resistance than direct protection against clinical infection. In conclusion, the existing body of literature cautiously endorses the notion that exercise can affect the composition and functionality of gut microbiota; nonetheless, these effects are context-dependent, exhibit variability in their magnitude, and remain incompletely elucidated. The extent to which exercise-associated microbial changes translate into clinically meaningful infection resistance in humans remains unresolved. Future longitudinal studies should incorporate standardized exercise protocols, controlled or carefully characterized dietary conditions, repeated assessments of the gut microbiome and metabolome, pathogen-specific outcomes, and clinically defined infection endpoints, and sufficiently long follow-up periods to determine the persistence, reversibility, and clinical relevance of exercise-associated microbial changes.