A narrative review of the gut microbiota-testis axis: mechanisms and implications for male reproductive health.

Background and Objective: Infertility affects approximately 8-12% of couples worldwide, with male factors accounting for nearly half of all cases. The gut microbiota is widely recognized as a key regulator of host physiology, yet its specific role in male reproductive health remains to be elucidated. This narrative review synthesizes current evidence on the "gut microbiota-testis axis" and provides a critical evaluation of the strength of evidence derived from different study designs. Methods: We conducted a comprehensive literature search in PubMed/MEDLINE up to May 2026 using Boolean search strategies combining terms related to the gut microbiota (e.g., "Gastrointestinal Microbiome", "gut microbiota", "gut microbiome") with terms related to male reproduction (e.g., "Semen Analysis", "Spermatozoa", "Infertility, Male", "semen quality", "sperm quality"). Separate search strategies were developed for human and animal studies. Literature screening and data extraction were performed independently by two reviewers according to predefined inclusion and exclusion criteria. Key Content and Findings: Although human studies remain preliminary, several genera, including Conclusions: The gut microbiota-testis axis has emerged as a critical physiological pathway linking the microbiome to male reproductive function. This review provides an evidence-based framework for understanding this axis and highlights key knowledge gaps that must be addressed. Future research should integrate large-scale longitudinal cohorts, mechanistic exploration, and randomized controlled trials with fertility outcomes as primary endpoints to advance the field from descriptive correlations toward mechanism-driven clinical translation.