The gut-bone axis in osteoporosis: microbial mechanisms, conflicting evidence, and translational priorities

Microbial effects on bone depend on the host's hormonal and inflammatory state, the microbial exposure and the tissue that receives it. This critical narrative review examines the gut-bone axis through those relationships, separating microbial associations from tested mechanisms and product-specific clinical effects. In mice, short-chain fatty acids can suppress osteoclast differentiation through metabolic reprogramming or promote bone formation through regulatory T-cell control of WNT10B production by CD8+ T cells. Intestinal T-cell trafficking also contributes to trabecular bone loss in an estrogen-deficiency model. These experiments identify causal links within defined systems; they do not establish equivalent pathways in patients. Bile acids, indole derivatives and bacterial extracellular vesicles provide further candidates, although evidence for tissue exposure and immune mediation varies. Human cohorts associate microbial features with bone density and incident fracture, but no consistent taxonomic signature has emerged across populations. Functional profiles may aid mechanistic interpretation, but their reproducibility across populations remains uncertain. Probiotic and synbiotic trials report benefit, no detectable effect or greater bone loss at particular sites. Discordance persists even between studies using the same strain accessions, making population, formulation and endpoint essential to interpretation. We distinguish microbial target engagement from bone-turnover responses, structural skeletal measures and fracture outcomes. The clinical evidence reviewed is insufficient to establish that microbiota-directed interventions reduce fragility-fracture risk. The recent, context-specific qualification of total-hip bone density as a surrogate nevertheless matters for future trial design. The most useful next studies will measure exposure and host response together, test the necessity of proposed mediators and prospectively examine host factors that may modify treatment response. This approach identifies where microbial mechanisms could complement established osteoporosis care without treating biological plausibility as clinical efficacy.