Targeting the Gut–Bone Axis with Prebiotics, Probiotics, Synbiotics, and Postbiotics: From Mechanistic Plausibility to Clinical Bone Outcomes

Osteoporosis is a major skeletal disorder. Increasing evidence suggests that gut microbiota-derived mechanisms may contribute to bone remodeling. The biological rationale for a gut–bone axis is well supported, but the clinical relevance of microbiome-targeted interventions remains uncertain. In this narrative review, we focus on prebiotics, probiotics, synbiotics, and postbiotics. We searched in the literature the progress from mechanistic observations to measurable skeletal outcomes in humans. We searched PubMed, Scopus, and Web of Science for literature published mainly between 2021 and 2026. We included also earlier studies when they were important for the mechanistic framework. The evidence was evaluated according to intervention type and outcome. We focused on bone turnover markers, bone mineral density, and the duration of exposure required for these outcomes to become clinically meaningful. Recent randomized trials and meta-analyses were also compared in a semi-quantitative synthesis to show where findings are consistent and where they remain conflicting.