Immune and environmental determinants of diabetic bone fragility: bone microenvironment remodeling, organ crosstalk, and therapeutic perspectives

Diabetic bone disease is increasingly recognized as a complication in which fracture risk exceeds what bone mineral density (BMD) predicts: in type 2 diabetes mellitus (T2DM) despite preserved or elevated BMD, and in type 1 diabetes mellitus (T1DM) beyond its BMD deficit. We re-examine this problem through the immune-metabolic axis, using the gut-bone axis as an organizing framework that links the bone microenvironment to multi-organ communication, and we assign every mechanistic claim an explicit evidence level. Gut dysbiosis, increased intestinal permeability and loss of microbial metabolites are treated as candidate upstream amplifiers rather than established drivers: hyperglycemia-induced barrier failure and metabolic endotoxemia are documented in diabetes, whereas the short-chain fatty acid (SCFA)-regulatory T cell-WNT10B pathway and gut-to-marrow T-cell trafficking have been shown principally in non-diabetic rodents, and no longitudinal or interventional study has yet linked diabetes-associated dysbiosis to a skeletal endpoint. We summarize how advanced glycation end products (AGEs), oxidative stress, NF-κB/NLRP3 signaling, AMPK-mTOR imbalance and epigenetic memory suppress bone formation while priming osteoclastogenic signaling, and we propose a stage- and context-dependent model of remodeling uncoupling that reconciles this signaling with the low bone turnover measured in human T2DM. The osteoimmune niche is then considered across four compartments: primed macrophage-osteoclast signaling, Th17/Treg disequilibrium, stromal-cell fate shift with marrow adiposity, and osteocyte dysfunction. We set out the diabetes-specific molecular alterations of the gut-bone axis, including GLUT2-dependent barrier failure, convergence of lipopolysaccharide-TLR4 and AGE-RAGE signaling on NF-κB, and loss of SCFA-GPR43/GPR109A signaling. Environmental and systemic determinants, including metabolic dysfunction-associated steatotic liver disease and stage-dependent chronic kidney disease, are integrated with five bone-organ axes (pancreas, gut, adipose, liver, kidney) to show how skeletal injury becomes embedded in multi-organ inflammation. Therapeutic directions are grounded in clinical evidence and safety: licensed antiresorptive agents retain their efficacy in T2DM, whereas microbiota-directed, immune-cell and epigenetic strategies remain experimental. Framing diabetic bone disease as evidence-graded osteoimmune remodeling, with dysbiosis treated as a candidate amplifier, may refine risk assessment, biomarker development and the design of trials that test combination hypotheses.