The FGF23–Undercarboxylated Osteocalcin Axis in Predialysis Chronic Kidney Disease: Linking Mineral Homeostasis and Bone Turnover

Introduction: FGF23 and osteocalcin are important regulators of mineral and bone metabolism. The association between FGF23 and undercarboxylated osteocalcin (Glu-OC) in chronic kidney disease (CKD) remains insufficiently understood. This study aimed to investigate the association between FGF23 and Glu-OC and assess whether it is independent of kidney function. Methods: This cross-sectional study included 80 adults: 12 healthy controls and 68 patients with non-dialysis CKD stages G1–G4. FGF23, Glu-OC, calcium, phosphate, PTH, and bone metabolism markers were measured. Associations were assessed using Spearman correlation, partial Spearman correlation adjusted for eGFR, and multivariable linear regression. Results: FGF23 was inversely correlated with eGFR (ρ = −0.346; p = 0.004) and positively correlated with creatinine (ρ = 0.299; p = 0.013). FGF23 was inversely associated with Glu-OC (ρ = −0.344; p = 0.004), with the association becoming stronger after adjustment for eGFR (partial ρ = -0.477; p < 0.001). In complementary multivariable models, the inverse association between FGF23 and Glu-OC persisted: Glu-OC was associated with FGF23 (β = −0.258; p = 0.024), along with ALP (β = 0.369; p = 0.001), while FGF23 was associated with Glu-OC (β = −0.266; p = 0.020), along with calcium (β = −0.366; p = 0.002). Conclusion: FGF23 is independently and inversely associated with Glu-OC in non-dialysis CKD, and this relationship persists after adjustment for kidney function. These findings support an association between FGF23 and Glu-OC in CKD–mineral and bone disorder (CKD-MBD).