Bone Abstracts

Introduction: Fibroblast growth factor (FGF23) is a hormone derived from osteoblasts and osteocytes being involved in calcium and phosphate homeostasis, where serum iron (S-Fe) has been suggested as a potential mediator of FGF23 regulation. The aim was to determine whether iron status is a determinant of FGF23 in elderly men.

Methods: The MrOS (osteoporotic fractures in men is a population based study of elderly men, in the Gothenburg part, (median age of 75.3, range 70.5–81.0 years) 955 men without ongoing medication or supplementation with iron were included. Serum intact FGF23 was analyzed with a two-site monoclonal antibody based ELISA (Kainos Laboratories International; Tokyo, Japan). Baseline data included serum levels of intact FGF23, S-Fe, transferrin saturation (TS) and ferritin before and after adjusting for potential confounders.

Results: Log FGF23 correlated (age adjusted) negatively with S-Fe (r=−0.15, P<0.001), TS (r=−0.16, P<0.001), and ferritin (r=−0.07, P=0.03). There was a significant difference in mean S-Fe between quartiles 1 and 3 of FGF23 compared with quartile 4 (20.0 μmol/l vs 18.5 μmol/l, P<0.001), still significant after further adjustment for age, BMI, Cystatin C, hs-CRP, 25 hydroxyl vitamin D, phosphate and PTH (P=0.045). Subjects with TS<15% (3.2%, 31/955) had higher mean FGF23 compared with subjects with TS≥15% (59.0 μmol/l vs 46.1 μmol/l, P=0.008), still significant after adjustment for age (P log=0.007).

Multiple stepwise linear regression analyses, (adjusted for age, BMI, smoking, cystatin C, hs-CRP, 25 hydroxyl vitamin D, phosphate, calcium, PTH, erythropoietin, hemoglobin, total cholesterol, were performed in three separate models. S-Fe and TS were then independent predictors of FGF23 (standardized β −0.11, P<0.001 and −0.10, P<0.001 respectively) whereas ferritin was not.

Conclusions: Low S-Fe is in old men associated with high levels of intact FGF23, independently of markers for inflammation and renal function, suggesting an iron related pathway in FGF23 regulation.