AGE-RELATED-CHANGES IN SERUM UNDERCARBOXYLATED OSTEOCALCIN AND ITS RELATIONSHIPS WITH BONE-DENSITY, BONE QUALITY, AND HIP FRACTURE

The effect of the degree of carboxylation of osteocalcin (OC) on the p roperties of bone is unclear. The aim of this study was to relate seru m concentrations of total OC (tOC) and undercarboxylated OC (ucOC), me asured with a two-site immunoassay, to bone mineral density (BMD) at t he femoral neck and ultrasonic transmitted velocity (UTV) at the os ca lcis in 257 women aged 60-99 years, 22 of whom had sustained a hip fra cture. There was an increase in tOC (r = 0.19, P = 0.003) and ucOC (r = 0.20, P = 0.002) with age. No significant difference in tOC or ucOC between subjects with and without hip fracture was found. Serum tOC wa s negatively correlated with femoral neck BMD (r = -0.23, P = 0.0001) and os calcis UTV (r = -0.29, P = 0.0001) and partial correlations ind icated that these relationships were independent of age. Serum ucOC al so correlated negatively with os calcis UTV (r = -0.21, P = 0.001) and less strongly with femoral neck BMD (r = -0.13, P = 0.052). After adj usting for age, only the relationship between ucOC and os calcis UTV r emained significant (r = -0.16, P = 0.017). It is concluded that in wo men over 60 years, the increase in tOC reflects an age-related rise in bone remodeling, whereas the increase in ucOC reflects an age-related fall in vitamin K status. The stronger relationship of ucOC with UTV than BMD suggests that the rise in ucOC may perhaps relate more to cha nges in bone quality than mineral content. Higher serum ucOC concentra tions in subjects with a history of hip fracture could not be confirme d.