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 properties of bone is unclear. The aim of this study was to relate serum concentrations of total OC (tOC) and undercarboxylated OC (ucOC), measured with a two-site immunoassay, to bone mineral density (BMD) at the femoral neck and ultrasonic transmitted velocity (UTV) at the os calcis in 257 women aged 60-99 years, 22 of whom had sustained a hip fracture. 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 was 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 indicated that these relationships were independent of age. Serum ucOC also 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 adjusting for age, only the relationship between ucOC and os calcis UTV remained significant (r = -0.16, P = 0.017). It is concluded that in women 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 changes in bone quality than mineral content. Higher serum ucOC concentrations in subjects with a history of hip fracture could not be confirmed.