Osteocalcin-insulin relationship in obese children: a role for the skeleton in energy metabolism

Objective Osteocalcin is a bone-specific protein secreted by osteoblasts and often used as a bone formation biomarker. Rodent studies have reported a hormonal role of osteocalcin on glucose metabolism, increasing insulin secretion and sensitivity and increasing energy expenditure. However, it is unknown whether osteocalcin fulfils the same function in humans. Methods We investigated the relationship between serum osteocalcin and insulin concentrations in 27 prepubertal obese children (9-12 years old) randomly divided into two groups, one of which entered a physical training programme, and 16 nonobese control children. Whole body bone mineral density (WB-BMD), serum osteocalcin, circulating insulin and adiponectin were measured at baseline and after 6 months. Results Trained and untrained obese children had higher WB-BMD than controls at baseline. Trained children also displayed a significant insulin increase and a significant adiponectin decrease while osteocalcin was increased compared to untrained obese children. Significant linear correlations between WB-BMD and adiponectin, delta BMD (variation between baseline and after-training values) and delta adiponectin, insulin and osteocalcin, delta insulin and delta osteocalcin, delta insulin and delta under-carboxylated osteocalcin were found only in trained obese children with no significant relationship in control and untrained obese children. Conclusions In trained obese children, correlations indicate that when BMD is increased, osteocalcin is increased and insulin lowered. This suggests that increased BMD is associated with increased energy metabolism and a decreased level of insulin. We thus report statistically significant relationships between the skeleton (osteocalcin) and energy metabolism (insulin), suggesting a regulatory hormonal loop including osteocalcin and insulin.