Suppressed bone turnover in obesity: a link to energy metabolism? A case-control study.

Context: Observations in rodents suggest that osteocalcin (OC) participates in glucose metabolism. Based on human studies, it remains unclear whether circulating OC is simply a bone turnover marker (BTM) or also a mediator in interactions between the skeleton and glucose homeostasis.
Objective: The objective of the study was to determine the responses of BTMs, including OC, to oral glucose tolerance test (OGTT) in a case-control setting.
Design and Patients: Thirty-four normoglycemic young adults [mean age 19 y (SD 2.3)] with severe childhood-onset obesity and their gender- and age-matched nonobese controls underwent a standard 2-hour OGTT.
Main Outcome Measures: Glucose, insulin, and six BTMs including total and carboxylated OC (cOC) were determined at baseline and at 30, 60, 90, and 120 minutes during OGTT.
Results: The obese and control subjects were similar in height; the mean body mass indices were 40.4 and 21.9 kg/m(2), respectively. The homeostasis model assessment index was 2.7 times greater in the obese subjects. All BTMs, except bone-specific alkaline phophatase, were lower in the obese subjects compared with the controls: the differences at baseline were 40%, 35%, 17%, 31%, and 32% for N-terminal propeptides of type I collagen, cross-linked telopeptides of type I collagen, tartrate-resistant acid phosphatase, total OC, and carboxylated OC (P < .05 for all) after adjusting for whole-body bone area. All BTMs decreased during OGTT. The relative values for the OGTT responses for total, but not for cOC (measured as area under the curve) differed between the two groups (P = .029 and P = .139, respectively): the decrease in total OC during the OGTT was less pronounced in the obese subjects. Responses in other BTMs were similar between the groups. No associations were observed between glucose metabolism and OCs during OGTT with linear regression.
Conclusions: Bone turnover markers were substantially lower in obese subjects compared with controls. Total OC and cOC showed less pronounced decrease during the OGTT in obese subjects compared with controls, whereas other BTMs responded similarly in the two groups. The role of OC, if anything, in glucose homeostasis is indirect and may be mediated via other factors than glucose or insulin.