Osteocalcin is independently associated with body mass index in adolescent girls

Objective: Osteocalcin is a bone-related protein, recently found to correlate with body mass index (BMI), waist circumference, fat percentage and metabolic syndrome in adults. The aim of this study was to determine the relationship between osteocalcin and BMI in adolescence, a time of significant bone accrual, while considering possible confounders related to bone and body composition. Methods: We analyzed data from 160 female adolescents (mean age 15.1 ± 0.7 years), which were divided into tertiles by osteocalcin levels. Across these three groups, we examined the differences in BMI with relation to age, total daily energy intake, calcium intake, physical activity (PA), total body bone mineral density, parathyroid hormone (PTH), 25(OH)-vitamin D, bone alkaline phosphatase and body fat percentage. Results: Mean BMI values differed significantly between participants in the three osteocalcin tertiles, including after adjustment for age, PA, PTH, energy and calcium intakes. Post-hoc analysis revealed that girls in the highest osteocalcin tertile, had a significantly lower BMI than those in the two lower ones (19.3 ± 2.2 vs. 20.6 ± 3.0 and 20.7 ± 2.9 kg m-2, respectively, P = 0.018). There was no significant difference in energy and calcium intakes, bone mineral density, 25(OH)-vitamin D levels and PTH between study groups. Conclusions: In female adolescents, BMI is inversely related to osteocalcin, even after consideration of several factors that may affect bone and fat mass. As bone mineral density, 25(OH)D and PTH did not differ between groups, it is possible that the relation between osteocalcin and BMI could be unrelated to bone tissue itself.