High bone density in adolescents with obesity is related to fat mass and serum leptin concentrations

Obesity has been associated with increased bone mass, but the mechanisms involved are still poorly understood. We aimed to explore the relation between bone mineral density and factors known to influence bone formation in obese and lean adolescents.

We recruited 24 obese and 25 lean adolescents in a case-control study. Total body bone mineral density (TB-BMD) z scores and body composition were determined using dual-energy x-ray absorptiometry. We measured 25-hydroxyvitamin D (25-OH-D), glucose, insulin, and leptin concentrations. Physical activity (PA) level was quantified using accelerometer.

TB-BMD z score was higher, whereas 25-OH-D and PA levels were lower in obese compared with lean subjects (TB-BMD z score 1.06 ± 0.96 vs 0.26 ± 0.91, P = 0.004; 25-OH-D 9.9 ± 6.4 vs 18.5 ± 7.4 ng mL, P < 0.001; PA level 308.3 ± 22.1 vs 406.8 ± 29.2 count min, P = 0.01). TB-BMD z score was not related to 25-OH-D or PA levels, but was positively correlated with leptin concentration and fat mass (P < 0.05). Vitamin D concentration was negatively correlated with fat mass (P < 0.001).

Despite lower serum vitamin D and PA levels, BMD was higher in adolescents with obesity and associated with higher serum leptin concentrations. Furthermore, adolescents with obesity have lower vitamin D serum concentrations than lean controls, probably owing to its distribution in adipose tissue.