Sex Differences in Bone Size and Bone Mineral Density Exist before Puberty. The Copenhagen School Child Intervention Study (CoSCIS)

Background The aim of this study was to provide normative data of bone mineral density (BMD; g/cm²) of the forearm and the calcaneus, evaluated by peripheral dual X ray absorbtiometry (DXA), in children aged 6 to 7 years of age and to evaluate the association with anthropometrics and sex. Material and methods 368 boys and 326 girls with a mean age of 6.7 ± 0.4 years participated. BMD was measured by DXA in the forearms and the os calcanei, with average values presented in this report. Measurements of weight, height, skinfolds, the width of distal radius and ulna, and the femur condyles were collected and body composition estimated from skinfolds measurements. Results There was no difference in calcaneus BMD when comparing boys and girls, whereas the boys had 4.5% (0.013 g/cm²) higher forearm BMD than the girls (P < 0.001). Calcaneal BMD (mean 0.318 g/cm²) was 11% higher than forearm BMD (mean 0.283 g/cm²). Linear relationship was found between calcaneus BMD and weight (partial r = 0.50), Fat free mass (FFM) (partial r = 0.50), Fat mass (FM) (partial r = 0.45), % body fat (partial r = 0.29) and knee width (partial r = 0.46), all P < 0.000 respectively. Adjusted for weight the relationship between calcaneus BMD and FFM, FM, %body fat and knee width disappeared. There were significant relationships between the forearm BMD and weight (partial r = 0.37), FFM (partial r = 0.39), FM (partial r = 0.28), %body fat (partial r = 0.14) and wrist width (partial r = 0.24), all P < 0.000 respectively. Adjusted for body weight, the relationship remained between forearm BMD and FFM (r = 0.10), FM (R = −0.10) and % body fat (r = −0.12), all P < 0.000 respectively. Children measured in the spring had 3.5% (P < 0.01) higher calcaneus BMD than children measured in the winter. Conclusion Seven year old boys have higher BMD in the forearm but not in the calcaneus in comparison with girls of a similar age. Body weight is the best predictor of calcaneus BMD, accounting for 25% of the variance whereas body weight and FFM are the best predictors of forearm BMD, each accounting for 17% of the variance, respectively.