Relationship Between Hormonal Variables and Bone Mineral Density, Muscle Force, and Fat Mass in Peripubertal Girls

The muscle-bone unit represents an evolutionary system, in which both of its components are under the common control of the insulin-like growth factor I (IGF-I), sex hormones, and vitamin D. The mutual interactions between these hormones maintain integrity, growth and maturation of pubertal bone mass. Thus, insufficiency of any of these hormones will negatively influence development of the skeleton during puberty. The aim of the study as to analyse the correlation between muscle mass, total bone mineral content (BMC), bone mineral density (BMD) of the lumbar spine (BMD L1-L4), and serum or urine hormones. Total BMC (g) and areal BMD L1-L4 (g/cm2 and Z-score) as well as muscle mass and fat mass (g) were assessed by means of dual-energy X-ray absorptiometry (DXA). The Z-score is the number of standard deviations a patient´s BMD which differs from the average BMD of their age, sex, and ethnicity. This Parameter is used in children. Muscle force (N) was measured using a dynamometer. The simple correlations showed strong positive associations between BMC or BMD L1-L4 (g/cm2) and serum phosphate, estradiol, insulin-like growth factor (IGF-I), leptin and fat masses, and muscle force (P < 0.001 for all parameters). Positive correlations were also observed between BMD and serum phosphate (P < 0.01), IGF-I (P < 0.01), estradiol (P < 0.001), leptin (P < 0.01), fat and lean mass (P < 0.001 and P < 0.001, respectively) and muscle force (P < 0.001). The partial correlations, after eliminating the impact of height, Tanner stage, and physical activity level, confirmed positive relationships between either BMC or BMD L1-L4 and lean mass (P < 0.001 and P < 0.001, respectively) and fat mass (P < 0.001 for BMC and BMD). Furthermore, a positive relationship was observed between serum leptin and both BMC and BMD (Z score) (P < 0.05 and P < 0.05, respectively). After removing the effects of height, Tanner stage, and physical activity, positive associations were observed between lean mass and IGF-I (P < 0.01), leptin levels (P < 0.05), and muscle force (P < 0.01). On the basis of the study results, it can be expected that low values of lean or fat mass, and insufficient production of IGF-I or leptin, could negatively influence bone development in pubertal girls.