M. A. Petit1, T. J. Beck2, H. Lin*1, C. Bentley*1, T. Lloyd1. 1Dept. of Health Evaluation Sciences, Penn State University, Hershey, PA, USA, 2Dept. of Radiology, Johns Hopkins University, Baltimore, MD, USA.

We used longitudinal data from the 10-year Penn State Young Women’s Health Study to establish determinants of adult bone mass, structure and strength. 112 participants were enrolled in the study at age 12 and measurements made bi-annually for 6 years and annually thereafter. Proximal femur DXA scans (Hologic QDR 2000) were taken from age 17-22y and analyzed using a hip structure analysis program to assess areal BMD (g/cm2), subperiosteal width, cortical thickness, bone axial strength (cross-sectional area, CSA) and bone bending strength (section modulus) at the femoral neck and femoral shaft regions. Height and weight were measured by standard procedures and body composition taken from DXA total body scans. Other variables were measured at each time point and the average of all data from age 12-22y used including: calcium and total calorie intake from 3-day food records; estradiol and testosterone values converted to log transformed and then standardized scores; sports exercise score by questionnaire; and age at menarche. Backward regression models were used to assess determinants of change in bone variables and absolute bone values at age 21-22y, controlling for height and weight in each model. Models were run both with and without lean mass. Femoral neck section modulus (+3.5%) and width (+1.6%), but not BMD (-0.8%), increased significantly from age 17-22y. At the shaft, all variables increased (+1.0-4.0%, p < 0.01) from age 17-22y. After controlling for changes in body weight and height, none of the variables contributed significantly to predicting change. Absolute bone values at age 22y were predicted primarily by lean mass, sports exercise scores, and average estradiol levels (controlling for height and weight). At both the neck (r2 = 0.48, p < 0.01) and the shaft (r2 = 0.67, p < 0.01), only lean mass predicted section modulus. When lean mass was removed from the model, sports exercise score replaced lean mass as a predictor at both neck (r2 = 0.41, p < 0.01) and shaft (r2 = 0.61, p < 0.01) sites. For neck cortical thickness, BMD, and CSA, both estradiol and sports exercise history were significant predictors (r2 = 0.20-0.40, p < 0.01), and for bone width testosterone (negative) and lean were significant (r2= 0.48). Results were similar for each geometric variable at the shaft site. These data suggest bone adapts its bending strength primarily to mechanical loading either through lean body mass or adolescent exercise.