Lean mass and not fat mass is associated with male proximal femur strength

Obesity is suspected to confer protection against fracture, but evidence is mixed. We examined proximal femur geometry and body composition measures in a diverse group of 1171 men (30-79 yr of age). Analyses showed that nonbone lean mass, but not fat mass, is independently associated with measures of proximal femur density, axial and bending strength, and resistance to buckling. Introduction: Obesity is often said to confer protection against fracture, but the mechanisms driving such an association remain poorly understood. We hypothesized that the effect of increased body mass on bone structure would be accounted for by total and/or appendicular nonbone lean mass, and that once these trends were removed, fat mass would show no protective influence. To test this hypothesis, we examined body composition and geometric indices of proximal femur strength in an ethnically diverse (black, Hispanic, and white) sample of randomly selected men, 30-79 yr of age. Materials and Methods: Data were obtained from N = 1171 community-dwelling subjects enrolled in the cross-sectional Boston Area Community Health/Bone study. Body composition was obtained by DXA. Hip geometry parameters at the narrow neck, intertrochanter, and shaft were obtained using Hip Structural Analysis of DXA images. These measures included BMD, bone material in cross-sections (cross-sectional area), bending strength (section modulus), and propensity to buckle under compression (average buckling ratio). Analyses controlled for age, race/ethnicity, height, and physical activity. Results: In exploratory analyses, lean mass, fat mass, and BMI were each positively associated with hip strength. However, controlling for lean mass was sufficient to remove the positive, and induce a negative, association for fat mass or BMI. Associations between lean mass and hip strength were strongest and resistant to control for other measures. Lean mass alone was sufficient to account for a substantial proportion of racial/ethnic difference in hip strength measures, whereas fat mass exhibited no comparable explanatory power. Conclusions: The positive association between relative weight and proximal femur strength is accounted for by lean mass, suggesting that, in men, the protective effect of BMI in preventing fracture is mediated not by adipose tissue but by the influence of increased muscle mass accompanying elevated BMI.