Physical Activity Associations with Bone Mineral Density and Modification by Metabolic Traits

OBJECTIVE: To assess the relationship of physical activity with bone mineral density (BMD) at various sites and examine potential modifying metabolic factors. METHODS: Responses from physical activity questionnaires were used to determine total physical activity (PA), moderate physical activity (mod-PA), and sedentary time. Regression analyses were performed to evaluate association of activity traits with insulin sensitivity by euglycemic clamp, adiponectin, C-reactive protein (CRP), and plasminogen activator inhibitor-1 (PAI-1) in 741 healthy subjects. RESULTS: The cohort was relatively sedentary. Activity level was associated with arm, pelvis, and leg BMD in univariate analyses. In multivariate association analyses of arm BMD, only female sex (β = -0.73, P < 0.0001) and adiponectin (β = -0.076, P = 0.0091) were significant. Multivariate analyses of pelvis BMD found independent associations with body mass index (BMI) (β = 0.33, P < 0.0001), adiponectin (β = -0.10, P = 0.013), female sex (β = -0.18, P < 0.0001), sedentary time (β = -0.088, P = 0.034), PA (β = 0.11, P = 0.01), and mod-PA (β = 0.11, P = 0.014). Age (β = -0.10, P = 0.0087), female sex (β = -0.63, P < 0.0001), BMI (β = 0.24, P < 0.0001), and mod-PA (β = 0.10, P = 0.0024) were independently associated with leg BMD. CONCLUSIONS: These results suggest that BMD increases with physical activity in the arms, legs, and pelvis and is inversely related to sedentary time in the pelvis and legs; these associations may be modified by age, sex, BMI, and adiponectin, depending on the site, with physical activity being more important to pelvis and leg BMD than arm BMD and sedentary time being important for pelvis BMD. Moreover, we demonstrated that CRP, PAI-1, and insulin sensitivity play a minor role in BMD.