PURPOSE: The purpose of this analysis was to examine the associations of body composition and physical activity (PA) with markers of bone health and to additionally explore the influence of age, race and sex.
METHODS: Cross-sectional analyses were conducted between measures of body composition, PA, and markers of bone health from two population-based surveys: National Health and Nutrition Examination Survey (NHANES; 2003-2004 & 2005-2006), and the Third Generation Cohort of Framingham Heart Study (FHS). Participants in each survey were included if they had DXA scans (bone mineral density (BMD) g/cm2, fat mass (FM) kg, and lean mass (LM) kg); accelerometer derived PA (moderate-to-vigorous-PA; MVPA and steps), and had complete data for all covariates. The main dependent variable in these analyses was BMD; exposure/predictor variables were LM, FM, MVPA, and steps/day. Nested hierarchical modeling was conducted unadjusted Model 1, Model 2 was adjusted for demographic covariates (age, sex, race, and height), and Model 3 was additionally adjusted for behavioral covariates (smoking status, daily calcium intake, and MVPA). Effects of independent variables were assessed using regression coefficients. Receiver operating characteristic (ROC) curve analyses were applied to calculate relationships between MVPA and steps with BMD. MVPA and steps entered as dichotomized variables whether the guidelines were met, adjusting for the same covariates mentioned above.
RESULTS: BMD was negatively related with FM and positively related with LM (p\u3c0.001). Significant coefficients for FM with BMD increased slope in men {β(SE)=-0.004(0.002)} while women decreased {β(SE)=-0.002(0.001)} when analyzed separately by sex. For PA models, positive relationships were found between steps/day and BMD in NHANES women (p=0.001), but not in FHS. A positive relationship between MVPA and BMD was found in both FHS and NHANES women (p\u3c0.04). To predict high BMD approximately 279-385 MVPA mins/week or 14,246-14,245 steps/day is needed.
CONCLUSION: LM displayed a consistent relationship with BMD throughout all models and separate analyses, which could be suggestive of a systemic effect on bone health. Relationships between FM and BMD varied between sex warranting more investigation. Although association between PA guidelines and BMD was positive, the current guidelines may be insufficient in eliciting osteogenic benefits
