Investigating the association between reproductive characteristics, injury history, physical fitness, and bone mineral density in female Canadian Armed Forces members

Introduction: This study examined the relationship between reproductive health, body composition, physical fitness, and bone mineral density (BMD) among female Canadian Armed Forces (CAF) members. Methods: Ninety female CAF members aged 18 to 55 years completed a demographics and reproductive health survey, objective InBody body composition assessments (e.g., height, weight, fat mass/lean mass), a blood draw (e.g., plasma), and physical testing (e.g., VO _2max ). The UltraScan650 (CyberLogic Inc., Brooklyn, NY) was used to measure BMD at the one-third radius position. Analysis of variance (ANOVA) examined differences in BMD across reproductive variables. Pearson correlations and forward selection multivariate linear models were used to examine the association of predictor variables with BMD. The estradiol-to-progesterone ratio was forced into the model. Results: Members who have given birth had higher BMD than nulliparous members ( p = 0.031). Those using hormonal birth control (HBC) showed a trend toward higher BMD ( p = 0.069). A two-way ANOVA indicated an interaction between parity and HBC use ( F = 4.718, p = 0.033), with parous HBC use having the highest BMD (adjusted mean = 0.730, 95% CI 0.717-0.743). VO _2max (β = 0.003, p <0.001) and the estradiol-to-progesterone ratio (β = 0.127, p = 0.041) were significant predictors of BMD, explaining 38.3% of the variance. Discussion: The interaction between parity status and the use of HBC is linked to higher BMD among female CAF members. Further, physical fitness (VO _2max ) remains the largest influencer of BMD in female CAF members; however, the ratio of estradiol to progesterone is associated with BMD at the one-third radius position.