Lean Mass and Fat Mass as Mediators of the Relationship Between Physical Activity and Bone Mineral Density in Postmenopausal Women

Background: The relationship between physical activity (PA) and bone health is well known, but the role of lean mass (LM) and fat mass (FM) in this relationship remains uncertain. Therefore, the aim of this study was to examine the mediating effect of LM and FM on the relationship between PA and bone mineral density (BMD) in postmenopausal women. Materials and Methods: This cross-sectional study involved 282 postmenopausal women aged between 50 and 65 year, who were randomly selected from Hongqi community of Harbin City in China. PA was measured using an International PA Questionnaire. Body composition, BMD of the lumbar spine, hip, and total body were measured using dual-energy X-ray absorptiometry. Mediation analysis was performed to investigate the mediating effect of LM and FM on the relationship between PA and BMD. Results: In partial correlation analysis, PA, LM, and FM were positively related to BMD. Positive correlation was found between PA and LM. There were significant differences in BMD between different categories of PA, but the differences disappeared after adjusting for LM. Mediation analysis showed that LM and FM played a mediating role in the relationship between PA and BMD. LM appeared to mediate the effect of BMD in the spine, hip, and total body by 26.91%, 19.55% and 47.98%, respectively; and FM was 22.23%, 27.97%, and 33.02%, respectively. Conclusion: LM and FM affected the relationship between PA and BMD as mediator. Postmenopausal women with high LM and FM had more BMD.