Is There a Causal Relationship Between Physical Activity and Bone Microarchitecture? A Study of Adult Female Twin Pairs.

The reasons for the association between physical activity (PA) and bone microarchitecture traits are unclear. We examined whether these associations were consistent with causation and/or with shared familial factors using a cross-sectional study of 47 dizygotic and 93 monozygotic female twin pairs aged 31-77 years. Images of the nondominant distal tibia were obtained using high-resolution-peripheral quantitative computed tomography. The bone microarchitecture was assessed using StrAx1.0 software. Based on a self-completed questionnaire, a PA index was calculated as a weighted sum of weekly hours of light (walking, light gardening), moderate (social tennis, golf, hiking), and vigorous activity (competitive active sports) = light +2*moderate +3*vigorous. We applied Inference about Causation through Examination of FAmiliaL CONfounding (ICE FALCON) to test whether cross-pair cross-trait associations changed after adjustment for within-individual associations. Within-individuals distal tibia cortical cross-sectional area (CSA) and cortical thickness were positively associated with PA (regression coefficients (β) = 0.20 and 0.22) while porosity of the inner transitional zone was negatively associated with PA (β = -0.17), all p < 0.05. Trabecular vBMD and trabecular thickness were positively associated with PA (β = 0.13 and 0.14), and medullary CSA was negatively associated with PA (β = -0.22), all p ≤ 0.01. Cross-pair cross-trait associations of cortical thickness, cortical CSA and medullary CSA with PA attenuated after adjustment for the within-individual association (p = 0.048, p = 0.062 and p = 0.028 for changes). In conclusion, increasing PA was associated with thicker cortices, larger cortical area, lower porosity of the inner transitional zone, thicker trabeculae, and smaller medullary cavities. The attenuation of cross-pair cross-trait associations after accounting for the within-individual associations were consistent with PA having a causal effect on the improved cortical and trabecular microarchitecture of adult females, in addition to shared familial factors. This article is protected by copyright. All rights reserved.