Associations of Body Mass Index With Incident Fractures and Hip Structural Parameters in a Large Canadian Cohort

Context: Fractures in obese individuals are of public health importance, but the relationship between obesity and fracture is complex and remains poorly understood. Objective: The study examined the association of body mass index (BMI) with bone structural and strength parameters and incident fracture. Design and Setting: We performed cross-sectional and longitudinal analyses using data from the Manitoba Bone Density Program. Participants: We included 51 313 women and 4689 men aged 50 years or older referred for dual-energy X-ray absorptiometry scans. For 41 919 women and 4085 men, we were able to derive hip structural parameters. Main Outcome Measure: Cross-sectional moment of inertia, cross-sectional area, and femoral strength index were derived from dual-energy X-ray absorptiometry. Health service records were assessed for incident major osteoporotic fractures (MOFs) (mean follow-up 6.2 y in women and 4.7 y in men). Results: Among individuals with a BMI of less than 30 kg/m2, increasing BMI was associated with progressive increases in bone mineral density (BMD), cross-sectional moment of inertia, and cross-sectional area. The relationship reached a plateau around a BMI of 30 kg/m2, with little additional increment with further increases in BMI (all P for interaction < .0001, obese vs nonobese). Increasing BMI was linearly associated with decreases in strength index in both women and men. MOFs were ascertained in 3721 women and 276 men (1027 female and 75 male hip fractures). Higher BMI was associated with a lower risk of MOF in women in multivariable models, but this association was largely explained by their higher BMD. Protective association of higher BMI with hip fracture were stronger and only partially explained by BMD (hazard ratio [95% confidence interval] 0.79 [0.73–0.99] for obese I and 0.67 [0.46–0.98] for obese II). Higher BMI was not significantly associated with a risk of MOF or hip fracture in men. Conclusions: Despite structural and biomechanical disadvantages, obese women were at lower risk of fracture.