The association between fracture and obesity is site-dependent: a population-based study in postmenopausal women

The association between obesity and fracture is controversial. We investigated the relationship between body mass index (BMI) and fracture at different skeletal sites in women aged ≥50 years using data from the Sistema d' Informació per al Desenvolupament de la Investigació en Atenció Primària (SIDIAP) database. SIDIAP contains the computerized medical records of >3400 general practitioners in Catalonia (northeastern Spain), with information on a representative 80% of the population (>5 million people). In 2009, 1,039,878 women aged ≥50 years were eligible, of whom 832,775 (80.1%) had a BMI measurement. These were categorized into underweight/normal (302,414 women), overweight (266,798), and obese (263,563). Fractures were ascertained using the International Classification of Diseases, 10th revision (ICD-10) codes. Multivariate Poisson regression models were fitted to adjust for age, smoking, high alcohol intake, type 2 diabetes, and oral corticosteroid use. Hip fractures were significantly less common in overweight and obese women than in normal/underweight women (rate ratio [RR] 0.77 [95% confidence interval (CI) 0.68 to 0.88], RR 0.63 [95% CI 0.64 to 0.79], p < 0.001, respectively). Pelvis fracture rates were lower in the overweight (RR 0.78 [95% CI 0.63 to 0.96], p = 0.017) and obese (RR 0.58 [95% CI 0.47 to 0.73], p < 0.001) groups. Conversely, obese women were at significantly higher risk of proximal humerus fracture than the normal/underweight group (RR 1.28 [95% CI 1.04 to 1.58], p = 0.018). Clinical spine, wrist, tibial, and multiple rib fracture rates were not significantly different between groups. An age-related increase in incidence was seen for all BMI groups at all fracture sites; obese women with hip, clinical spine, and pelvis fracture were significantly younger at the time of fracture than normal/underweight women, whereas those with wrist fracture were significantly older. The association between obesity and fracture in postmenopausal women is site-dependent, obesity being protective against hip and pelvis fractures but associated with an almost 30% increase in risk for proximal humerus fractures when compared with normal/underweight women. The reasons for these site-specific variations are unknown but may be related to different patterns of falls and attenuation of their impact by adipose tissue.