Relationship of body mass index with main limb fragility fractures in postmenopausal women

Body mass index (BMI) has been found to be related to the risk of osteoporotic hip fractures in women, regardless of bone mineral density (BMD). The same relationship is under debate for other limb fragility fractures. Very few studies have investigated the comparison of fracture risk among BMI categories, classified according to the WHO criteria, despite the potential usefulness of such information for clinical purposes. To address these issues we studied 2,235 postmenopausal women including those with fragility fractures of the hip (187), ankle (108), wrist (226) and humerus (85). Statistical analyses were performed by logistic regression by treating the fracture status as the dependent variable and age, age at menopause, femoral neck BMD and BMI as covariates. BMI was tested as a continuous or categorical variable. As a continuous variable, increased BMI had a protective effect against hip fracture: OR 0.949 (95% CI, 0.900-0.999), but carried a higher risk of humerus fracture: OR 1.077 (95% CI, 1.017-1.141). Among the BMI categories, only leanness: OR 3.819 (95% CI, 2.035-7.168) and obesity: OR 3.481 (95% CI, 1.815-6.678) showed a significantly higher fracture risk for hip and humerus fractures, respectively. There was no relationship between ankle and wrist fractures and BMI. In conclusion, decreasing BMI increases the risk for hip fracture, whereas increasing BMI increases the risk for humerus fractures. Leanness-related low BMD and obesity-related body instability might explain the different BMI relationships with these two types of fracture.