Overweight/obesity and underweight are both risk factors for osteoporotic fractures at different sites in Japanese postmenopausal women

This cohort study of 1,614 postmenopausal Japanese women followed for 6.7 years showed that overweight/obesity and underweight are both risk factors for fractures at different sites. Fracture risk assessment may be improved if fracture sites are taken into account and BMI is categorized. The effect of body mass index (BMI) on fracture at a given level of bone mineral density (BMD) is controversial, since varying associations between BMI and fracture sites have been reported. A total of 1,614 postmenopausal Japanese women were followed for 6.7 years in a hospital-based cohort study. Endpoints included incident vertebral, femoral neck, and long-bone fractures. Rate ratios were estimated by Poisson regression models adjusted for age, diabetes mellitus, BMD, prior fracture, back pain, and treatment by estrogen. Over a mean follow-up period of 6.7 years, a total of 254 clinical and 335 morphometric vertebral fractures, 48 femoral neck fractures, and 159 long-bone fractures were observed. Incidence rates of vertebral fracture in underweight and normal weight women were significantly lower than overweight or obese women by 0.45 (95 % confidence interval: 0.32 to 0.63) and 0.61 (0.50 to 0.74), respectively, if BMD and other risk factors were adjusted, and by 0.66 (0.48 to 0.90) and 0.70 (0.58 to 0.84) if only BMD was not adjusted. Incidence rates of femoral neck and long-bone fractures in the underweight group were higher than the overweight/obese group by 2.15 (0.73 to 6.34) and 1.51 (0.82 to 2.77) and were similar between normal weight and overweight/obesity. Overweight/obesity and underweight are both risk factors for fractures at different sites. Fracture risk assessment may be improved if fracture sites are taken into account and BMI is categorized.