High bone mineral density in women with obesity who sustain a hip fracture: a cross-sectional study.

Background: In the presence of obesity, high bone mineral density (BMD) seems not to protect against fragility fractures at selected body sites. The aim of this study was to compare femoral BMD in hip- fracture women with versus without obesity.

Methods: At a median of 19 days after an original hip fracture due to fragility we measured BMD by dual-energy X-ray absorptiometry at the non-fractured femur. Densitometric osteoporosis was diagnosed with a T-score <-2.5. Obesity was defined with a Body Mass Index ≥30 kg/m².

Results: We studied 674 women. Mean T-scores were significantly higher in the 55 women with obesity than in the 619 without obesity, with a between-group difference of 0.79 (95% CI from 0.53 to 1.05, P<0.001). Adjustments for age, type of hip fracture, concomitant neurologic impairment, 25-hydroxyvitamin D, parathyroid hormone and estimated glomerular filtration rate did not erase the significant association between obesity and high T-scores (P<0.001). A woman without obesity had an adjusted odds ratio of 2.7 (95% CI from 1.4 to 5.2, P=0.002) for having densitometric osteoporosis compared to a woman with obesity.

Conclusions: BMD was higher in the hip-fracture women with versus without obesity. High BMD levels may not protect obese women against hip fractures and may falsely reassure patients and clinicians. To avoid underestimation of hip-fracture risk due to high BMD, we suggest an adjustment of risk calculation based on the 0.79 T-score difference between hip-fracture women with and without obesity. Adjustment validation by robust prospective studies is needed.