Association between low lean mass and low bone mineral density in 653 women with hip fracture: does the definition of low lean mass matter?

Loss of both muscle and bone mass results in fragility fractures with increased risk of disability, poor quality of life, and death. Our aim was to assess the association between low appendicular lean mass (aLM) defined according to different criteria and low bone mineral density (BMD) in hip-fracture women. Six hundred fifty-three women admitted to our rehabilitation hospital underwent dual energy X-ray absorptiometry 19.1 +/- 4.1 (mean +/- SD) days after hip-fracture occurrence. Low aLM was identified according to either Baumgartner's definition (aLM/height(2) less than two standard deviations below the mean of the young reference group) or FNIH criteria: aLM < 15.02 kg, or aLM adjusted for body mass index (BMI) < 0.512. Low BMD was diagnosed with a T-score <-2.5 at the unfractured femoral neck. Using Baumgartner's definition, the association between low aLM/height(2) and low BMD was significant: chi (2)(1, n = 653) = 8.52 (p = 0.004), but it was erased by adjustments for age and fat mass. Using the FNIH definition the association between low aLM and low BMD was significant: chi (2)(1, n = 653) = 42.5 (p < 0.001), and it was confirmed after adjustment for age and fat mass (p < 0.001). With the FNIH definition based on aLM/BMI ratio the association between low aLM/BMI ratio and low BMD was nonsignificant: chi (2)(1, n = 653) = 0.003 (p = 0.957). The association between low aLM and low BMD in women with hip fracture dramatically depends on the adopted definition of low aLM. FNIH threshold for aLM (< 15.02 kg) emerges as a useful tool to capture women with damage of the muscle-bone unit.