Skeletal muscle mass, fat mass, and hip bone mineral density in elderly women with hip fracture

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Abstract
Soft tissue body composition strongly affects bone health. Our aim was to investigate the relationship between both skeletal muscle mass (SMM) and fat mass (FM) and femoral bone mineral density (BMD) in a sample of elderly women with hip fracture. We assessed 293 of 325 hip fracture women admitted consecutively to a rehabilitation hospital. Soft tissue body composition and BMD were assessed by dual-energy X-ray absorptiometry (DXA), 23.2 ± 7.7 (mean ± SD) days after fracture occurrence. BMD was measured at four sites (neck, total femur, trochanter, intertrochanteric area) in the unfractured femur. Appendicular lean mass (aLM) was calculated as the sum of LM in arms and legs. We used two approaches to adjust aLM for body size: aLM divided by height squared (aLM/ht2), and aLM adjusted for height and FM (residuals). Both FM and aLM were significantly correlated with femoral BMD. However, the correlation coefficients for aLM were lower than for FM; they further decreased after adjustment for height squared, and were no longer significant after correction for both height and FM (residuals). When FM, aLM/ht2, age, and time spent between fracture occurrence and DXA assessment were included together as the independent variables in a regression model, FM was the only independent variable significantly associated with BMD. The coefficients of partial correlation ranged from 0.414 to 0.647 depending on the femoral region of BMD assessment (P < 0.001). FM, but not SMM emerged as a pivotal determinant of BMD in our sample of hip fracture women.