RESUMO
Black women have lower fracture rates, primarily due to bone-dependent factors; however, the racial differences in quantitative ultrasound (QUS) for older black and white women are poorly characterized, especially in regard to the mediating effects of physical activity. The primary aim of this study was to determine if QUS measures differ in age- and body-size-matched older black and white women and to explore if the racial differences were mediated by differences in current habitual physical activity. We performed dual-energy X-ray absorptiometry (DXA) bone mineral density (BMD) measures and QUS measures [broadband ultrasound attenuation (BUA), speed of sound (SOS), quantitative ultrasound index (QUI)] of the calcaneus in 30 black and 30 white women matched in age (68.3 years) and body mass index (30.0 kg/m2). Black women had greater QUS measures of SOS (1,563.6+/-31.7 vs 1,541.2+/-23.4, p=0.003) and QUI (102.6+/-18.5 vs 90.4+/-15.4, p=0.008), with a strong trend for greater BUA (79.2+/-15.1 vs 71.9+/-15.1 m/sec, p=0.066) compared with white women. As expected, black women had greater BMD measures at all sites; whole body (6.8%, p<0.010), lumbar spine (10.4%, p=0.008), proximal femur (14.2%; p<0.001) and femoral neck (20.3%; p<0.001), compared with white women. Although white women reported expending 45% more energy (p=0.03) in moderate intensity physical activity than black women on a weekly basis, current physical activity did not attenuate the difference in QUS measures between the races. The relations between QUS and BMD measures were similar in black compared with white women. After controlling for BMD, the racial differences in QUS measures were not apparent, precluding the conclusion that bone quality independently contributes to the reduction in fracture risk in older black women. The impact of current physical activity on QUS measures in older black and white women is negligible.
