Body composition and physical function in the Women's Health Initiative Observational Study.

Physical function is critical for mobility and quality of life. We hypothesized that higher total lean mass is associated with higher physical function, and body fat inversely associated, among postmenopausal women. Women's Health Initiative Observational Study participants at Pittsburgh, PA; Birmingham, AL; and Tucson-Phoenix, AZ (1993-1998) completed dual-energy X-ray absorptiometry scans and the Rand SF-36 questionnaire at baseline and 3 y (N = 4526). Associations between quartiles (Q1-4) of lean or fat mass and physical function were tested using linear regression, adjusted for demographics, lifestyle factors, medical history, and scanner serial number. At baseline, participants had a mean ±â€¯SD age of 63.4 ±â€¯7.4 y and BMI of 27.4 ±â€¯5.8 kg/m2. Higher percent lean mass was positively associated with physical function at baseline (Q4, 83.6 ±â€¯0.6 versus Q1, 74.6 ±â€¯0.7; p < 0.001), while fat mass (kg and %) was inversely associated (e.g., Q4, 73.7 ±â€¯0.7 versus Q1, 84.2 ±â€¯0.7 kg; ptrend < 0.001). Physical function had declined across the cohort at 3 y; the highest relative lean mass quartile at baseline conferred a lesser decline in physical function than the lowest (Q4, -3.3 ±â€¯0.6 versus Q1-7.0 ±â€¯0.6; ptrend < 0.001), while the highest fat mass quartile (% and kg) conferred greater decline (ex. Kg Q4, -6.7 ±â€¯0.7 versus Q1-2.8 ±â€¯0.6; ptrend < 0.001). Increased fat mass (≥5%), but not lean mass, was associated with lower physical function at 3 y (p < 0.001). Adiposity, as well as lean mass, requires consideration in the prediction of physical function among postmenopausal women over time.

Dual-energy X-ray absorptiometry is a valid tool for assessing skeletal muscle mass in older women.

Assessing skeletal muscle mass (SMM) is critical in studying and detecting sarcopenia. Direct measurements by MRI or computerized tomography are expensive or high in radiation exposure. Dual-energy X-ray absorptiometry (DXA) is promising for body composition assessments, but the validity of DXA for predicting SMM in the elderly is still under investigation. The objective of this study was to assess the relationship between DXA-derived measurements of lean soft tissue mass (LSTM) and SMM in older women. Study participants were postmenopausal women (n = 101) recruited in southern Arizona. Total and regional body composition was measured using MRI and DXA (QDR4500w). The participants' mean age was 70.7 +/- 6.4 y and their mean BMI was 27.4 +/- 5.1 kg/m2. DXA-derived LSTM was highly correlated with MRI-derived SMM for the whole body (r = 0.94; P < 0.001) and leg region (r = 0.91; P < 0.001). In multivariate models, adjusting for age and DXA-derived percent fat slightly increased the amount of variance in SMM that can be explained by the DXA-derived LSTM assessments for the leg region but not for the total body. In conclusion, although the relationships between DXA measures and MRI-derived SMM vary by region of interest, the overall prediction of SMM by DXA is excellent. We conclude that DXA is a reliable method for cross-sectional assessments of SMM in older women.