Impact of physical function on health outcomes in older community-dwelling women
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In previous epidemiologic studies, poor physical function has been associated with increased risks of nursing home placement, hospitalization, and mortality in older adults. However, these associations are subject to confounding and misclassification. Studies to date do not adequately account for these biases; previous studies have evaluated only cross-sectional associations, followed participants for less than ten years, or inadequately controlled for confounders by using only baseline values of characteristics that vary over time.5,75,106 In addition, no study has finely controlled for age, the strongest predictor of both physical function and health outcomes such as mortality and institutionalization in older adults. This dissertation is comprised of three studies that evaluated the associations between physical performance and skeletal health, respectively, with mortality and long-term nursing home residence while utilizing age-based risk set sampling, evaluating mediation by osteoporotic fractures, and controlling for death as a competing risk. All studies in this dissertation use data from the Study of Osteoporotic Fractures, a longitudinal epidemiologic study of older women with over 20 years of follow-up. Study 1 evaluated the association between physical performance and incident disability, using time-dependent exposures and confounders, and age-based risk sets to control for age. Women with poorer performance based on individual measures of physical function had an increased risk of incident disability over follow-up. Similarly, a whole body summary physical performance score was linearly associated with increased risk of death. Study 2 evaluates the association between low bone mineral density and mortality. Women with low bone mineral density were more likely to experience a fracture and to die compared to women with normal bone mineral density. Mediation analyses suggested that incident fracture had a measureable impact on this association, though this varied by fracture site. Study 3 evaluates the association between slow gait speed and risk of long-term nursing home placement while controlling for death as a competing risk. Women with slow gait speed had an increased risk of long-term nursing home residence, which was slightly attenuated when considering death as a competing risk. These results extend previous studies of the health effects of physical function among older women. The findings underscore the clinical importance of physical function and bone mineral density (BMD) for identifying older adults for whom interventions to improve their physical function may prolong their independence and optimize health.