Conceptual problems in identifying risk factors for functional decline in the elderly: a commentary.

Assessment of risk factors for chronic disease and disability among the elderly is a complex issue. Findings may be affected by the age at which the putative risk factor was measured, interval between the time of measurement of risk and outcome, possibility of changing patterns of exposure throughout the life span, and selective survival. We illustrate the complexity of risk assessment with findings from the Iowa 65+ Rural Health Study on the relationship between body mass and changing physical function. In baseline cross-sectional analyses, both high and low body mass were associated with poorer function. In 6-year longitudinal analyses limited to persons who were functionally intact at baseline, high body mass was associated with poorer lower-extremity function but relatively preserved upper-extremity function. High body mass was also associated with elevated rates of transient impairments. Extremes of teenage body habitus were associated with impaired physical function in old age. Function was poorest among those with a lifetime history of high body mass, and changes in body mass were complexly associated with subsequent function. Thus conceptual and analytic models of chronic illness and disability need to consider multiple, changing exposures over the lifetime, as well as the impact of illness, disability, and their treatment on subsequent risk factor exposure.