The Effect of Stability and Change in Health Behaviors on Trajectories of Body Mass Index in Older Americans: A 14-Year Longitudinal Study

Overweight and obesity are increasingly prevalent among older Americans. Excess weight in older age has been associated with increased mortality (1), physical and cognitive morbidity (2), and higher utilization of health care services (3), accounting for approximately half of the total $147 billion in obesity-attributable annual U.S. medical expenditures (4). Reducing the risk factors for and the consequences of excess weight in older adults is therefore a critical clinical and public health goal. Smoking (5), physical inactivity (6), and alcohol use (7) are significant health issues among older adults. Investigations into the relationship between health behaviors and body weight in older adults consist predominantly of cross-sectional or short-term longitudinal studies, and older age groups have largely been excluded from clinical trials aimed at reducing health risk behaviors (8). Consequently, the long-term effect of health behaviors and modifications thereof on body weight in older age is still unclear. Three considerations based in the existing literature motivate our study. First, to our knowledge, no study to date has examined the impact of health behaviors on the trajectory (ie, intercept and rate of change) of body weight over an extended period of time. Trajectories of health indicators (in this case, body weight) are useful outcomes in any age group, mainly because health is the result of long-term accumulation of biopsychosocial processes that occur over time (9). In older adults, there is a particular interest in understanding the long-term processes (trajectories) that eventually lead to poor health, disability, or death, and in evaluating how such trajectories can be modified through interventions (behavioral or others) at various points in the life course. The main reason for focusing on trajectories, as opposed to two-point transitions, is that transitions between a starting and an end point are often nonlinear (10) and that two observations do not yield sufficient information for the identification of the functional form depicting the rate of change. Although linear trajectories can be defined by three observation points, a greater number of observation points allows us to postulate and test for discontinuous (ie, nonlinear) individual change and more flexibility in defining the shape of change trajectories (11). Second, most epidemiological studies assume that individuals have a stable lifestyle and do not account for fluctuations in health behaviors over time (12) or for the observed intraindividual clustering of risky health behaviors (high cigarette consumption, high alcohol intake, and low physical activity [PA] [13]). Health behaviors are hardly invariable over time (ie, smokers or drinkers who quit and relapse), especially in old age (14). Trajectory analysis allows the concomitant incorporation of multiple health behaviors as time-varying covariates to more accurately describe the actual change in behavioral patterns over the life course. Finally, over the last few decades, the prevalence of smoking has declined steadily (5,15), as the reported rates of involvement in physical activities have increased among all age groups, including older adults (6). The strong correlation between the rise in obesity and the reduction in smoking in particular has led to suggestions of a causal relationship. Although population-level studies have examined this link (16), no study to date has evaluated whether the individual-level association between obesity-related behaviors and body weight increased or decreased over time. This study has two aims. The first aim is to estimate the effects of smoking, PA, and alcohol use status and variation over time on the long-term trajectory of body mass index (BMI) starting in middle age. Second, the study examines whether the intraindividual association of aforementioned health behaviors with the trajectory of BMI changed (increased or decreased) over time.