Obesity Epidemiology

PHYSICAL ACTIVITY, SEDENTARY BEHAVIORS, AND OBESITY 313Because chronic diseases can lead to a decline in physical activity as well as weightloss, confounding by existing chronic diseases may distort a relationship between physicalactivity and obesity. It is therefore desirable to analyze the relationship betweenphysical activity and weight gain in a healthy cohort that excludes those who reporta diagnosis of chronic conditions (especially heart disease and cancer) at baseline orduring follow-up. This often requires a very large initial sample. Another strategy is toconduct stratified analyses by the presence or absence of chronic diseases at baseline,especially in a cohort of older individuals. In this way, one can specifically examinewhether physical activity attenuates weight loss among those who are fragile.Longitudinal Data Analysis StrategiesThe repeated measurements of physical activity and weight that are available in longitudinalstudies provide opportunities to test various hypotheses regarding short-term andlong-term effects of physical activity on obesity. Several common strategies have beenemployed to analyze the relationship between physical activity and body weight. First,baseline physical activity can be used to predict subsequent weight gain or onset of obesityduring follow-up. For example, using data from the NHS, we examined whether walkingand sedentary behaviors assessed in 1992 predicted incident obesity between 1992 and1998 in women who were not obese at baseline. 23 This classic method clearly establishesthe temporal relationship between the exposure (i.e., physical activity at baseline) and theoutcome (weight gain or onset of obesity).A second strategy, concurrent analyses of changes in physical activity and bodyweight over time, is also commonly used. Many studies reviewed above examinedwhether changes in physical activity predicted weight gain during follow-up. Such analysesmimic an intervention study by looking at the associations between changes in anexposure variable and changes in outcome. As discussed earlier, the reciprocal relationshipbetween physical activity and body weight often makes it difficult to distinguishcause from effect. However, if more than two repeated measures are collected, a laggedanalysis can be conducted to examine whether changes in physical activity in the earlyfollow-up period predict subsequent weight change. With multiple time points, one canalso compare weight trajectories of those who are consistently active with those whoare consistently sedentary. For example, Coakley et al. 32 found that those who maintainedhigh-activity levels had the lowest BMI at baseline and the smallest incrementalincrease in BMI over time, whereas those who maintained low-activity levels were notonly heavier at baseline, but also experienced the greatest weight gain during follow-up.The third strategy, random effects or mixed models, has become increasingly popular inanalyzing the longitudinal relationship between physical activity and weight changes. Thismethod has the advantage of examining both cross-sectional and longitudinal associationssimultaneously while taking into account correlations among the repeated measures of thedependent variable. 71 It also offers flexibility in handling missing data with the assumptionof missing at random (MAR). 72 In a typical longitudinal analysis, repeated measures ofBMI or body weight are modeled as the dependent variable, while repeated measuresof physical activity are entered as an independent variable along with repeated measuresof other covariates. A random intercept is typically used to take into account correlationsamong repeated measures of weight. Two-way interactions of follow-up time with baselinephysical activity can be used to estimate whether the trajectory of body weight varies withbaseline physical activity levels. If repeated measures of weight change and physical activityare modeled, the interaction between follow-up time and changes in physical activityestimates the impact of changes in physical activity on trajectories of weight gain over