Lawrence Berkeley National Laboratory

Background: The predicted health benefits of becoming physically active or fit will be exaggerated if health outcomes cause fitness and activity rather than the converse in prospective and cross-sectional epidemiological studies. Objective: Assess whether the relationships of adiposity to fitness and activity are explained by adiposity prior to exercising. Design: Cross-sectional study of physical fitness (running speed during 10km foot race) and physical activity (weekly running distance) to current BMI (BMIcurrent) and BMI at the start of running (BMIstarting) in 44,370 male and 25,252 female participants of the National Runners' Health Study. Results: BMIstarting explained all of the association between fitness and BMIcurrent in both sexes, but less than a third of the association between physical activity and BMIcurrent in men. In women, BMIstarting accounted for 58 percent of the association between BMIcurrent and activity levels. The 95th percentile of BMIcurrent showed substantially greater declines with fitness and activity levels than the 5th percentile of BMIcurrent in men (i.e., the negative slope for 95th percentile was 2.6-fold greater than the 5th percentile for fitness and 3-fold greater for activity) and women (6-fold and 3.4-fold greater, respectively). At all percentiles, the regression slopes relating BMIstarting to fitness were comparable or greater (more negative) than the slopes relating BMIcurrent to fitness, whereas the converse was true for activity. Conclusion: Self-selection bias accounts for all of the association between fitness and adiposity and probably a portion of other health outcomes, but has less affect on associations involving physical activity