Self-selection accounts for inverse association between weight and cardiorespiratory fitness

Background: Men and women who exercise regularly and who are physically fit tend to be leaner than those who are sedentary and not fit. Although exercise is known to attenuate weight gain and promote weight loss, there may also be a propensity for leaner men and women to choose to exercise vigorously (self-selection). Pre-exercise body weights have been shown to account for all the weight differences between fast and slow walkers, but seem to account for only a portion of the weight differences associated with walking distances. Whether these results apply to maximum exercise performance (i.e., cardiorespiratory fitness) as well as to doses of vigorous exercise (metabolic equivalents >6) remains to be determined. Objective: Assess whether the cross-sectional relationships of BMI to cardiorespiratory fitness and vigorous activity are explained by BMI prior to exercising. Methods and Procedures: Cross-sectional study of the relationships between cardiorespiratory fitness (running speed during 10 km foot race) and vigorous 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 accounted entirely for the association between fitness and BMIcurrent in both sexes, but only a quarter of the association between vigorous physical activity levels and BMIcurrent in men. In women, BMIstarting accounted for 58% of the association between BMIcurrent and vigorous activity levels. Discussion: Self-selection based on pre-exercise BMI accounts entirely for the association found between fitness and BMI (and possibly a portion of other health outcomes).