Self-selection contributes significantly to the lower adiposity of faster,
longer-distanced, male and female walkers
Although cross-sectional studies show active individuals are leaner than their sedentary counterparts, it remains to be determined to what extent this is due to initially leaner men and women choosing to exercise longer and more intensely (self-selection bias). In this report walking volume (weekly distance) and intensity (speed) were compared to current BMI (BMIcurrent) and BMI at the start of walking (BMIstarting) in 20,353 women and 5,174 men who had walked regularly for exercise for 7.2 and 10.6 years, respectively. The relationships of BMIcurrent and BMIstarting with distance and intensity were nonlinear (convex). On average, BMIstarting explained >70 percent of the association between BMIcurrent and intensity, and 40 percent and 17 percent of the association between BMIcurrent and distance in women and men, respectively. Although the declines in BMIcurrent with distance and intensity were greater among fatter than leaner individuals, the portions attributable to BMIstarting remained relatively constant regardless of fatness. Thus self-selection bias accounts for most of the decline in BMI with walking intensity and smaller albeit significant proportions of the decline with distance. This demonstration of self-selection is germane to other cross-sectional comparisons in epidemiological research, given self-selection is unlikely to be limited to weight or peculiar to physical activity.