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

Predictors of Physical Activity in Community-dwelling Elderly White Women

OBJECTIVE: To describe patterns of physical activity and to determine factors associated with engaging in regular exercise, especially walking, in elderly white women. DESIGN: Cross-sectional study of 9,442 independently living elderly white women aged 65 years and over participating in the Study of Osteoporotic Fractures. MEASUREMENTS AND MAIN RESULTS: We studied the association between lifestyle habits, social factors, health status and self-reported physical activity (assessed by modified Paffenbarger scale) during the past twelve months. Walking was the most common form of exercise: 4,837 (51%) women reported doing so a mean of 12 (SD = 10) blocks per day, 3.9 (SD = 2.9) times per week. Other common activities were gardening (35%), swimming (16%), and bicycling (13%). Less than a third of women reported engaging in medium- or high-intensity exercise in the past year. In a multivariate age-adjusted analysis, factors independently (P < .01) associated with walking for exercise included greater than high school education (52% vs 48%), history of physical activity for exercise at ages 30 years (51% vs 46%) and 50 years (51% vs 45%), and stronger social network (51% vs 47%). Women who were current smokers, obese, or depressed were less likely to take walks for exercise. Marital status, self-reported arthritis, current estrogen use, and a history of falls in the past year were not independently associated with taking walks for exercise. CONCLUSIONS: In this healthy cohort, walking for exercise is associated with other positive health behaviors. Given the mounting evidence about the health benefits of walking, and since many of these community dwelling women can and do walk for exercise, but rarely engage in other common prescribed physical activities, clinicians might best focus their efforts on encouraging walking