Physical activity measured by accelerometer predicts body composition in free-living adults

Governmental recommendations for the duration and intensity of physical activity (PA) conducive to optimal health emphasize the need for objective measurements of PA that can be compared to data collected by commonly used questionnaires. One potential objective methodology for monitoring PA is the use of accelerometers (activity monitors), electronic devices worn by volunteers that track the intensity and duration of free-living PA. To determine the relationship between body composition measured by dual energy x-ray absorptometry and PA measured by accelerometry, we placed accelerometers on a large population of free-living adults (245 women, 243 men) over a 2-week period, in conjunction with concurrent measures of resting energy expenditure and total energy expenditure. Models with accelerometer data alone (P=0.001 for men, P<0.0001 for women) and with lean body mass (P= 0.0002 for men, P<0.0001 for women) were significant predictors of body fat. The variance of body fat percentage was highest for the most sedentary subjects, and decreased markedly with more PA (P=0.007). Comparisons between body fat percentage and five categories of average time spent in daily moderate-to-vigorous PA (MVPA; ranging from none to ≥90 min/day) found that body fat percentage was negatively (P<0.001) related to time spent in MVPA. Although sedentary subjects tend to have higher body fat percentages than subjects with higher PA, body fat percentages were also more variable; many sedentary subjects had low body fat. This suggests that some subjects have adaptive mechanisms that do not involve PA. With increasing physical activity, subjects converged to about 24% body fat.