Trends in Spontaneous Physical Activity and Energy Expenditure among Adults in a Respiratory Chamber

Background: A rapid rise in the prevalence of obesity has occurred in the U.S. during a time frame consistent with environmental rather than genetic causes. Increases in sedentary behavior could be one contributing factor. We measured 24-h spontaneous physical activity (SPA) and energy expenditure (EE) inside a respiratory chamber over two decades. We examined secular trends in these determinants of obesity. Methods: A total of 678 healthy adults (aged 28.8 ± 7.1 years; 405 men and 273 women; BMI 33.9 ± 8.6 kg/m2; 522 Indigenous American, 129 white, and 27 black) were admitted to the metabolic unit from 1985-2005. During a 24-h stay in the respiratory chamber equipped with radar sensors, SPA, energy cost of SPA, EE, and respiratory quotient (RQ) were measured. Body composition was assessed by underwater weighing or DXA. Results: SPA adjusted for covariates declined over the study period (r = −0.30, p < .0001) with a concomitant rise in the energy cost of SPA (r = 0.30, p < .0001). 24-h EE adjusted for covariates increased (r = 0.31, p < .0001), which was reflected in increases in EE during sleep (r = 0.18, p < .0001) and during the awake, fed condition (r = 0.28, p < .0001). No secular trend was observed in 24-h RQ (p = 0.53). Conclusions: Secular trend analyses showed a decline in directly measured SPA, reflecting fidgeting-like behavior. This decline may be contributing to the obesity epidemic and this finding supports public health efforts to reduce sedentary behaviors. However, this decline in SPA may be counterbalanced in part by an increase in energy cost of this activity and a rise in EE.