It is natural to assume if you exercise harder and longer, you will burn more calories. However, recent research found, in reality, the relationship between physical activity and total energy expenditure is actually more complex. 
The study, led by Professor Pontzer of the Department of Anthropology at City University of New York, recruited 332 adult women and men from 5 different countries and measured their daily energy expenditure for 7 days using the Doubly Labeled Water method (the gold standard method to measure energy expenditure). Participants’ physical activity was measured using wearable tri-axial accelerometer over the same period of time (data expressed as counts per minute per day, or CPM/day).
The researchers found that, among those engaging in lower activity levels (i.e., up to 230 CPM/day), the total energy expenditure correlated with the physical activity. However, for participants in the higher activity levels (above 230 CPM/day), their total energy expenditure plateaued and no longer increased linearly. The researchers hypothesized that the body metabolically adapts to higher activity levels in order to keep total energy expenditure within a relatively narrow range. The adaptation may include behavior change (e.g., sitting instead of standing, or fidgeting less) and reductions in non-muscular metabolic activity and basal metabolic rate. In other words, engaging in tougher and longer workouts does not help burn more calories.
An important limitation of this study was its cross-sectional design. A future study that utilizes a longitudinal design, such as measuring total energy expenditure as participants increase physical activity over a long period of time, would provide more solid evidence of what was observed in the current study.
The study results were published in the journal Current Biology (February 2017).
Why is this Clinically Relevant?
- To address the current obesity epidemic, increasing daily physical activity remains absolutely essential
- However, a common misconception is that more vigorous exercise will burn more calories and increase the amount of weight loss
- Healthcare providers should caution their patients that ‘more’ does not always necessarily means ‘better.’ Maintaining a regular schedule for moderate exercise will result in greater health benefits
- Further, a successful and health weight management strategy requires a combination of dietary modification, smart physical activity, and behavior therapy
 Pontzer, H., et al., Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans. Curr Biol, 2016. 26(3): p. 410-7.