Vigorous-intensity exercise has been trending in recent years as it is shorter in duration and yields quick results. A popular article making its’ way around the internet has shown that frequent exercisers may hit a plateau at which they do not burn additional calories, past a certain threshold. Pontzer et al. found that total energy expenditure (TEE) plateaued at higher activity levels because the body adapts to keep TTE within a specific range.
What is exactly is TEE?
Total Energy Expenditure = Resting metabolic rate + thermic effect of meals + non-exercise activity thermogenesis + exercise
Resting metabolic rate is the natural amount of calories your body burns at rest. The amount of calories it takes the body to digest food refers to the the thermic effect of meals. Activity energy expenditure relates to non-exercise activity and exercise. Examples of non-exercise activities include fidgeting, walking, etc. Together these make up TEE.
In the cross-sectional study with 322 adults from the United States, Ghana, South Africa, Seychelles, and Jamaica; participants wore accelerometers for one week and took a survey regarding their current physical activity level. A day where the accelerometer was worn for at least 62% of the maximal wear time per day to be considered valid. Results showed that those who exercised at low levels had increased TEE, but those who exercised at higher levels had leveled off TEE. So even though participants worked at different exercise intensity levels, roughly the same number of calories were being burned.
Although researchers could not explain the results, there are theories that the body adapts to higher levels of physical activity. Additionally, this phenomenon occurs as it is the body’s way to keep from starving if the TEE is always in a certain range.
Self reported data is a limitation of this study, as people tend to overestimate the amount of physical activity they engage in. Additionally, dietary composition was not captured; this plays a role in TEE because of the thermic effect of food. This will vary among participants but does affect the overall TEE. To measure TEE, doubly labeled water(DLW) was used which is an expensive and time consuming method. However, it is important to note DLW is considered the gold standard of measurement.
Regardless, the take home message is that exercise is still beneficial. However, if you hit a plateau in your fitness journey- try something new to change up your routine.
Pontzer et al., Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans, Current Biology (2016), http://dx.doi.org/10.1016/j.cub.2015.12.046