The effects of physical activity don't stop when the movement does.
In a new study published in the Proceedings of the National Academy of Sciences, Virginia Tech researchers in collaboration with researchers at the University of Aberdeen and Shenzhen University found that being active adds to the total energy you use every day without causing the body to conserve energy in other ways.
This is important because the health benefits of increasing physical activity are already well-documented, but there is less research about how exercise impacts a person's "energy budget," or the allocation of energy to different bodily functions.
It has been thought that a person's energy budget functions in one of two ways: like a fixed salary where energy is redistributed from other functions to cover the cost of movement, or like a flexible, commission-based system that is additive and allows for increased energy expenditure. The team wanted to determine which model better explains how the energy budget actually changes across different levels of physical activity.
To do this, researchers evaluated the total energy expenditure, or the total calories burned in a day, in participants across a wide spectrum of physical activity.
"Our study found that more physical activity is associated with higher calorie burn, regardless of body composition, and that this increase is not balanced out by the body reducing energy spent elsewhere," said Kevin Davy, professor in the Department of Human Nutrition, Foods, and Exercise and the principal investigator of the study.
Participants were asked to drink isotopes of oxygen and hydrogen and then to have their urine samples collected over a period of two weeks. Oxygen is eliminated as water and carbon dioxide, and the hydrogen is eliminated only as water. The difference in the amount of each isotope lost is proportionate to the amount of carbon dioxide produced and, therefore, energy expended. Physical activity was measured using a small sensor worn at the waist that measures motion in multiple directions.
Participants' physical activity levels varied widely, from sedentary to ultra-endurance running. There were 75 participants between the ages of 19 and 63.
Instead of saving energy in one area to make up for the calories burned through physical activity, the study found that the body continues to function at its usual rate, which means that overall energy use rises in direct response to increased movement.
For example, when the body is maintaining basic functions such as breathing, circulating blood, and regulating temperature, the amount of energy it uses remains stable, the study found. This means the body doesn't obviously compensate or "cancel out" the extra calories burned through activity.
"Energy balance was a key piece of the study," said Kristen Howard, senior research associate at Virginia Tech and the article's lead author. "We looked at folks who were adequately fueled. It could be that apparent compensation under extreme conditions may reflect under-fueling."
The research also found a clear link between being more active and spending less time sitting still. In simple terms, people who are more physically active are less likely to spend long periods of time being inactive.
As it turns out, the idea that moving more is associated with burning more may not be such a metabolic myth, despite uncertainty among experts on the topic. Though their findings seem to confirm the additive model, the researchers said there is still room for further work in this area. "We need more research to understand in who and under what conditions energy compensation might occur," said Davy.
