More might be less when it comes to exercise, according to a new study that shows there is a limit to how many calories we can burn through physical activity. The study, published in the journal Current Biology, found that our bodies stop burning extra calories beyond a certain threshold of activity, providing a scientific explanation for why exercise alone is rarely effective for long-term weight loss.
While the health benefits of physical activity are well established, the effects of exercise on total energy (i.e., caloric) requirements are much less clear. Basic observational studies and evaluations of exercise interventions have shown positive associations between physical activity and energy expenditure, leading many to conclude that increasing activity levels would produce corresponding linear gains in total energy expenditure. This “additive model” of total energy expenditure has informed most current approaches to weight loss and obesity prevention, which emphasize exercise as an effective means to increase overall caloric expenditure and achieve a healthy energy balance (“calories in, calories out”).
However, a growing line of evidence suggests that the assumptions of this traditional model do not capture the true complexity of the long-term relationship between physical activity and energy expenditure. In contrast to the predominant view that this relationship is linear, a number of large ecological studies have found that total average energy expenditure is remarkably stable across populations — in other words, more active populations do not have higher total energy expenditures.
Now, we may have an explanation for these seemingly contradictory findings: “Rather than increasing total energy expenditure linearly in response to physical activity, individuals tend to adapt metabolically to increased physical activity, muting the expected increase in daily energy output,” lead author Dr. Herman Pontzer and colleagues concluded in Current Biology.
The body’s “calorie budget”
To reach their conclusions, Dr. Pontzer’s team followed a group of 332 adults drawn from five countries across Africa and North America, some of them more sedentary and some more active. Over the course of a week, the researchers tracked participants’ activity levels using personalized accelerometers. They also measured participants’ total energy expenditure for the week using the “doubly labeled water” (DLW) method, which yields a measurement based on metabolic rate.
The analysis found that physical activity did have a weak effect on daily energy expenditure, but further assessment revealed that the relationship only applied to subjects who were in the lower half of the spectrum of physical activity. For those taking part in the most strenuous activities, however, energy expenditure plateaued so that additional exercise did not yield an increase in overall energy expenditure. Detailed results showed that study subjects with moderate activity levels had daily energy expenditures that were about 200 calories higher than the most sedentary group. However, those who engaged in more than moderate activity did not burn any more calories than those who stayed within moderate activity levels.
“The most physically active people expended the same amount of calories each day as people who were only moderately active,” Dr. Pontzer explained. “As we move from moderate activity levels up to more and more activity, our bodies adapt, so that energy expenditure per day stays basically the same, even as we’re more and more active.”
After uncovering these initial findings, the researchers took a closer look at the types of calories the participants in the study were burning. They found that the participants did actually continue to burn additional so-called “activity calories” as they exercised more, but above a certain threshold, their bodies compensated by burning fewer “resting calories”, which are used for carrying out basic biological functions. Basically, it is as if we have a set number of calories in the bank that our bodies let us burn. If we blow too much of that budget on physical activity, our bodies may keep us from spending too many calories doing other things like ramping up our immune system or stockpiling reproductive resources.
Finding the “sweet spot” for physical activity
The results could help explain why people who start exercise programs with the aim of shedding pounds often see initial results but then experience a steep decline in weight loss — or even a reversal — after a few months. While someone who goes from a sedentary lifestyle to a more active lifestyle is likely to see an initial increase in total energy expenditure, going beyond a moderate level of physical activity does not appear to increase overall energy expenditure, the researchers said.
However, they emphasized that their study does not call into question the many health benefits of staying active, which has been shown to reduce the risk of serious conditions including cardiovascular disease, type 2 diabetes, dementia, and certain cancers.
“Exercise is really important for your health. That’s the first thing I mention to anyone asking about the implications of this work for exercise,” Dr. Pontzer said. “There is tons of evidence that exercise is important for keeping our bodies and minds healthy, and this work does nothing to change that message. What our work adds is that we also need to focus on diet, particularly when it comes to managing our weight and preventing or reversing unhealthy weight gain.”
The researchers suggest there may be a “sweet spot” for physical activity: too little results in an unhealthy, sedentary lifestyle, but too much makes the body adjust by burning fewer resting calories, rendering the activity counterproductive for weight loss. Each of us probably has a slightly different calorie-burning plateau, the researchers said, noting that factors such as body fat, metabolism, hormone levels, muscle mass and genetic differences all impact energy expenditure.
In light of their findings, the authors suggest revising the World Health Organization guidance on how to prevent weight gain and obesity, which currently recommends 150 minutes of activity a week for adults (although it also includes dietary advice). New guidelines, the authors say, should “better reflect the constrained nature of total energy expenditure and the complex effects on physical activity on metabolic physiology”.