Why exercise burns fewer calories than we think
New research suggests the body may limit how many calories you burn in a workout—helping explain why weight loss is often smaller than expected.
The logic feels airtight: Move more, burn more, weigh less. But for many people, the math never quite adds up.
In controlled studies, exercise often results in less weight loss than calorie models predict. Even when people add aerobic exercise such as walking, jogging, and cycling, most lose an average of 3.5 pounds over six months. It’s a modest return for a significant investment of time and effort, and one that has long puzzled researchers.
Part of the answer may be familiar: working out can make you hungrier, making it easier to eat back the calories burned. But experts are also trying to understand a more counterintuitive phenomenon at play.
A 2025 analysis suggested people only burned about a third of the extra calories their workouts theoretically demanded. In other words, a run that should have burned 500 calories, only added around 165 calories to the daily budget. The body seems to compensate for increased physical activity by reducing energy spent elsewhere—but the extent to which it does this, and how, is “still a mystery,” says Vincent Careau, an evolutionary biologist at the University of Ottawa.
This emerging idea, known as energy compensation, is reshaping a long-held assumption at the heart of fitness culture: that exercise is a straightforward engine for weight loss. Instead, researchers are finding it may be far more effective at something else—helping the body maintain its weight and protect long-term health.
The body’s hidden energy trade-offs
The idea that the body might ‘compensate’ emerged after a groundbreaking 2012 study found that a hunter-gatherer population in Tanzania burned roughly the same number of calories as an average sedentary person in industrialized countries, even though they walked many more miles a day to forage and hunt.
That counterintuitive finding is explained by the constrained energy expenditure model, which proposes that the body compensates for exercise by dialing down energy spent on other physiological processes. In doing so, it keeps our total daily expenditure—the cost of pumping blood, digesting food, walking around, as well as exercising intentionally—within a relatively narrow range.
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That “fine-tuning” can take many forms, says Leanne Redman, an expert in human physiology and energy balance at the University of Sydney. Redman’s own 2021 study found that adults who burned about 1,800 calories per week through exercise lost only about half as much weight as standard calorie models would predict.
Changes in movement might explain some of this difference, though findings in this study weren’t consistent. “People might take a nap on the couch because they're tired, they might walk slower to the bus that day, they may get an Uber,” Redman says.
But the adjustments may run deeper than behavior alone. There’s other evidence that the body allocates less energy to non-exercise activity thermogenesis after exercise, such as fidgeting and restlessness, walking around while on the phone, or tapping your leg. Other studies also link exercise to reduced immune, thyroid, and reproductive hormone activity—all of which carry metabolic costs.
At the same time, not everyone compensates in the same way—or at all. In follow-up analyses, Redman found that only about half of the participants showed clear signs of compensation. When it did happen, the researchers think it came from people becoming more efficient at the exercise itself, with their muscles and cells adapting to do the same work with less fuel.
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“I just can’t believe that someone that's running ultra marathons hasn't had adaptive changes,” says Redman. Interestingly, people with higher baseline energy expenditure were more likely to compensate for exercise.
Where the science still disagrees
But not everybody is convinced that compensation is really happening. A recent study, led by Kristen Howard at the University of Alabama, examined mostly sedentary volunteers and ultra-marathon runners who clocked more than 70 kilometers a week.
Across both groups, total energy expenditure rose in a straight line with physical activity—just as traditional calorie models would predict. “We saw exactly what the historical model would predict, it was just a straight line,” Howard says. There was no compensation through behavioral changes or trade-offs in immune or thyroid function among both people who move and those who don’t.
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This aligns with earlier findings among older adults: greater activity generally led to more calories burned. But among those in a negative energy balance—meaning they were losing weight from a calorie deficit—their total daily energy expenditure plateaued at around 2,500 calories regardless of how much they exercised.
In other words, compensation seemed to happen when calories were scarce. But when people were at maintenance or surplus, the body didn’t compensate. Other findings also suggest that energy balance could explain why some people compensate for exercise, and others don’t.
“We have an innate sense of matching our calories, it's remarkable,” says Howard. “If we weren't good at that, we would be either losing weight or gaining weight on a massive scale, but year after year, changes and weight really are pretty small.”
Even in Howard’s study, no one exceeded a total daily energy expenditure of about 2.5 times their resting metabolic rate, which is the theoretical ceiling that the constrained model predicts. Most people tend to operate at up to twice that rate, meaning the ceiling might be real but perhaps not observable for most people.
But at some point, the energy does have to come from somewhere else. “There probably isn’t an infinite amount of calories that a human being can burn,” says Redman.
For now, many experts suspect the truth lies somewhere between the two models. But they caution against drawing the wrong conclusion. “Don’t use the concern for compensation as a reason to believe that exercise is not going to benefit you,” Howard says.
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Even though it might not drive significant weight loss, it might help maintain weight loss by improving insulin sensitivity, for example. Physical activity is also linked to a host of other benefits, such as improving cholesterol and lowering inflammation, associated with a lower risk of disease in later life.
“Anyone studying this would want to underscore that there are phenomenal benefits to exercise and being physically active,” Howard says.
