Burn more, weigh less. Sounds simple, right? Not exactly.
A new study is challenging conventional wisdom about exercise and weight loss, suggesting your workout may not burn as many calories as you think.
The findings could help explain why so many people don’t see the scale budge, even when they’re regularly hitting the gym and watching what they eat.
It all comes down to math.
Over the course of a day, your body’s natural calorie burn without any formal exercise can range from about 1,300 to more than 2,000, depending on age, sex and other factors, according to the Cleveland Clinic.
For years, scientists assumed any extra calories you burned — like from running a mile or swimming laps — would simply add to that total and lead to weight loss.
But recently, some researchers have been questioning that “additive model,” suggesting the body may follow a “constrained” approach instead.
That theory says your body has a built-in energy cap. So when you burn more calories during exercise, your body makes up for it by saving calories elsewhere — like cutting back on cellular repairs and other internal tasks.
Intrigued, two Duke University researchers decided to put the models head-to-head.
They reviewed 14 exercise studies involving 450 people, along with several animal trials, and compared the calories subjects were expected to burn with the calories they actually burned.
On average, the researchers found that only 72% of the calories burned during exercise actually showed up in total daily burn — the other 28% was quietly offset elsewhere in the body.
From an evolutionary standpoint, it makes sense. Our ancestors had to trek for miles without exhausting their energy reserves, according to Herman Pontzer, lead author of the study and an evolutionary anthropologist.
He witnessed this firsthand in Tanzania, where he lived among the Hadza, one of the last hunter-gatherer communities on Earth. Every day they trek miles across the dry savannah, hunting game and foraging for food.
Pontzer expected them to burn far more calories than notoriously sedentary Americans, but he found they actually burned about the same amount.
Our flexible metabolism — which lets us adapt to different diets and store fat for hard times — helped humans survive and thrive, and even shaped the way we age, Pontzer explained in an interview with Duke’s Magnify Magazine.
Notably, however, this compensation effect wasn’t universal across all workouts.
The researchers found the body only seemed to compensate during aerobic exercise like running. When it came to lifting weights or resistance training, the three strength studies they reviewed showed people burned more calories than expected based on how much they exercised.
The team isn’t exactly sure why — but they have a few theories.
For one, it’s tough to measure calories burned while lifting. The methods used in the studies are likely better suited for steady cardio, so the numbers might be off.
It’s also possible that heavy lifting doesn’t trigger the same compensation response as long, sweaty aerobic sessions. And the act of repairing muscle damage after strength training may require extra energy as well.
Diet also seemed to play a key role in how the body compensates.
The researchers found that if people cut calories while ramping up their workouts, their total burn often didn’t budge at all.
“The real killer here is that if you pair exercise with diet, your body goes, ‘Fine, well, then I’m going to compensate more,’” Pontzer told the New Scientist. “It’s still good for you, just not for weight loss.”
But that doesn’t mean you should cancel your gym membership.
Regular movement is still essential for our health — lowering chronic inflammation, stabilizing hormones and reducing the risk of chronic illnesses.
“You have to think about diet and exercise as two different tools for two different jobs,” Pontzer said.
“Diet is the tool for managing your weight. Exercise is the tool for everything else related to health — from mental health to cardiometabolic disease.”
