Most people tend to view exercise through the lens of the ‘additive’ energy model: More exercise = more calories burned.
But back in 2016, Herman Pontzer and colleagues published a study that put forth the ‘constrained’ energy model: Energy expenditure does increase with added activity, but only to a point. 1
In other words, if you go from being sedentary to active, you’ll see an increase in energy expenditure. But if you’re already very active and increase physical activity levels further, you aren’t likely to see energy expenditure increase linearly. This happens because the body adjusts other processes to maintain total energy expenditure within a narrow range.
For example, a recent paper found weight loss was only half of what was expected in a group burning 2000-2500 kcals/wk. The researchers explained this was likely due to behavioural adaptations that reduced 24-hour energy expenditure. 2
At the time, this was an interesting finding. It seemed to offer a tentative explanation for why people don’t always lose the expected amount of weight despite doing a bunch of physical activity.
Then, this area of research went pretty quiet, and there were questions about the validity of the constrained energy model. After all, it was just one study––did we jump the gun?
Well, two recent studies have added to the constrained energy model.
In the first study, Careau and colleagues examined total energy expenditure in 1,754 adults living ‘normal lives’. They found energy compensation averaged 28%.
In other words, if someone increased their physical activity levels by 500 kcals, you’d assume this would mean their total energy expenditure also increased by 500 kcals. However, according to this study, the actual increase in energy expenditure would be only 360 kcals. 3
But the researchers also found something else. Leaner individuals compensated less (29.7% compensation) than individuals with more body fat (45.7% compensation).
As the researchers explain:
It appears then, that either individuals with greater fat levels are predisposed to increased adiposity because they are stronger energy compensators or because they become stronger compensators as they get fatter. If the former is true, then two people can be equally active yet one puts on fat mass while the other stays lean. If the latter is true, then such a positive feedback loop may imply that using exercise as a strategy to escape high adiposity becomes less and less effective.
In the second study published earlier this year, researchers found energy balance influenced total energy expenditure such that when participants were at maintenance or a surplus, the additive model held true (to a point), whereas when participants were in a deficit, the constrained model held true. 4
This idea is reinforced by new findings from Howard and colleagues, who found no evidence of constraint or compensation across a wide range of activity levels, from sedentary individuals to ultra-endurance runners. Importantly, all participants were weight-stable and in energy balance, meaning any changes in energy expenditure weren’t influenced by a calorie deficit or surplus.
Using doubly labelled water (the gold standard for measuring energy expenditure), the researchers observed a clear linear relationship between physical activity and total energy expenditure, even after adjusting for fat-free mass and resting metabolic rate. In other words, the more active people were, the more calories they burned, without signs that the body was “saving” energy elsewhere.
They also found no reduction in resting metabolism, no physiological trade-offs in immune or thyroid function, and no behavioural compensation (i.e. active people weren’t compensating by sitting more).
That being said, while activity levels varied dramatically, participants’ total daily energy expenditure never exceeded 2.5 times their resting metabolic rate—the theoretical ceiling proposed by the constrained energy model. This suggests that the body doesn’t impose meaningful constraints until much higher, unsustainable levels of output are reached. For context, most people in everyday life show typical energy expenditure hovering around 1.6–2.0 times BMR (see panel C in the image below).
Adapted from Thurber et al., 2019.
In short, within the range of what most humans can reasonably do, total energy expenditure appears additive, not constrained when at energy balance, suggesting that the body’s energy “budget” is more flexible than the constrained model implies.
Relatedly, a 2024 study by Flanagan and colleagues also found a similar result. Over 24 weeks of supervised aerobic training, approximately half of the participants exhibited some “energy compensation,” meaning their total daily calorie burn didn’t rise as much as expected. However, this compensation wasn’t tied to changes in their resting and sleeping metabolism. The researchers suggested that the effect was due to improved exercise efficiency: as people became fitter, they burned fewer calories doing the same amount of work.
What to do with this information
There’s a lot to unpack here. However, a few key points can be taken away from all of this.
Trying to ‘out-train’ your diet is a losing strategy
Not just because the amount of exercise required to achieve this is impossible for most people, especially when you’re not paying attention to how much you eat. But because the number of calories you think you burned during exercise isn’t how many calories you really burned, due to the compensation that takes place. This compensation seems to increase as fat mass increases.
The goal of training during a fat loss phase is to maintain muscle and strength. If you enjoy cardio, do it for the health benefits, not to burn more calories. Of course, cardio can be implemented to increase the deficit, but be mindful of how much you’re doing so you don’t inadvertently end up in too big a deficit, which can then increase fatigue and result in less movement for the rest of the day.
Your energy balance status likely impacts the degree of compensation
If you’re at maintenance or a surplus, you could probably get away with a bit more physical activity without experiencing the same degree of compensation you would during a deficit.
This also adds to the point above: If you do more exercise in the hopes you’ll ‘speed’ up fat loss, the more likely the compensation is to kick in, hence why the goal of a fat loss diet is to coax (not force) fat loss.
Stop paying attention to your smartwatch
While this point isn’t directly related to the topic, I think it’s still worth mentioning. Or, well, mentioning again because I still get questions from people about whether they should ‘eat back’ their exercise calories.
Let’s forget that smartwatches are terrible at accurately estimating energy expenditure (see this and this). Even if they were accurate, they wouldn’t be able to account for the compensation that occurs.
That said, smartwatches appear to be pretty accurate for tracking step counts, and you can utilise this to your advantage. If you know how many steps you’re doing at the start of a diet, you can try to maintain the number of steps throughout the diet to help keep energy expenditure up.
Finally, before wrapping up, I want to emphasise that physical activity is important.
I’ve seen many people misinterpret the original Pontzer study to imply that physical activity is pointless.
But that isn’t what the constrained energy model suggests. As I mentioned above, the constrained energy model states that physical activity does increase energy expenditure, but only up to a point. It’s when physical activity becomes excessive that the increase in energy expenditure plateaus.
Putting aside all the obvious benefits, one of the most significant benefits of regular exercise is its role in maintaining weight. In fact, according to the latest systematic review on the subject, it’s the “most consistent positive correlate of weight loss maintenance.” 5
So exercise is definitely important. And contrary to what some people say, exercise will increase your energy expenditure. However, the extent to which it increases will depend on several factors, including your energy status, body fat, and the impact of the exercise session on your activity levels throughout the day.
It’s time to shift the focus away from using exercise as a means to burn calories and place more emphasis on using physical activity to improve our health and support our fitness goals. Any increase in energy expenditure should be viewed as a nice bonus, not the sole reason for exercising.
Thanks for reading. If you enjoyed this, you’d love the Vitamin
95% of my new content is only being sent to my email list. One email every Thursday, filled with actionable, evidence-based fitness advice to help you with your goals. If you enjoyed this, you’ll love my emails. You can learn more and subscribe for free here.