Photo: Nestor Rizhniak (Shutterstock)
Exercise offers any number of benefits, helping to prevent or lessen the effects of chronic health conditions like anxiety, depression, heart disease, and high blood pressure, among others. It is, in so many ways, an essential component of good health.
However, as scientists have discovered in recent years, what staying active does not do is lead to a long-term, significant increase in the number of calories we burn in a day.
“Your lifestyle doesn’t significantly affect how many calories you burn, at least not in a simple one-to-one way,” said Herman Pontzer, an evolutionary anthropologist at Duke University and author of the book Burn: New Research Blows The Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy.
Instead, our bodies are adapted to burn a relatively fixed amount of energy every day—an amount which doesn’t significantly vary between people of the same weight who are sedentary versus those who are active.
Daily calories burned doesn’t vary much
If you’re starting up a new exercise program, you’ll probably burn extra calories in the short-term, but within a few months, your body will adapt, keeping your total energy expenditure within a relatively narrow margin.
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“The body accommodates these long-term changes in how you spend calories,” said Samuel Urlacher, an evolutionary anthropologist at Baylor University who collaborates with Pontzer. “If you are a regular exerciser, you’re not spending as many calories as those predictive equations say you are.”
This concept, called constrained daily energy expenditure, is a relatively recent one in the field of fitness and nutrition, and has been verified by comparing the energy requirements of people living a traditional hunter-gatherer lifestyle, which includes a lot of physical activity, with the energy requirements of people with a sedentary lifestyle.
The average number of calories burned is the same for sedentary and active people
What Pontzer, Urlacher and others have discovered is that both of these populations—active hunter-gatherers and sedentary people—burn, on average, the same number of calories a day, once you adjust for weight. The difference appears to be in how they burn calories, rather than how many.
Although the original idea was that the similarities are due to active people either compensating by being less active during the rest of the day, or because they are incredibly efficient with their movement, the answer looks to be a little more complicated.
It turns out the body uses a lot of energy for critically important tasks that aren’t quite as visible or as obvious as our activity levels. “Even if you exercise all the time, you still spend well over 50% of your calories on just resting,” Pontzer said.
This includes expending energy on your immune system, your stress response, your reproductive response, as well as your brain. “Your brain burns the equivalent of 300 kilocalories every day,” Pontzer said. “That’s the equivalent of running a 5K.”
Our bodies spend the energy that isn’t used on exercise on other tasks
The thought is that, when faced with an excess of energy, which has been a relatively rare situation for most of human history, the body will use it on processes that are useful, but normally a low priority for the body. “Now that we live in a high energy environment all the time, your body can do those low priority things all the time,” Pontzer said.
A helpful analogy would be to think of this concept as similar to a household budget. If you’ve always made just barely enough to survive, only to come into some extra money, you’ll probably spend it on something that will really help you but is way out of your normal budget. You won’t change your overall budget, because you can’t count on getting that extra money again.
With our bodies, which adapted to survive on an energy budget that was really tight and often unpredictable, the extra energy that a sedentary lifestyle provides is treated like a one-time surplus, which it either has to use or lose.
Sedentary lifestyles lead to increased inflammatory and stress responses
What this means is that if your body isn’t burning those calories on exercise, it will use those calories on low-priority things that are useful but have a high energy cost, such as the inflammatory and stress response.
In small amounts, the inflammatory and stress responses protect us from pathogens and help us escape from danger. At chronic levels, this can damage blood vessels and other tissues, leading to a number of health issues.
Highly sedentary people show higher levels of chronic inflammation, as well as an elevated stress response, including increased levels of cortisol and adrenaline. People who are very active show lower levels of inflammation, as well as a reduced response to stress. Active people who aren’t eating enough can take longer to recover from injuries or infections, because their bodies can’t devote enough energy to the immune system.
When it comes to how our bodies use energy, we are still stuck in an evolutionary past in which energy was always in tight supply, and we needed whatever inflammatory or stress responses we could afford.
The difference is that we are now living in an energy-rich environment where we can afford so much, it’s damaging our bodies. “We call this an evolutionary mismatch,” Urlacher said. “Because the changes were so abrupt, our biology is not optimized to our new environments.”
Choose exercise based on what you enjoy, not calorie counts
In some ways, this knowledge is freeing: We know that exercise can help our health and happiness in some very important, tangible ways that go far beyond just burning calories. Exercise improves your mood, helps prevent or mitigate chronic health conditions, plus there is a lot of joy in having the strength and energy to live the kind of life you want.
With the goal of burning extra calories out of the way, your priorities can be solely about discovering what types of exercise you enjoy the most—the kinds of movement that you enjoy doing, that make you happy, that you’ll want to do regularly, calorie counts be damned.
So stop counting your exercise calories, and instead focus on finding activities that work for you, so that you can access the many other benefits associated with regular physical activity.
And as for what you should be eating and in what quantities? That should be a relatively constant amount, with a focus on healthy foods.
“You can’t outrun a bad diet,” Urlacher said.
