Exercise is often associated with caloric burn, but new research shows that active people actually use the same amount of energy as those who are sedentary. Here’s why that’s good news.
Despite all the benefits of exercise, it’s still often solely portrayed as a way to burn calories. One person on a mission to change that is Dr Herman Pontzer, whose new book, Burn: The Misunderstood Science of Metabolism, is going to change the way we view energy intake and expenditure.
Talking about the book on the Feel Better Live More podcast, he explained his groundbreaking research that shows that exercising for caloric burn is altogether pointless. There’s so much that factors into your personalenergy balance of intake and expenditure (such as your diet or whether you’re increasing or decreasing your usual rate of training). But it seems that across the average population and life span, our energy expenditure doesn’t change that much.
“That goes against what we’re taught both in nutrition classes and basic biology lessons you might have had. [Everyone] talks about exercise as a way to lose weight… [so] it was a game-changer for us. I can tell you that it was a total sea change in the way that I think about metabolism,” Dr Pontzer said of his research.
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That’s because it showed that, on average, pretty much everyone burns the same amount of calories a day. That’s true even when comparing active versus sedentary people, because our metabolisms are very well set.
The conclusion came from spending time with the Tanzanian Hadza tribe, one of the last hunter-gatherer tribes in the world. “Hadza men and women get between five and 10 times more physical activity every day than many women in the US or Europe. They are incredibly physically active. If you’re a step counter, the Hadza people get about 13,000 to 19,000, steps a day. Plus they’re digging up tubers and they’re chopping into trees… and they’re carrying babies on their back. I mean, it’s a lot of work,” Dr Pontzer explains.
“We were sure going into that that they would be spending tons more calories every day than we do in the west. That was the whole premise of the work, to go and document the deficit that we seem to have in the west. And instead what we found was that their total energy expenditure, or total calories, spent per day is the same – indistinguishable from folks in the US and Europe.”
While “the biggest predictor of how many calories you’re going to burn every day is how big you are” – as bigger people have more cells – and how much fat-free mass (muscle) you have, on average the human population burns the same amount every day. Men burn roughly 3,000 calories a day, whereas all women burn around 2,400 calories.
How does that happen? Well, Dr Pontzer and his team came to realise that metabolism is moderated, and the brain makes “economic decisions about how to spend calories. Someone who lives in the population like the Hadza actually doesn’t have a top-line number of calories per day that’s any different than you and me. They’re obviously [just] spending those calories very, very differently. They spend lots on activity and less on other stuff.”
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What impacts energy expenditure?
In order to understand this, it’s important to know what ‘metabolism’ really is. “It’s just an umbrella term for all the work that your cells do all day. You’ve got 37 trillion cells, and they are all doing work all day… all of that work requires energy.
“Most of that work we’re only dimly aware of, or maybe even not aware of at all, so things like your liver [function], digesting your food, your brain working… the immune system, your reproductive system. Then there’s the part of it that we’re aware of. The exercise part, when we get moving, we get sweaty, we get our heart rate up. We tend to equate energy expenditure with the activity we can see, we can feel,” he says.
This is why we see such similar energy expenditure in those with completely different lifestyles – because the less active you are, the more energy your body spends on the processes we can’t see. “What’s exciting for me as an evolutionary biologist is figuring out what it is the body prioritises,” says Dr Pontzer.
“We know that some of the things the body reduces expenditure on are some of the really good things that exercise does for you. So if you look at that person who’s running 20 miles a week, they’re going to have lower inflammation levels. What is inflammation? Inflammation is actually an energetically costly immune function that your body doesn’t need to do. It’s your innate immune function always on being on high alert when it doesn’t need to be, and it’s actually bad for you to have high background levels of inflammation stress.”
As well as reducing inflammation to reduce caloric spend, the body also lowers cortisol – the stress response – as well as healthily reduces testosterone, oestrogen and progesterone levels. Lower levels of these things improve our overall health (to a point – if the body starts to save too much energy by lowering hormone levels, women can end up with amenorrhea or REDS).
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“It’s a long-term time frame that your body’s working under – [regulating over] a couple of weeks or even maybe a couple of months,” says Dr Pontzer. “If you change that by starting your exercise programme you might lose a little bit of weight early on, but as your body adjusts to that new level of activity, your total energy expenditure will moderate and end up not a whole lot different than before you started.
“Day to day, you might still fluctuate in your expenditures – so if you always run 10 miles on Tuesdays and you sit on the couch all your other days, then yeah, on Tuesdays you’ll burn more calories because your body can’t adjust in 24 hours. But your body gets used to a lifestyle.”
Remember: energy intake and food quality is a whole other subject here. But the news that exercise doesn’t mean burning more fuel shouldn’t be disheartening – it’s actually incredible that the body can regulate its own metabolism regardless of how active our lifestyles are. And the most important takeaway is that there’s so much more to exercise than caloric burn – less inflammation, better hormones responses and less stress are reasons to move that we can get on board with.
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