Gym-goers might think that if they huff it on a treadmill for two hours every day, they will burn more calories overall than if they sneak in just 30 minutes.
But according to a study published Thursday in the journal Current Biology, as long as people are doing at least some baseline level of activity, they will expend about the same amount of energy each day no matter how much exercise they do — suggesting that exercise alone cannot be relied upon as a way to control weight at a time when the majority of American adults are overweight or obese.
“We should get out of our notion is that if we eat a chocolate cake, all we have to do is get on the treadmill and we’ll burn it off,” said Bill McCarthy, a nutrition and lifestyle researcher at the University of California, Los Angeles, who was not involved with the study. “The body doesn’t work that way — sorry.”
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For the study, a team led by Herman Pontzer sought to test whether energy expenditures increased as physical activity did, or if those expenditures plateaued no matter how much activity people were doing.
Researchers looked at 332 people in five different populations around the world: a mostly agrarian group in Ghana; people living in a township in South Africa; urban residents in Jamaica; island dwellers in the Seychelles; and suburbanites in the United States.
For about a week, they tracked physical activity using wearable devices akin to Fitbits, and measured energy expenditures through a specialized urine test.
They found that the amount of spent energy does increase with physical activity levels, but only in the low ranges of exercise overall. Pontzer said someone who does even just a little bit of activity — say, walking to work or exercising a few days a week — can burn about 200 more calories per day than a total couch potato.
But for participants whose activity levels ranged from moderate to intense, the number of calories shed stopped rising even as fitness levels soared, showing that overall energy expenditures tend to be constrained.
The study did not delve into why energy levels peak with only moderate levels of activity, but Pontzer, an anthropology professor at City University of New York’s Hunter College, offered two possibilities.
At the cellular level, the biological processes that pulse through our bodies all day may demand less energy in people who are more active, he said. And behaviorally, people who are more active during parts of the day may use up less energy during other parts of the day, perhaps by resting more or fidgeting less.
“You see the body works pretty hard to keep energy expenditures in check,” Pontzer said.
This helps explain why energy usage tends to be the same across populations, experts said. People living similarly to traditional hunter-gatherers in African countries, for example, burn roughly the same number of calories as desk jockeys hunched over keyboards for 12 hours a day.
And it’s not just people: Studies have found the same pattern in animals living in the wild versus those in captivity — meaning a polar bear chasing seals in the Arctic expends the same amount of energy as one snoozing in the sun at the zoo who has moderate activity levels.
Physical activity is, of course, still incredibly important for overall health. Among its benefits, it helps the heart and joints and promotes better aging.
But previous research has similarly found that, above a certain exercise level, additional activity does not further lower the risk of death from cardiovascular disease or cancer. And the new study complements other research showing that diet may be more important than exercise when it comes to losing and maintaining weight.
Those findings have arrived as health-promoting messages still frequently emphasize physical activity above other lifestyle changes.
“Evidence has been accumulating over the last 10 years or so that [exercise] shouldn’t be the focus of public health professionals,” said study author Amy Luke, a nutrition researcher at Loyola University Chicago. “It should be on the intake side.”
Luke has been following the five study populations for about six years and plans to publish research looking more deeply at how diets and environments in the different places affect obesity rates. With that information, she hopes to encourage better eating habits — not just in the United States, but around the world as the global burden of obesity rises.
“Because we’re going across this broad range of populations,” she said, “I’d like to be able to inform public health policies in countries that have experienced levels of obesity like we have in the US.”