https://www.npr.org/2025/07/24/nx-s1-5477662/diet-exercise-obesity-nutrition

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Diet, not lack of exercise, drives obesity, a new study finds One
explanation for the rise in obesity in industrialized countries is
that people burn fewer calories than people in countries where
obesity is rare. A major study finds that's not the case.
 
Special Series

Living Better

Science for a healthy life

You can't outrun a bad diet. Food -- not lack of exercise -- fuels
obesity, study finds

July 24, 20255:00 AM ET
Maria Godoy at NPR headquarters in Washington, D.C., May 22, 2018.
(photo by Allison Shelley) (Square)

Maria Godoy

Diet, not lack of exercise, is driving obesity

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A new study shows people in countries with different obesity rates
burn about the same number of calories.

A new study shows people in countries with different obesity rates
burn about the same number of calories. PCH-Vector/iStockphoto via
Getty Images hide caption

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PCH-Vector/iStockphoto via Getty Images

Back in the 1800s, obesity was almost nonexistent in the United
States. Over the last century, it's become common here and in other
industrialized nations, though it remains rare among people who live
more traditional lifestyles, such as the Hadza hunter-gatherers of
Tanzania.

So what's changed? One common explanation is that as societies have
developed, they've also become more sedentary, and people have gotten
less active. The assumption is that as a result, we burn fewer
calories each day, contributing to an energy imbalance that leads to
weight gain over time, says Herman Pontzer, a professor of
evolutionary biology and global health at Duke University who studies
how human metabolism has evolved.

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But in a major new study published in the journal PNAS, Pontzer and
an international team of collaborators found that's not the case.
They compared the daily total calorie burn for people from 34
different countries and cultures around the world. The people
involved ran the spectrum from hunter-gatherers and farming
populations with low obesity rates, to people in more sedentary jobs
in places like Europe and the U.S., where obesity is widespread.

"Surprisingly, what we find is that actually, the total calories
burned per day is really similar across these populations, even
though the lifestyle and the activity levels are really different,"
says Pontzer.

What we know about the health risks of ultraprocessed foods

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What we know about the health risks of ultra-processed foods

And that finding offers strong evidence that diet -- not a lack of
physical activity -- is the major driver of weight gain and obesity in
our modern world.

"This does sort of really fly in the face of what a lot of us
anecdotally assumed was driving a lot of the weight gain and obesity
today," says Deirdre Tobias, an obesity and nutritional
epidemiologist at the Brigham and Women's Hospital in Boston and the
Harvard T.H. Chan School of Public Health. Tobias was not involved in
the new research.

Different activity levels, same calorie burn

In the study, researchers examined data for more than 4,200 adult men
and women. Study participants were given a special water to drink
that contained isotopes that came out in their urine. These isotopes
allow scientists to determine exactly how much energy a person
expends -- not just in active movement, but also the energy it takes
to keep our hearts beating, our nervous systems functioning and just
generally stay alive.

There's a way to get healthier without even going to a gym. It's
called NEAT

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There's a way to get healthier without even going to a gym. It's
called NEAT

"This allows us to get a really accurate measure of how many calories
people burn per day over the course of about a week," Pontzer says.

When they adjusted for body size, Pontzer and his colleagues found
that people from populations with higher obesity rates burned only
slightly less total energy per day than those from populations that
weighed less. These differences in energy expenditure didn't
contribute much to the differences in obesity rates among
populations, he says.

While it may seem implausible that someone who is out foraging for
berries all day burns about the same amount of daily calories as,
say, your typical office worker, Pontzer says it fits with what
scientists have been learning about how our bodies burn calories.
Pontzer's past research has suggested that our bodies tend to adjust
how we expend energy to keep our total calorie burn stable within a
pretty narrow range.

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"So if we burn more of our energy every day on physical activity, on
exercise, after a while our bodies will adjust and spend less energy
on the other tasks that we sort of don't notice going on in the
background," Pontzer says.

Changing the message on obesity

Less snacking, more satisfaction: Some foods boost levels of an
Ozempic-like hormone

Shots - Health News

Less snacking, more satisfaction: Some foods boost levels of an
Ozempic-like hormone

The new findings have big implications for obesity. If differences in
calorie burn can't explain why some countries have higher obesity
rates than others, then it must be something else. "And that would be
diet," says Tobias, who praised the design of the new study.

"It's 100% the diet," agrees Dr. Dariush Mozaffarian, director of the
Food is Medicine Institute at the Friedman School of Nutrition
Science and Policy at Tufts University. "And I think then the
question is, what is it about the diet?"

Mozaffarian was not involved in the study, but he says it adds to
other recent research that suggests food is the biggest driver in
obesity. He points out there's been a major shift in our food supply
in recent decades -- which is now dominated by ultra-processed food.
In a subanalysis of the data for some of the populations, Pontzer and
his colleagues found that people in countries that got more of their
calories from ultra-processed foods tended to have more obesity and
higher body fat percentages.

"For decades we've been telling Americans that you're lazy, it's your
fault, you're not moving enough, you're eating too much," Mozaffarian
says. "And I think what this study shows is that there's really
complicated biology happening and that our food is driving this."

Now, this doesn't mean there's no reason to exercise. After all, it's
good for our mental and physical health in so many ways that have
nothing to do with weight.

People who exercised the equivalent of about an hour of walking a
day, six days a week saw the most benefit.

Living Better

Colon cancer survivors who exercise regularly live longer

But it does mean we can't outrun a bad diet. Pontzer says if we want
to tackle obesity, the public health message should focus on changing
what's on our plates.

Edited by Jane Greenhalgh

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