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.

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.

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.

"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.

"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

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.

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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