What causes obesity? A major new study is upending common wisdom.

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Obesity is uncommon among Hadza hunter-gatherers in Tanzania, Tsimane forager-farmers in Bolivia, Tuvan herder-farmers in Siberia, and other people in less-developed nations. But it's widespread among those of us in wealthy, highly industrialized nations.
Why? A major study published this week in PNAS brings surprising clarity to that question. Using objective data about metabolic rates and energy expenditure among more than 4,000 men and women living in dozens of nations across a broad spectrum of socioeconomic conditions, the study quantified how many calories people from different cultures burn most days.
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For decades, common wisdom and public health messaging have assumed that people in highly developed nations, like the United States, are relatively sedentary and burn far fewer daily calories than people in less-industrialized countries, greatly increasing the risk for obesity.
But the new study says no. Instead, it finds that Americans, Europeans and people living in other developed nations expend about the same number of total calories most days as hunter-gatherers, herders, subsistence farmers, foragers and anyone else living in less-industrialized nations.
That unexpected finding almost certainly means inactivity is not the main cause of obesity in the U.S. and elsewhere, said Herman Pontzer, a professor of evolutionary anthropology and global health at Duke University in North Carolina and a senior author of the new study.
What is, then? The study offers provocative hints about the role of diet and some of the specific foods we eat, as well as about the limits of exercise, and the best ways, in the long run, to avoid and treat obesity.
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Is diet or inactivity causing obesity?
'There's still a lively debate in public health about the role of diet and activity' in the development of obesity, Pontzer said, especially in wealthy nations. Some experts believe we're exercising too little, others that we're eating too much, and still more that the two contribute almost equally.
Understanding the relative contributions of diet and physical activity is important, Pontzer noted, because we can't effectively help people with obesity unless we first tease out its origins. But few large-scale studies have carefully compared energy expenditure among populations prone to obesity against those more resistant to it, which would be a first step toward figuring out what drives weight gain.
So, for the new study, Pontzer and his 80-plus co-authors gathered existing data from labs around the world that use doubly labeled water in metabolism studies. Doubly labeled water contains isotopes that, when excreted in urine or other fluids, allow researchers to precisely determine someone's energy expenditure, metabolic rates and body-fat percentage. It's the gold standard in this kind of research.
They wound up with data for 4,213 men and women from 34 countries or cultural groups, running the socioeconomic gamut from tribes in Africa to executives in Norway. They calculated total daily energy expenditures for everyone, along with their basal energy expenditure, which is the number of calories our bodies burn during basic, biological operations, and physical activity energy expenditure, which is how many calories we use while moving around.
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A new theory of how our metabolisms work
After adjusting for body size (since people in wealthy nations tend to have larger bodies, and larger bodies burn more calories), they started comparing different groups. Anyone expecting a wide range of energy expenditures, with hunter-gatherers and farmer-herders at the high end and deskbound American office workers trailing well behind, would be wrong.
Across the board, the total daily energy expenditures of the 4,213 people were quite similar, no matter where they lived or how they spent their lives. Although the hunter-gatherers and other similar groups moved around far more throughout the day than a typical American, their overall daily calorie burns were nearly the same.
The findings, though counterintuitive, align with a new theory about our metabolisms, first proposed by Pontzer. Known as the constrained total energy expenditure model, it says that our brains and bodies closely monitor our total energy expenditure, keeping it within a narrow range. If we start consistently burning extra calories by, for instance, stalking prey on foot for days or training for a marathon, our brains slow down or shut off some tangential biological operations, often related to growth, and our overall daily calorie burn stays within a consistent band.
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The role of ultra-processed foods
The upshot is that 'there is no effect of economic development on size-adjusted physical activity expenditure,' Pontzer says. In which case, the fundamental problem isn't that we're moving too little, meaning more exercise is unlikely to reduce obesity much.
What could, then? 'Our analyses suggest that increased energy intake has been roughly 10 times more important than declining total energy expenditure in driving the modern obesity crisis,' the study authors write.
In other words, we're eating too much. We may also be eating the wrong kinds of foods, the study also suggests. In a sub-analysis of the diets of some of the groups from both highly and less-developed nations, the scientists found a strong correlation between the percentage of daily diets that consists of 'ultra-processed foods' - which the study's authors define as 'industrial formulations of five or more ingredients' - and higher body-fat percentages.
We are, to be blunt, eating too much and probably eating too much of the wrong foods.
'This study confirms what I've been saying, which is that diet is the key culprit in our current [obesity] epidemic,' said Barry Popkin, a professor at the Gillings School of Global Public Health at the University of North Carolina at Chapel Hill and an obesity expert.
'This is a well-done study,' he added.
Other experts agree. 'It's clear from this important new research and other studies that changes to our food, not our activity, are the dominant drivers of obesity,' said Dariush Mozaffarian, director of the Food is Medicine Institute at Tufts University in Boston.
The findings don't mean, though, that exercise is unimportant, Pontzer emphasized. 'We know that exercise is essential for health. This study doesn't change that,' he said.
But the study does suggest that 'to address obesity, public health efforts need to focus on diet,' he said, especially on ultra-processed foods, 'that seem to be really potent causes of obesity.'
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