Obesity is linked more to food than fitness

Obesity is rare among Hadza hunter‑gatherers in Tanzania, Tsimane forager‑farmers in Bolivia, or Tuvan herder‑farmers in Siberia. Yet it’s common across rich, industrialized nations.

For decades, the explanation has been inactivity: people in wealthy countries sit too much and therefore burn too few calories. However, a new study argues that this explanation is wrong – or at least badly incomplete.

Researchers analyzed objective metabolic data from 4,213 men and women spread across 34 countries and cultural groups, running the gamut from small‑scale subsistence societies to office‑dwelling professionals in Europe and North America.

The result: people in highly developed nations burn about the same total number of calories per day, once you adjust for body size, as people in far more physically demanding settings.

That finding forces a rethink. If most of us burn similar daily energy regardless of lifestyle, then widespread obesity in wealthy nations must come primarily from what we eat, not how little we move.

Rethinking obesity’s root cause

“There’s still a lively debate in public health about the role of diet and activity in the development of obesity,” said senior author Herman Pontzer, a professor of evolutionary anthropology and global health at Duke University.

Untangling the two matters: prevention and treatment strategies hinge on knowing which lever – calories in or calories out – has moved the most.

To tackle the question, Pontzer and more than 80 collaborators pooled gold‑standard measurements collected with “doubly labeled water,” a tracer technique in which volunteers drink water marked with rare, harmless isotopes.

By tracking how those isotopes leave the body in urine or breath, scientists can calculate how much carbon dioxide a person produces – and from that, determine days‑long averages of total energy expenditure.

The dataset also included basal energy expenditure (calories burned just staying alive) and the extra calories used in physical activity.

Calorie burn stays steady

After statistically adjusting for body size – people in affluent countries tend to be taller and heavier, and bigger bodies naturally burn more – the expected pattern vanished.

Hunter‑gatherers who walk miles each day, herder‑farmers who haul and dig, and office workers who sit long hours all landed in roughly the same range for total daily energy burn.

Pontzer interprets this through his “constrained total energy expenditure” model. When physical activity rises, the body trims energy elsewhere (for example, dampening some growth or immune processes) to keep total burn within a narrow band.

The practical takeaway is that ramping up movement does not necessarily translate into a large increase in total calories burned over the long haul.

“There is no effect of economic development on size-adjusted physical activity expenditure,” noted Pontzer. In other words, rich‑country living hasn’t slashed our activity calories the way many assumed.

Food drives obesity now

When the team compared energy intake estimates, a pattern emerged. “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 authors wrote.

More simply put, overeating swamps any small changes in how much we burn.

Diet quality also mattered. In a subset of the sample where dietary data were available, higher intake of ultra‑processed foods correlated strongly with higher body fat percentages.

The study defined these products as “industrial formulations of five or more ingredients.” Think packaged snacks, sugary cereals, instant noodles, processed meats, and ready‑to‑eat meals – items engineered for shelf life and low cost.

“This study confirms what I’ve been saying, which is that diet is the key culprit in our current [obesity] epidemic. This is a well-done study,” said Barry Popkin, a nutrition researcher at the University of North Carolina.

“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,” added Dariush Mozaffarian, a cardiometabolic expert at Tufts University.

Exercise still improves health

“We know that exercise is essential for health. This study doesn’t change that,” Pontzer said. Regular movement improves heart health, mood, blood sugar control, and longevity – even if it doesn’t melt off pounds as easily as hoped.

Still, when the goal is lowering obesity rates, Pontzer argues that nutrition must take center stage. Public guidance, food policy, and healthcare counseling need to tackle obesity by focusing on dietary choices – what’s on the plate, how much is consumed, and how often.

“To address obesity, public health efforts need to focus on diet,” especially on the products “that seem to be really potent causes of obesity,” concluded Pontzer.

The study is published in the journal Proceedings of the National Academy of Sciences.

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