Why is obesity widespread in developed countries but uncommon among traditional hunter-gatherer or forager communities? A new global study published in the Proceedings of the National Academy of Sciences (PNAS) offers a surprising answer: it’s not because we’re more sedentary, the Washington Post reported.
We burn the same calories as hunter-gatherers
The study, which analysed data from over 4,200 adults across 34 countries, found that people in wealthy nations burn about the same number of calories daily as those in less-developed societies — including Tanzanian Hadza hunter-gatherers, Bolivian Tsimane farmers, and Siberian Tuvan herders. This challenges the long-standing belief that people in industrialized countries are less active and thus more prone to obesity.
The research used the “doubly labelled water” method, the gold standard in energy expenditure measurement. After adjusting for body size, researchers found total daily energy expenditure to be surprisingly uniform across populations, regardless of lifestyle or socioeconomic status.
Why movement alone isn’t the answer
Even though groups like the Hadza are physically active for hours each day, their total energy use remains close to that of office workers in the U.S. This supports the "constrained total energy expenditure" theory — the idea that the body maintains a consistent energy budget. When physical activity rises, energy is saved elsewhere, such as in immune or reproductive functions.
The implication? Exercise alone, while vital for health, is unlikely to significantly reduce obesity unless paired with changes in diet.
Ultra-processed foods: the real problem
The researchers found that food quality — not just quantity — plays a crucial role. Populations with high consumption of ultra-processed foods had significantly higher body-fat percentages. These foods, defined as industrial formulations with five or more ingredients, now make up a large share of diets in wealthier nations and are closely tied to rising obesity.
The study estimates that increased calorie intake — largely from processed foods — has played a 10-times greater role in the global obesity surge than reductions in physical activity.
Experts agree: diet must be the focus
“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 global obesity expert at the University of North Carolina. Dariush Mozaffarian of Tufts University echoed the point: “It’s clear... that changes to our food, not our activity, are the dominant drivers of obesity.”
Still, physical activity remains important. “We know that exercise is essential for health,” said Herman Pontzer, the lead author. “This study doesn’t change that.”
But when it comes to addressing obesity, he added, “public health efforts need to focus on diet — especially on ultra-processed foods that seem to be really potent causes of obesity.”
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