New research upends obesity myth: It’s the calories, not the lifestyle

An international study led by Duke University researchers challenges the widespread belief that modern sedentary lifestyles are primarily to blame for the global obesity epidemic. Instead, the study finds that rising obesity in economically developed countries is driven largely by increased caloric intake rather than reduced physical activity.

The findings, published July 14 in the Proceedings of the National Academy of Sciences (PNAS), suggest public health efforts should refocus on diet as the central driver of obesity while still promoting physical activity for its many other benefits.

The findings arrive amidst the federal government’s realignment of national health priorities under the Make America Healthy Again (MAHA) initiative. Led by Health and Human Services (HHS) Secretary Robert F. Kennedy Jr., MAHA seeks to reduce chronic disease by promoting better nutrition, transparency and root-cause prevention strategies.

“It’s clear that changes in diet, not reduced activity, are the main cause of obesity in the U.S. and other developed countries,” said Herman Pontzer, Ph.D., professor of evolutionary anthropology at Duke and senior author of the study.

Energy output steady across economies, despite rising obesity

The study, which analyzed data from 4,213 adults across 34 populations on six continents, found that people in industrialized nations expend roughly the same — or even more — daily energy as those in less-developed societies.

Using doubly labeled water to measure total energy expenditure (TEE), the researchers compared individuals from hunter-gatherer and subsistence farming communities with those in low-, middle- and high-income countries.

Although physical activity patterns varied widely, total energy expenditure remained relatively consistent after adjusting for body size and composition.

Body size-adjusted TEE declined slightly — by just 6% to 11% — across the economic development spectrum.

While physical activity energy expenditure (AEE) and physical activity level (PAL) did not significantly decline, body fat and BMI rose sharply with affluence.

“While we saw a marginal decrease in size-adjusted total energy expenditure with economic development, differences in total energy expenditure explained only a fraction of the increase in body fat that accompanied development,” said Amanda McGrosky, Ph.D., lead author of the study and assistant professor of biology at Elon University.

Dietary intake is the primary driver

By comparing energy expenditure with body composition data, the researchers concluded that increased caloric intake — not reduced expenditure — accounts for most of the rise in obesity associated with economic development.

The study estimates that differences in energy expenditure explain only about one-tenth of the increase in BMI and body fat observed in wealthier populations. The remaining gap, the authors argue, is attributable to increased calorie consumption, including a rise in ultraprocessed food intake.

Among the 25 populations for which dietary data was available, the proportion of daily calories from ultraprocessed foods was significantly correlated with higher body fat percentages, even after controlling for energy expenditure and other demographic factors.

A wake-up call for chronic condition prevention

Though the study stops short of prescribing policy, it lends weight to the recent MAHA platform’s call to reframe health care around root causes. The emphasis on ultraprocessed foods and caloric overload as key contributors to chronic disease echoes key MAHA talking points and the movement’s goals to shift from disease management to disease prevention, address environmental and dietary drivers of illness and reduce America’s dependence on high-cost medical interventions.

That said, the authors caution that the findings should not be interpreted as a reason to downplay the importance of physical activity.

“Diet and physical activity should be viewed as essential and complementary, rather than interchangeable,” the study notes.