Study Finds Overeating, Not Inactivity, Drives Obesity in Wealthier Societies

New international research has revealed that excess calorie intake — rather than declining physical activity — is the primary factor driving rising obesity rates in industrialized societies. The findings challenge long-held assumptions about the global obesity epidemic and underscore the importance of dietary interventions over exercise-focused strategies.

The study, published in the Proceedings of the National Academy of Sciences (PNAS), draws on data from the International Atomic Energy Agency (IAEA)’s Doubly Labelled Water (DLW) Database and involved more than 50 research institutions across 19 countries.

Unpacking the Obesity Crisis

Obesity has become one of the most pressing public health issues worldwide. By 2022, nearly one in eight people globally were classified as obese — a figure that has more than doubled among adults and quadrupled among adolescents since the 1990s. Obesity is strongly linked to chronic diseases including diabetes, cardiovascular illnesses, and several cancers.

Although obesity rates soar in industrialized countries, traditional farming and subsistence communities rarely experience the same prevalence. This contrast has long fueled the debate over whether reduced activity levels or greater calorie consumption are to blame for the condition’s rise alongside economic development.

“Despite decades of trying to understand the root causes of the obesity crisis in economically developed countries, the relative importance of diet and physical activity has remained uncertain,” said Herman Pontzer, professor of evolutionary anthropology and global health at Duke University and one of the article’s co-authors. “The IAEA’s database enabled a collaborative global effort to resolve this pressing public health question.”

How the Research Was Conducted

The study analyzed 4,213 adults between the ages of 18 and 60, covering 34 populations across six continents with diverse economic and lifestyle contexts. The researchers used the DLW stable isotope technique, considered the gold standard for measuring total daily energy expenditure, to capture reliable and precise metabolic data.

Key findings included:

• People in wealthier, industrialized societies burn more absolute energy — largely because of larger average body sizes — but when adjusted for body mass, their energy expenditure is only slightly lower than in less developed populations.

• At most, only 10% of BMI and body fat increases associated with industrialization could be attributed to differences in activity-related energy expenditure.

• The primary driver of weight gain was increased calorie intake, inferred from the gap between measured energy expenditure and actual weight change over time.

These results directly challenge the assumption that sedentary lifestyles alone explain obesity trends.

Why Diet Matters More

“Policies which focus on improving diet quality and reducing the consumption of high-calorie, ultra-processed foods are likely to be more effective in combating obesity than those centred solely on increased physical activity,” explained Cornelia Loechl, Head of Nutritional and Health-related Environmental Studies at the IAEA and a co-author of the study.

This does not mean that physical activity lacks importance. Exercise remains vital for cardiovascular health, mental well-being, and chronic disease prevention. However, when it comes to controlling weight in wealthy societies, diet emerges as the dominant factor.

The Role of the IAEA’s DLW Database

The IAEA’s Doubly Labelled Water Database is a global repository of high-quality measurements of human energy expenditure. With data from 45 countries, it has become a cornerstone for metabolic research. Beyond this obesity study, the database has been instrumental in:

• Developing predictive models to assess self-reported diet and activity data.

• Informing global standards for human energy requirements.

• Advancing the science of human energy metabolism and nutrition policy.

The methodology, first developed in the 1980s, involves individuals drinking water enriched with stable isotopes of hydrogen and oxygen, which can then be traced through the body to calculate precise energy expenditure over time.

Global Impact and Public Response

Since publication, the PNAS article has been viewed more than 72,000 times and covered by over 165 international news outlets, highlighting the global interest in obesity research and its implications for public health.

Experts believe these findings could reshape health campaigns, government nutrition policies, and international strategies to combat obesity. Instead of focusing narrowly on fitness promotion, interventions may need to prioritize reducing calorie intake, improving food quality, and addressing the spread of ultra-processed diets.

As the obesity epidemic continues to rise, particularly in countries undergoing rapid economic development, the study underscores a sobering but critical truth: it is not how little we move, but how much we eat that matters most in the battle against obesity.