With nearly one in eight people around the world living with obesity in 2022, the disease has more than doubled among adults and quadrupled among adolescents over the past three decades. This complex and chronic condition of excessive body fat increases the risk of non-communicable diseases such as diabetes, heart disease and cancer. Yet despite its surge in industrialized populations, obesity is seldom seen?in traditional and farming communities — a contrast commonly attributed to greater physical activity.??

At its root, obesity stems from an imbalance between calories consumed and the energy the body burns.?Public health experts often point to two culprits —?overeating and insufficient physical activity. Yet the exact role each factor plays remain debated, since lower activity levels do not always translate into less energy expended over the course of a day.?

The lack of diverse, reliable data on calorie intake, energy expenditure and body composition has further complicated research. Past studies have tried to address the debate, but most focused on nonindustrial populations, lacked body fat measurements, or relied on limited information from country-level consumption data and surveys.???

To close this gap, 68 researchers turned to the IAEA’s Doubly Labelled Water (DLW) Database — a global collection of energy expenditure measurements that have been collected via the DLW stable isotope technique. With datapoints spanning 45 different countries, the database has previously been used by scientists to conduct groundbreaking research on human energy metabolism, develop a predictive equation to assess self-reporting and inform ongoing revisions of human energy requirements.??

The researchers analysed 4213 adults aged?18 to 60 across six continents, reflecting 34 populations from different economies and lifestyles. They observed a higher energy expenditure (both activity-related and in general) among populations from more industrialized economies. However, when adjusting for their larger body sizes, energy expenditure decreased only slightly — not enough to account for the overall increases in body mass index and fat percentage that are associated with economic development. At most, only ten per cent of these increases could be explained by the differences in energy expenditure between groups.?Instead, energy intake — which researchers estimated from measures of total energy expenditure and weight change — was the primary factor driving obesity.??

“For public health professionals and nutrition specialists, these findings offer novel insights on the importance of diet. 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 in the IAEA Division of Human Health and one of the article’s authors. “More broadly, these insights are a testament to the impactful scientific research the IAEA’s human health databases enable.”??

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