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It's common knowledge that obesity is a global epidemic, particularly in industrialized countries, and that it is a major cause of disease and poor overall health. However, there has been a tug-of-war in the debate of whether the main cause of obesity is diet or a lack of exercise.

Calories consumed must be expended as energy, whether through physical activity or basal activity—the inherent processes within the body that use up energy, such as breathing or digesting food. When it comes to obesity, it has been unclear if too many calories are being consumed on average to be used by normal activity or if there is not enough activity to expend a reasonable amount of calories.

A new study, published in the journal Proceedings of the National Academy of Sciences, focuses on the problem of diet vs. exercise. Amanda McGrosky and her team analyzed data from 4, 213 adults, aged 18 to 60, across 34 populations in six continents. The sample is impressively diverse and covers people from hunter-gatherer groups, pastoralists, farmers, and industrialized societies. This includes people with a wide range of diets and activity levels.

The researchers examined total energy expenditure (TEE), activity energy expenditure (AEE), basal energy expenditure (BEE) and two measures of obesity: body fat percentage and body mass index (BMI). AEE is a representation of energy expended by exercise and was found by subtracting BEE from the TEE. The results were categorized into groups by the level of economic development, using the UN Human Development Index (HDI), due to the general lifestyle and dietary differences among groups at different levels of economic development.

The results initially found TEE, AEE and BEE to be higher in more developed populations, along with body mass, BMI and body fat, meaning obesity was more prevalent in more economically developed countries, but it appeared that they were also expending more energy overall. However, this is not the total picture.

Body size and composition across economies. Cohorts are ordered from lowest to highest HDI score. Bars indicate mean and quartiles. Credit: Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2420902122

The researchers acknowledged that body size in general, like height, was overall larger in industrialized places. There are also fluctuations in weight and energy expenditure due to age and sex. The researchers further refined their data by adjusting for these factors.

The results then showed that TEE and BEE decreased slightly, by around 6–11%, with economic development. After adjusting for age, sex and body size, AEE was still higher overall in more economically developed populations, indicating that it is not likely to be a lack of exercise that is causing higher BMI or body fat.

The researchers found that total energy expenditure is only weakly associated with obesity, accounting for about 10% of the increased incidence of obesity in more economically developed countries. Instead, they indicate that a higher consumption of ultra processed foods (UPF), like processed meats, ready-made meals and snack cakes, is to blame, noting that "the percent of UPFs in the diet was positively correlated with body fat percentage."

Despite exercise not being the main driver of obesity, the study authors encourage regular exercise, as it is still known to be key in preventing disease and maintaining better mental health. The study also highlights the need for an emphasis on reducing calories from highly processed foods to help address the obesity crisis, but also to continue to study what makes certain foods lead to obesity, as UPFs appear to affect the body in unique ways.

They write, "The hyperpalatability, energy density, nutrient composition, and appearance of UPF might disrupt satiety signaling and encourage overconsumption. Processing has also been shown to increase the percentage of calories consumed that are absorbed into the body rather than excreted."

Written for you by our author Krystal Kasal, edited by Gaby Clark, and fact-checked and reviewed by Andrew Zinin—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

More information: Amanda McGrosky et al, Energy expenditure and obesity across the economic spectrum, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2420902122  Journal information: Proceedings of the National Academy of Sciences

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