The global obesity epidemic has long been attributed to two main factors: reduced physical activity and increased caloric intake. Public health narratives often emphasize that sedentary lifestyles are at the core of modern weight gain. However, recent evidence challenges this assumption.
A comprehensive study analyzing more than 4,200 adults across 34 populations on six continents found that diet, not declining activity, is the dominant driver of obesity worldwide (McGrosky et al., 2025).
The study revealed that individuals in developed nations actually burn more calories overall than those in less-developed regions, largely due to increased body size. When researchers adjusted for body size, total and basal energy expenditure decreased modestly with economic development—approximately 6 to 11%. Crucially, physical activity expenditure remained stable across all levels of industrialization.
This suggests that declines in physical activity do not meaningfully explain the surge in obesity rates. At most, reduced basal energy expenditure (BEE) could account for about 10% of the increase in body mass index (BMI) and body fat observed with economic development.
In contrast, dietary intake emerged as a far stronger predictor of obesity. Estimated energy intake was roughly ten times more influential than reduced expenditure in explaining higher body fat and BMI. The study also found a strong positive correlation between consumption of ultraprocessed foods (UPFs) and body fat percentage, independent of age, sex, or activity levels.
UPFs—industrial formulations typically high in sugar, fat, salt, and additives—appear to undermine normal satiety signals, encourage overconsumption, and increase the proportion of calories absorbed rather than excreted. These foods are abundant in high-income countries and are rapidly penetrating developing markets, making them central to the global obesity crisis.
While physical activity remains critical for cardiovascular health, mental wellbeing, and overall longevity, it is unlikely to be the decisive factor in rising obesity rates. Instead, excess caloric intake, particularly from poor-quality diets, should be seen as the primary culprit.
This does not mean that exercise should be deprioritized, but rather that public health strategies must focus more heavily on regulating food environments, limiting ultraprocessed food consumption, and improving overall diet quality.
References
Caballero, B. (2007). The global epidemic of obesity: An overview. Epidemiologic Reviews, 29(1), pp. 1–5.
Komlos, J. & Brabec, M. (2010). The trend of mean BMI values of US adults, birth cohorts 1882–1986 indicates that the obesity epidemic began earlier than hitherto thought. American Journal of Human Biology, 22(5), pp. 631–638.
McGrosky, A., Luke, A., Arab, L., & The IAEA DLW Database Consortium. (2025). Energy expenditure and obesity across the economic spectrum. Proceedings of the National Academy of Sciences, 122(29), e2420902122. Available at: https://doi.org/10.1073/pnas.2420902122
[Accessed 8 Sept. 2025].
Stanaway, J. D., et al. (2018). Global, regional, and national comparative risk assessment of 84 behavioural, environmental and occupational, and metabolic risks for 195 countries and territories, 1990–2017: A systematic analysis for the Global Burden of Disease Study 2017. The Lancet, 392(10159), pp. 1923–1994.
