A global study reveals that eating more ultraprocessed foods, not burning fewer calories, explains why obesity rates soar as economies grow.
Study: Energy expenditure and obesity across the economic spectrum. Image Credit: Valente Romero / Shutterstock
In a recent article published in the journal PNAS, researchers examined how economic development affects obesity by analyzing energy expenditure and weight indicators from more than 4000 people around the world.
Their findings reveal that energy expenditure was higher in economically developed populations, primarily due to greater body size.
However, once they adjusted for body size, total and basal energy expenditure decreased slightly with development while activity energy expenditure remained unchanged, and did not align closely with lifestyle. In contrast, higher energy intake, particularly from ultraprocessed foods (UPFs), was strongly linked to obesity.
Background
Obesity is a growing global health crisis, contributing to over four million deaths annually. Its rapid rise is closely linked with economic development. In contrast to the rarity of obesity in pre-industrial societies and traditional communities, modern industrialized societies show high prevalence.
While weight gain results from consuming more calories than are expended, public health agencies often cite both decreased physical activity and increased food intake as causes. However, determining their relative importance has proven difficult.
Although industrialized societies are generally less physically active, past research indicates that declines in activity do not necessarily reduce total energy expenditure. Moreover, modern environments often promote calorie-dense diets, but the exact factors that contribute to the obesogenic nature of these diets remain unclear.
Complicating the issue further is the lack of reliable, population-wide empirical data on energy expenditure, energy intake, and body composition. Many obesity studies rely on indirect measures or surveys, limiting their precision.
About the Study
In this study, a team of over 60 researchers from around the world used direct measurements of total energy expenditure, basal energy expenditure, and body fat from a globally diverse dataset of over 4,000 adults.
By comparing populations along a spectrum of economic development, the researchers sought to clarify whether changes in energy expenditure or dietary intake better explain the increasing prevalence of obesity.
Researchers used data from the International Atomic Energy Agency's Doubly Labelled Water Database, which included 4,213 adults aged 18 to 60 from 34 global populations.
Total energy expenditure (TEE) was measured over 7–14 days using the doubly labeled water method, which tracks the loss of stable isotopes to estimate energy use and body composition (fat-free and fat mass).
Basal energy expenditure (BEE) was either directly measured using indirect calorimetry or estimated using predictive equations. Activity energy expenditure (AEE) was calculated as 90% of TEE minus BEE, and the physical activity level (PAL) was TEE divided by BEE.
Additional data were gathered from three pastoralist populations to enhance economic diversity.
Economic development levels were ranked using the United Nations Human Development Index (HDI). For comparisons, the researchers categorized populations into hunter-gatherer, pastoralist, horticulturalist, and low-, mid, and high-HDI groups.
The relationship between diet and obesity was explored using dietary data on UPF and meat consumption. These were modeled alongside energy expenditure and HDI rank to assess their influence on body fat percentage and body mass index (BMI).
Key Findings
As expected, individuals from more economically developed populations had higher body weights, BMIs, and body fat percentages. Obesity was more prevalent in high- and middle-HDI countries, especially among women.
While BMI increased with economic development, this was largely due to greater fat-free mass (muscle and bone), meaning BMI may overestimate obesity in these populations. In contrast, body fat percentage was a more direct indicator of obesity and was strongly linked to economic development.
Energy expenditure (total, basal, and activity-related) also increased with development, primarily driven by larger body sizes. However, when adjusted for body size, total and basal energy expenditure slightly declined with economic development (by approximately 6-11%), while activity energy expenditure and physical activity levels (PAL) remained unchanged.
Importantly, size-adjusted energy expenditure showed high variability among populations within the same economic group and did not vary systematically across different lifestyles. For example, some traditional populations had similar or higher adjusted energy expenditures than industrialized ones.
Moreover, energy expenditure was only weakly related to obesity measures. For instance, a one standard deviation increase in adjusted total energy expenditure corresponded to just a 1% decrease in body fat or a 0.6 drop in BMI, effects ten times smaller than those associated with economic development.
These findings suggest that changes in energy expenditure explain only a small portion (roughly one-tenth) of obesity increases, pointing instead to higher energy intake, likely from ultraprocessed foods, as a primary driver accounting for the remaining 90% of the effect.
Conclusions
This study challenges the notion that reduced physical activity is a primary cause of the rise in obesity associated with economic development. Although people in wealthier countries expend more energy overall, body size–adjusted energy expenditure decreases only slightly and inconsistently with development, and is mostly due to reduced basal metabolism, rather than reduced activity.
These small changes cannot explain the significant rise in obesity. Instead, the data suggest that increased calorie intake plays a much larger role, approximately 10 times greater than reduced energy expenditure.
The rise in obesity is likely linked to higher consumption of UPFs, which may disrupt appetite regulation, increase calorie absorption, and lead to overeating. In populations with dietary data, a higher proportion of UPFs in the diet was associated with higher body fat.
It's essential to acknowledge the study's limitations: its cross-sectional design cannot establish causation, and detailed dietary data were only available for 25 of the 34 populations.
While promoting physical activity remains important for health, tackling obesity will require greater focus on food quality, access, and regulation. Measuring body fat instead of BMI may offer better insight for public health interventions.