Why We Keep Gaining Weight: Science Reveals Ultra‑Processed Foods, Not Laziness
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| People in wealthier nations burn more calories—but gain more weight due to ultra-processed food consumption. / Pexels |
Key Takeaways
- New study of 4,213 adults across 34 societies finds no drop in calorie burn in wealthier countries.
- Americans burn as many—or more—calories daily as people in traditional lifestyles using doubly labeled water.
- Ultra-processed foods, not inactivity, are the primary drivers of rising obesity.
Total Calories Burned in Rich vs. Traditional Societies
Published July 14, 2025 in PNAS, the study titled “Energy expenditure and obesity across the economic spectrum” tracked 4,213 adults across 34 populations—from Tanzanian hunter-gatherers to Norwegian office workers—using the gold-standard doubly labeled water method .
Results show that individuals in industrialized nations burn as many—or more—total calories than those in less developed communities. After adjusting for body size, total daily energy expenditure dropped a modest 6–11%, insufficient to explain marked differences in body fat .
Physical Activity Levels: Still High Across the Board
Contrary to public perception, physical activity levels didn’t decline with development. The study found no significant difference in activity-related energy expenditure—rich or poor m.
Ultra-Processed Foods: The Hidden Culprit
Among the 25 populations with dietary data, a clear trend emerged: higher consumption of ultra-processed foods strongly correlated with higher body fat percentages, independent of physical activity .
These foods—high in sugar, fat, salt, and additives—are engineered for palatability and efficient absorption, promoting overeating and fat storage .
Implications for Public Health
The study suggests shifting public health focus: move beyond “eat less, move more” to policies that reduce ultra-processed food intake. While physical activity remains vital for overall health, combating obesity means improving diet quality .
Study Overview & Limitations
| Method | Doubly labeled water tracking for 7–14 days; body composition via deuterium dilution; HDI for economic development levels. |
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| Result | Body-fat % and BMI increased with development, but calorie burn stayed constant; diet accounted for ~10× more obesity variation than activity. |
| Limitations | Cross-sectional design; detailed dietary data from 25/34 populations; mechanisms of ultra-processed effects need more research. |
References
- McGrosky A. et al., “Energy expenditure and obesity across the economic spectrum”, PNAS, July 14 2025
- Washington Post, “What causes obesity? A major new study is upending common wisdom”, July 15 2025
- EurekAlert!, “New Duke study finds obesity rises with caloric intake, not couch time”, July 17 2025
- Wikipedia, “Ultra-processed food”, latest summary
- Wikipedia, “Health effects of ultra‑processed foods”