Why Americans Keep Gaining Weight Despite Burning More Calories Than Ever

Global study of 4,000+ people reveals processed food, not lack of exercise, drives obesity epidemic

DURHAM, N.C. — Americans burn more calories daily than people living traditional lifestyles in developing countries. Yet we’re also significantly fatter. How can that be?

A Duke University study of 4,213 people spanning 34 populations worldwide — from Tanzanian hunter-gatherers to Norwegian office workers — shows that expanding waistlines have little to do with becoming less active. What we eat, not how much we move, is the real driver.

The research, published in the Proceedings of the National Academy of Sciences, found that “daily energy expenditures are greater in developed populations, and activity energy expenditures are not reduced in more industrialized populations.” This directly challenges the common assumption that declining physical activity is the main cause of obesity.

As the authors write in the study’s significance section, these results “suggest that dietary intake plays a far greater role than reduced expenditure in the elevated prevalence of obesity associated with economic development.”

Developed Countries Actually Burn More Daily Calories

To measure how many calories people actually burn each day, the researchers used a technique called “doubly labeled water,” which tracks how quickly the body processes certain harmless isotopes over 7 to 14 days. It’s considered the most accurate method available for measuring total energy expenditure in real-world settings.

Surprisingly, the study found that people in more economically developed countries burned more calories per day than those in less developed ones. This wasn’t just because they were taller or heavier, though body size did explain part of the difference. Even after adjusting for weight and muscle mass, the difference in calories burned was modest, only about 6%.

The researchers also expected to see lower physical activity levels in wealthier nations, but that wasn’t the case. Across all 34 populations studied, people in industrialized societies were just as physically active as those in traditional or rural settings. There was “no significant change” in physical activity levels with economic development, meaning people across different lifestyles generally moved their bodies just as much.

How Ultra-Processed Foods Drive Weight Gain

The real difference between countries wasn’t how much people moved. It was what they ate.

Among the 25 populations where dietary data were available, one trend stood out clearly: the more ultra-processed foods people consumed, the more body fat they tended to carry. These foods — like sugary drinks, packaged snacks, and ready-to-eat meals — are industrially made and often contain long lists of ingredients.

According to the researchers, one likely explanation is that these foods are easier to digest, which means the body absorbs more calories from them. Less energy is lost during digestion, and more gets stored. Some studies suggest these foods may also interfere with the body’s hunger signals, causing people to eat more than they need.

Even though people in wealthier countries were burning more calories, they were also eating more; and not just more food overall, but more food that seems to encourage fat storage. Because participants in the study maintained a stable weight during the measurement period, the researchers were able to infer that calorie intake must have closely matched calorie expenditure at the time. That means the high calorie burn in developed countries is matched by equally high consumption, likely driven by food quality rather than quantity alone

What This Means for Fighting Obesity

The researchers emphasize that physical activity is still vital for overall health. It lowers the risk of heart disease, improves mental well-being, and helps maintain a healthy metabolism. But when it comes specifically to obesity, the biggest factor appears to be the kind of food people eat, not how much they move.

Interestingly, the study also found that much of the increase in body mass in wealthier countries came from muscle and lean tissue, not just fat. In other words, better access to nutrition helps support healthy growth, even as it contributes to weight gain. Still, body fat increased more than muscle mass, pointing to a mismatch between calorie intake and the body’s needs.

What stood out most was this: the differences in body fat between rich and poor countries were about 10 times greater than any differences linked to how many calories people burned. That’s a powerful argument for focusing on the food environment — what’s available, affordable, and marketed to people — rather than just telling individuals to exercise more.

For decades, public health campaigns have leaned on the slogan “eat less, move more.” This study suggests it’s time to shift focus. The root of the obesity crisis may not be laziness, but the global rise of ultra-processed foods that are cheap, convenient, and engineered to be overeaten.

Disclaimer: This study is observational and cross-sectional, meaning it cannot establish cause-and-effect relationships. While the data strongly support associations between economic development, ultra-processed food consumption, and obesity, the authors note that more research is needed to clarify the underlying physiological and environmental mechanisms. Additionally, dietary data were only available for a subset of populations.