Obesity is rare among traditional communities like Tanzania’s Hadza and Bolivia’s Tsimane, yet common in modern industrialized societies. A groundbreaking international study has revealed why: the problem lies more in what we eat than how little we move.
Published in Proceedings of the National Academy of Sciences (PNAS), the study analyzed data from over 4,200 men and women across 34 countries, ranging from foraging societies to urban professionals. It measured daily energy expenditure using the gold-standard “doubly labeled water” method—an advanced way to track calorie burn and metabolism.
Exercise Isn’t the Obesity Villain Many Believe
For years, public health campaigns have claimed that low physical activity is the major reason for rising obesity in wealthier nations. But the new study directly challenges that assumption. Researchers found that people in industrialized countries, like the United States and Europe, burn about the same number of calories each day as those living traditional lifestyles.
Even highly active groups, like hunter-gatherers or herder-farmers, don’t significantly outpace the daily energy burn of sedentary office workers when body size is factored in. This supports a theory called the “constrained total energy expenditure” model—our bodies tightly regulate calorie use, compensating for higher physical activity by reducing energy spent on other functions like immune activity or growth.
According to Dr. Herman Pontzer of Duke University, who co-led the study, this means obesity isn’t a result of laziness. “People in wealthy nations are not burning fewer calories,” he said. “The difference is what and how much they eat.”
Diet—Not Inactivity—Emerges as the Main Culprit
So, what’s really driving the obesity crisis? The study offers a clear answer: increased calorie consumption, especially from ultra-processed foods. These are items made with five or more industrial ingredients—such as additives, flavorings, emulsifiers, and preservatives—and often have little in common with whole foods.
In populations with high intake of ultra-processed foods, researchers found significantly higher body-fat percentages. The correlation held regardless of physical activity levels.
“Changes to our food, not our activity, are the dominant drivers of obesity,” said Dr. Dariush Mozaffarian, director of the Food is Medicine Institute at Tufts University.
Dr. Barry Popkin, a leading public health professor at the University of North Carolina, added, “This study confirms what I’ve been saying for years—diet is the key culprit in our obesity epidemic.”
The Practical Lesson: What to Do About Obesity
While exercise remains crucial for maintaining health, the researchers stress that diet must be the main focus in tackling obesity. The evidence points especially to the damaging impact of ultra-processed foods, which are cheap, convenient, and heavily marketed—but also calorie-dense and nutrient-poor.
What can individuals do?
Prioritize whole foods: Fresh fruits, vegetables, legumes, lean proteins, and whole grains.
Limit ultra-processed items: Reduce intake of packaged snacks, sugary drinks, frozen meals, and fast food.
Focus on food quality: Calories matter, but the source of those calories matters more.
Avoid excess portion sizes: Even healthy foods can lead to weight gain if consumed excessively.
Stay active: While exercise may not directly lead to major weight loss, it supports heart health, bone strength, mental clarity, and metabolic efficiency.
Public Health Should Rethink Its Strategy
This research challenges decades of messaging that emphasized “move more” as the solution to obesity. According to Pontzer, “To truly address obesity, public health efforts must shift from focusing primarily on physical activity to addressing the quality and quantity of our diets—especially the overconsumption of ultra-processed foods.”
The researchers aren’t dismissing the value of exercise. Rather, they want policies and programs to address root causes, such as unhealthy food environments and economic incentives that favor ultra-processed goods over fresh produce.
Key Takeaways:
Physical activity levels are similar across both traditional and modern societies.
Obesity is more closely linked to high calorie intake, especially from ultra-processed foods.
Exercise is important, but dietary change is critical for reducing obesity.
Public health messaging needs to focus more on food choices and consumption patterns.
