A new major study reveals that our assumptions about obesity may be all wrong—what’s causing it?

What causes obesity has been a mystery for decades, especially as waistlines grow in wealthy countries but not in traditional communities.

A groundbreaking metabolic study published in PNAS may now offer answers, pointing not to laziness or lack of movement, but to what’s on our plates, according to The Washington Post.

The research examined more than 4,200 people across 34 cultures, from the Hadza hunter-gatherers in Tanzania to office workers in Europe.

Scientists expected people in remote, physically active communities to burn far more calories than those in developed countries. But that’s not what they found.

Using a gold-standard method to measure calorie expenditure, researchers discovered that total energy use was surprisingly similar across all groups—regardless of lifestyle, location, or income level.

This unexpected result challenges a long-held assumption that people in wealthy nations are gaining weight because they don’t move enough.

The truth, researchers say, may be far more about food than fitness.

Reframing what causes obesity: More food, not less activity

For years, the idea that people in industrialized countries are too sedentary has dominated public health messaging.

However, this new metabolic study found that most people, regardless of their lifestyle, burn about the same number of calories each day once body size is considered.

This insight supports a newer theory called the “constrained total energy expenditure model.”

It suggests our bodies self-regulate energy use to stay within a tight range—even when we exercise more.

So, when physical activity increases, the body adjusts by slowing down other internal processes, like growth or reproduction.

That means even those who walk miles daily, like traditional farmers or foragers, don’t necessarily burn more daily calories than someone with a desk job.

“Our analyses suggest that increased energy intake has been roughly 10 times more important than declining total energy expenditure in driving the modern obesity crisis.”

According to the study, the rise in obesity is more strongly linked to how much—and what—we eat, rather than how much we move.

In other words, we consume far more calories than our bodies are designed to handle.

Diet vs. exercise: Experts weigh in on the real cause

This new research renews the long-standing debate over diet vs. exercise.

While both are important for overall health, the study shows that diet may have a larger role in determining what causes obesity.

“There’s still a lively debate in public health about the role of diet and activity” in the development of obesity, Dr. Herman Pontzer, a professor at Duke University and senior author of the study, said.

He also added that the confusion makes it harder to address obesity effectively.

Many believe exercise alone is the solution. However, this research suggests that focusing on food may offer a more effective path forward.

“We know that exercise is essential for health. This study doesn’t change that,” he said.

Physical activity still supports heart health, mental well-being, and long-term fitness.

But when it comes to preventing or reversing weight gain, food choices may matter much more than step counts or gym routines.

Ultra-processed foods: A powerful driver of obesity

So, if it’s not activity levels, what causes obesity to be so common in places like the U.S.? The study points to one major factor: the rise of ultra-processed foods.

In a deeper analysis, researchers looked at the types of food eaten across different countries.

They found a strong connection between body-fat percentage and the percentage of daily calories from ultra-processed products—defined as foods with five or more industrial ingredients, often loaded with sugars, fats, and preservatives.

These items are common in wealthier nations and less available in traditional communities.

This may help explain why groups like the Tsimane or Hadza—who eat simple, minimally processed diets—rarely experience obesity.

“This study confirms what I’ve been saying, which is that diet is the key culprit in our current [obesity] epidemic.”

Experts say improving our diets—eating fewer processed foods and focusing on whole, nutrient-rich options—could be a significant step in tackling obesity.

It’s a shift in focus from simply moving more to thinking more carefully about what’s on our plate.

Final takeaway: A more precise answer to what causes obesity

This metabolic study offers one of the most explicit pictures of how our bodies work and why so many are struggling with weight today.

By showing that calorie expenditure stays consistent across cultures and lifestyles, it points to diet—especially ultra-processed food—as the main force behind the obesity epidemic.

It also brings much-needed clarity to the ongoing diet vs. exercise conversation.

Exercise remains essential for health, but changing what we eat may be our most powerful tool.

As researchers continue to study what causes obesity, one thing is becoming more certain: it’s not just how much we move—it’s how much, and what, we eat.

Here’s Dr. Herman Pontzer discussing the truth about calories and weight loss in a podcast via Medcan: