We’ve said, experts have said it, but we are not afraid to say it again and again: you cannot outrun a poor diet. A recent groundbreaking study lays out all the evidence, confirming what wellness experts already suspected. 

The study was published in Proceedings of the National Academy of Sciences in the summer of 2025. Researchers examined energy use across lifestyles, from hunter-gatherers to industrialized urbanites. It is not the lack of movement that is driving weight gain; rather, it is the food that people are putting on their plates. Let’s explore what science found out. 

Study: Diet vs. Exercise In Obesity

Is weight gain more about sedentary lifestyles, or eating too much? Most people are of the mind that if they exercise more, they can balance the effects of poor eating. This recent study is changing that narrative in a big way. The study looked at data from 4,213 adults across 34 global populations, including hunter-gatherers, pastoralists, farmers and modern city dwellers. Using the gold-standard doubly labeled water method, researchers measured energy expenditure to track real calorie burn (total, basal, and activity-related). Researchers then compared those results against body fat percentage and body mass index (BMI). 

This was the largest study of its kind, using the doubly labeled water method to track actual amount of calories burned per day. Basically, the method involves participants drinking water that contains stable isotopes of hydrogen and oxygen. That allows scientists to accurately measure carbon dioxide production and total energy use over time. But researchers didn’t just look exercise; rather, they broke energy expenditure into three components:

• Basal energy expenditure (BEE): the calories burned at rest to maintain basic bodily functions
• Activity energy expenditure (AEE): the calories burned via movement and physical activity
• Total energy expenditure (TEE): overall amount of daily calories burned combining both BEE and AEE

What Did Researchers Find?

Researchers gathered numbers from BEE, AEE, and TEE and compared these measurements to participants’ BMI, body fat percentage, and dietary intake. The researchers noted the following:

• TEE generally increased with economic development, but that was because people were physically bigger (more lean mass and less fat). 
• After adjusting for body size, researchers found that BEE and TEE dropped roughly 6 to 11% in more developed populations. AEE did not vary that much in this group. 
• What was surprising was that energy expenditure only explained about 10% of the increase in body fat and BMI observed in wealthier populations. 
• As expected, bigger bodies burn more energy. Individuals in wealthier countries had higher TEE, but that was because they were bigger (more lean mass and body size), not because they were more active. 
• Activity levels were not super different, and AEE was pretty consistent across various populations. That means that hunter-gatherers did not burn a lot more calories than office workers. 
• The food people eat, especially ultra-processed foods, was linked to higher body fat percentages in populations in which dietary data was available. 
• Energy expenditure revealed very little about obesity. In fact, differences in energy expenditure only accounted for 10% of the increased BMI and body fat percentages in developed countries. 
• Diet quality, however, explained a lot more. Researchers noted that higher consumption of ultra-processed foods was strongly linked to increased body fat. Populations relying on traditional diets (fresh produce, whole grains, and lean proteins) displayed lower obesity rates, despite varying activity levels. 

After researchers analyzed data across dozens of populations, they concluded that diet, especially the rise in ultra-processed foods, is the primary driver of obesity. Although exercise remains beneficial for overall health, it matters less than diet when it comes to preventing or reversing obesity.

How To Help Prevent Obesity

This study reframes the way we think about weight management. For years, public health messages have focused on exercise. While movement is essential for optimal cardiovascular health, mood, longevity, and muscle strength, this study confirms that exercise alone is not a reliable tool to prevent obesity. 

Researchers suggest people limit their consumption of ultra-processed foods, which are rich in refined sugars, unhealthy fats, and additives. What’s worse is that they are low in fiber. These foods are fueling obesity rates, even in populations that remain active. If you want to maintain a healthy weight, diet is the key to success.

Vincent Stevens

Vincent Stevens is the senior content writer at Dherbs. As a fitness and health and wellness enthusiast, he enjoys covering a variety of topics, including the latest health, fitness, beauty, and lifestyle trends. His goal is to inform people of different ways they can improve their overall health, which aligns with Dherbs’ core values. He received his bachelor’s degree in creative writing from the University of Redlands, graduating summa cum laude. He lives in Los Angeles, CA.