Despite diet fads, exercise trends, and health and fitness influencers across social media, obesity rates continue to grow among adults in Westernized countries. In the U.S. alone, more than 40 percent of U.S. adults were considered obese in 2023, according to the U.S. Centers for Disease Control and Prevention.
Health researchers are working to understand what’s driving this public health problem. Last month, a team of 65 scientists from across the globe published a major study that better explains the root causes of obesity.
The study includes data from more than 4,500 adults living in 35 countries with a wide variety of cultural backgrounds. Study participants drank a special water containing isotopes that allowed scientists to measure exactly how much energy they used each day. These measurements accounted for the calories needed to perform basic bodily functions, such as their heartbeat and breathing, and physical activities, including any exercise regimes or manual labor.
Taking into account body size, the researchers found that people from populations with higher obesity rates and lower activity levels burned only slightly less energy compared to populations with lower obesity rates and higher activity levels. Essentially, a farmer with a physically active job expended about as much energy as an office worker.
This finding is strong evidence that diet – not a lack of exercise – is the main cause of obesity today, explained Herman Pontzer, principal investigator and professor in the Department of Evolutionary Anthropology at Duke University.
“Despite decades of trying to understand the root causes of the obesity crisis in economically developed countries, public health guidance remains stuck with uncertainty as to the relative importance of diet and physical activity,” he said. “This large, international, collaborative effort allowed us to test these competing ideas. It’s clear that changes in diet, not reduced activity, are the main cause of obesity in the U.S. and other developed countries.”
This tracks with some of Pontzer’s earlier research that suggests our bodies tend to adjust how we expend energy to keep our total calorie burn relatively stable. If we expend more energy on physical activity, our bodies will adjust to spend less energy on other tasks.
In addition, a second analysis of some of the study data found that in areas consuming more ultra-processed foods, which have undergone extensive industrial processing, more people tend to have higher body-fat percentages.
These findings don’t mean you should stop exercising. The evidence is crystal clear that exercise is one of the best ways you can improve your overall health, including delaying cognitive decline and promoting mental health.
But to address obesity, the take-home message is clear: It’s a healthy diet, not an exercise regimen, that helps promote weight loss.
