Why You’re Still Gaining Weight Even Though You’re Active, according to a New Study

A July 21, 2025 study came out of Duke University's Pontzer Lab, housed in the Department of Evolutionary Anthropology in Trinity College of Arts & Sciences. The study, published in PNAS and summarized in Science Daily, aimed to understand how activity levels and diet play a role in the obesity rates in developing countries.

So, why are you gaining weight even though you're active? The researchers found that higher caloric intake is the culprit, as opposed to lower physical activity levels.

It's a common thought that the cause of obesity in developing countries is lower activity levels--as societies become more industrialized, people can do less because of automation, delivery, and other simplified processes that allow humans to rest more.

But in the study, researchers found that what we eat plays a larger role in our society's overall weight management than our physical activity. They analyzed more than 4,200 adults ages 18 to 60--the participants were in 34 different populations and six continents.

They looked at factors like energy expenditure, body fat, and BMI in participants that came from a wide range of lifestyles and backgrounds. Some hunter-gatherer-type eaters, some processed food eaters, and all at various ranges of industrialization. Researchers gathered additional data on things like lifespan, education, and income from the United Nations Human Development Index (HDI).

After looking at all of these factors, researchers saw only a small increase in body fat that could be explained by the decreased energy expenditure in developed societies. This means the culprit is likely diet, not energy expenditure, causing the high obesity levels in such societies.

The study highlights that these results do not signify that people should not use physical activity as a means to promote health and weight maintenance. Rather, a combination of diet and exercise is ideal. Researchers will next aim to look even more into how diet impacts obesity in developed countries, looking at the specific diet factors that cause a societal-wide rise in obesity.