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A recent international study published in the Proceedings of the National Academy of Sciences has opened up an important conversation about obesity in America. The research, analyzing 4,213 adults across 34 populations worldwide — from Tanzanian hunter-gatherers to Norwegian office workers — reveals a surprising paradox: people in developed countries actually burn more calories daily than those living traditional lifestyles, yet they’re significantly heavier.
The study, led by researchers including Amanda McGrosky of Elon University, used the gold-standard “doubly labeled water” method to precisely measure daily energy expenditure. Their findings directly challenge the widespread belief that declining physical activity is the primary driver of obesity in wealthy nations.
Instead, the research points to a different culprit: ultra-processed foods. Across all populations studied, physical activity levels remained remarkably consistent regardless of economic development. The real difference wasn’t how much people moved, but what they ate. Among the 25 populations with available dietary data, consumption of ultra-processed foods — such as sugary drinks, packaged snacks, and ready-to-eat meals — showed a strong correlation with higher body fat percentages.
The implications are significant for public health policy. Rather than focusing primarily on encouraging more exercise, the research suggests that addressing the food environment and reducing ultra-processed food consumption may be more effective strategies for combating obesity.
Following the study’s publication on StudyFinds, we spoke with co-author Amanda McGrosky to dive deeper into these unexpected findings and explore what they mean for how we understand and address the global obesity crisis.
StudyFinds’ Q&A With Amanda McGrosky
Your study found that people in developed countries actually burn more daily calories than those in traditional societies. Were you surprised by this finding?
AM: We were fortunate in this study to have been able to pull together data from a large number of independent studies, and a number of these original studies on people living in traditional societies found no differences in body-size-adjusted total energy expenditures between those living more high-activity, traditional lifestyles and those living more “Westernized” lives. It turns out that the reason people in more economically developed countries burn more calories is simply because they’re larger, which makes good sense—it costs more to run a larger body. So, given the past work in this area, I wasn’t super surprised to see that those in developed countries, with their larger body sizes, burned more calories. That being said, when I initially started working in this field, I was surprised!
Many people assume obesity is driven mainly by inactivity. How do your results challenge that assumption?
AM: The assumption is that an increase in sedentary behavior results in lower total energy expenditure (i.e., that people who are more active burn more calories than those who are inactive), but this study (and others!) show that over the long term, size-adjusted total energy expenditures are similar across people with different habitual activity levels. Excess body fat is the result of consuming more calories than you burn, so because there aren’t meaningful differences in the total number of calories burned between people living very active lifestyles and people living more sedentary lifestyles, the increase in body fat that we see with increased economic development can’t be easily linked to differences in habitual activity and the total number of calories burned.
Instead, the increase in body fat and obesity that we see with increased economic development must be due to an increase in the number of calories consumed. In other words, because body fat is due to a mismatch between “calories in” and “calories out”, and we don’t see meaningful differences in (size-adjusted) calories out across populations, the increase in body fat that accompanies economic development is primarily due to increases in calories in.
You found no significant decline in physical activity levels with economic development. Can you explain why that might be, given what we hear about sedentary lifestyles?
AM: See above, but to clarify a bit more, it appears that once you adjust for body size, many people living more sedentary lifestyles burn just as many calories as people living highly active, traditional lifestyles. While we did find a slight decrease in body size-adjusted total energy expenditure with increased economic development, there weren’t consistent patterns across populations and in many cases, the body size-adjusted total energy expenditures of people living more “traditional” lifestyles, such as hunter-gatherers and pastoralists, overlapped with North American and European populations in the “high-HDI” category. In other words, hunter-gatherers with 40 kilograms of lean body mass often burn the same number of calories as American office workers with 40 kilograms of lean body mass. It appears that someone’s habitual physical activity doesn’t have much of an impact on the total number of calories that they burn per day; we don’t really understand why total energy expenditure appears to be constrained in this way, but it is an area of active research.
Ultra-processed foods stood out as a major factor linked to body fat. What exactly makes these foods so different from unprocessed or minimally processed foods?
AM: I’m not an expert on ultra-processed foods and will also note that we weren’t able to rigorously test the role that ultra processed foods may play in the obesity crisis, but ultra-processed foods are often hyper-palatable an calorie-dense, which can promote overconsumption. I know I myself have been guilty of eating 500 calories of chips in one sittings, and it’s a lot easier than accidentally eating 500 calories of carrots!
Your paper mentions that people in more developed countries are heavier partly because they have more lean mass. Can you talk about how this complicates our understanding of BMI and obesity?
AM: We see an increase in both body mass and height with economic development, but BMI (a measure of weight relative to height) is an imperfect tool for estimating body condition and how much body fat someone carries. It can’t differentiate between lean mass and fat mass, even though it is often used to determine whether someone has excessive body fat and is “obese”, and it is particularly bad at capturing the body condition of people with high muscle mass. A better way to assess obesity, or excess body fat, is to directly measure someone’s body fat percentage.
Some people might interpret this study as saying exercise doesn’t matter. What’s your response to that?
AM: Don’t believe them! Exercise and physical activity absolutely are important components of a healthy lifestyle. Physical activity is very important for cardiovascular and mental health, just to name a few examples, and should be considered as complementary to diet for overall health and well-being.
How should public health messaging shift in light of your findings, especially when it comes to addressing obesity in high-income countries?
AM: I think this study highlights the need to focus on calories in. Increased body fat is fundamentally the product of consuming more calories than are burned, and because it turns out that its quite hard to meaningfully change the total number of calories burned, the thing to adjust is the number of calories consumed. Knowing that differences in excess body fat across populations are primarily linked to differences in the number of calories that people take in and diet means that we (broadly speaking) have the capacity to change trajectories if we focus on ensuring that healthy, nutrient-rich foods are available and accessible.
Although this study wasn’t designed to establish causality in the relationships we observed, post-hoc analyses suggest that it’s very possible that part of the problem with modern diets is the prevalence of ultra-processed foods. Ultra-processed foods are hyperpalatable and calorie-rich, which can promote overconsumption of calories. They also tend to be more shelf-stable and lower in cost than foods such as fresh vegetables, fruits, and whole grains (especially when you consider cost-per-calorie), so the focus should be on ensuring that everyone has access to healthy, affordable food choices.
What are the next big questions this study raises? Where do you think research should go from here?
We still don’t understand how body-size-adjusted total energy expenditure remains fairly constant across different populations and individuals with very different activity levels. It’s thought that as energy expended on physical activity increases, bodies pull energy away from other physiological processes to keep the bottom line, total number of calories burned relatively constant, but we don’t know much about the mechanisms drive shifts in energy allocation.
“YOU CANNOT OUTWORK A BAD DIET!”
What a f’en distraction, its not calories, its fucking quality of food with added chemicals!
Processed food has abundantly more calories than if a person was to eat a whole plant based OR a Diet of Unprocessed Meats, Vegetables, Fruits and Grains similar to a Mediterranean style diet. That being said, an over intake of daily calories will always be the driving factor as to why someone gains weight. You could hypothetically eat a diet of donuts everyday consuming only 1400 calories per day and that individual will lose weight. Now this is certainly not healthy long term and can have detrimental health effects to your body and organs over time, but it will show that calories are ultimately the culprit for weight gain.