July 17, 2025, 1:20 pm | Read time: 5 minutes
Lack of exercise is often blamed as the main culprit of the global obesity epidemic. It’s easy to picture an office worker who rarely hits 10,000 steps a day. However, an international study shows that people in industrialized countries burn more calories than those in less economically developed nations. Yet, they are significantly more often overweight. Apparently, physical activity plays a less important role in the development of obesity than diet. This is no shock to FITBOOK nutrition expert Sophie Brünke, who knows the biggest culprits in the Western diet.
To settle the debate over the main culprit in the development of obesity, a research team from Duke University in the U.S. examined the energy expenditures and body compositions of 4,213 adults aged 18 to 60 from 34 population groups across six continents. The selection ranged from hunter-gatherers to farmers to residents of industrialized nations. The scientists compared not only total energy expenditure, basal metabolic rate, and physical activity but also body fat percentage and body mass index (BMI). The United Nations’ Human Development Index (HDI) was used to classify the level of development.1
Study Shows: Diet 10 Times More Important Than Exercise in Obesity
As expected, the study concluded that people in industrialized nations had higher BMI and body fat percentages. Curiously, they also had higher basal and total energy expenditures. The difference between these values provides insights into physical activity, which also slightly increased with a higher development index. The theory that people in industrialized countries simply move too little and therefore gain weight does not hold. Moreover, the differences in energy consumption between the population groups could only explain about one-tenth of the difference in body fat and BMI associated with economic development. Simply put, diet contributes about ten times more to the rise in obesity than lack of exercise.
The Culprit Tastes Good to Many
When the authors took a closer look at diet following this insight, it became clear that “the percentage of UPF (Ultra-Processed Foods, editor’s note) in the diet was positively correlated with body fat percentage.” So, if the proportion of processed meats, ready meals, or sweets was higher in the diet, weight also increased. The study defined UPFs as products with five or more ingredients.
Study Confirms What Is Seen in Practice
“I find the results only somewhat surprising. I often see people overestimating how many calories they can burn in a session of exercise. For example, you have to jog about five kilometers on average to burn off a croissant. For me, the study underscores many sayings like ‘You are what you eat’ or ‘Abs are made in the kitchen,’ which have long emphasized the importance of diet—even when it comes to a six-pack. And in an environment full of highly processed foods, it makes sense: A 500-milliliter ice cream tub is quickly devoured, but the approximately 1,000 calories must be painstakingly saved and worked off. The diet has to be right—especially in the fight against obesity—otherwise, even the best training won’t help.”
Contextualizing the Study
This study is the most comprehensive empirical investigation to date on the relationship between economic development, energy consumption, and obesity. However, it is a cross-sectional study, where all data were collected at a specific point in time. Causal statements—such as that higher UPF intake directly leads to higher body fat percentage—cannot be substantiated. Additionally, detailed dietary data were only available for 25 of the 34 groups, limiting the significance regarding the impact of individual food groups. Nevertheless, the results suggest that diet in industrialized countries seems to be the crucial factor.
3 Reasons Why I, as a Nutrition Expert, Advise Against UPFs
The fact that more UPFs end up on plates in industrialized countries could be due to their ubiquitous availability, long shelf life, and low prices. It sounds practical—but health-wise, they are to be avoided.
Disrupted Satiety
High-calorie UPFs taste particularly good due to simple sugars and lots of fat, tempting people to eat beyond the feeling of fullness. Moreover, a high-calorie diet disrupts the natural feeling of satiety because it reduces the sensitivity of insulin (the “satiety hormone”). This is particularly precarious for overweight individuals, as they often already have a disruption of hunger and satiety. A vicious cycle begins.
More Efficient Energy Absorption
UPFs rarely come with fiber, which regulates blood sugar levels and keeps you full for a long time. They are more likely to be quickly digested, and the calories efficiently extracted by our gut bacteria—fiber, on the other hand, provides little or no calories. A study examining the Western diet (characterized by UPFs) even concluded that industrially processed food is absorbed more efficiently by the body, with less energy excreted in stool. The difference compared to a gut-friendly diet was a whopping 116 calories per day!2
Nutrient Profiles from Hell
The lack of fiber is just the tip of the iceberg. Sugar, saturated fats, and trans fats are a combination that raises inflammation levels in the body and leads to chronic diseases.
Recently, a study compared the dietary composition of Western and North African diets (reported by FITBOOK). When participants who ate a Western diet switched to the Chagga diet, characterized by vegetables, whole grains, legumes, and fermented foods, their inflammation levels improved within two weeks.
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Anyone who now thinks they can hang up their gym bag for good is mistaken. The study authors still recommend regular exercise, as it helps prevent physical and mental illnesses. Exercise can, for example, extend life, make you happy in small doses, and help even if you are already ill, such as with Type 2 diabetes.