Why Do We Gain Weight As We Get Richer? A Groundbreaking Study Busts Myths About Obesity and Energy Use

Imagine this: You’re sitting at your desk in a modern office building, nibbling on a snack while scrolling through emails. Thousands of miles away, a Yanomami indigenous woman living on the shores of the Orinoco river with her baby strapped to her back treks through the jungle, harvesting tubers and berries by hand. Both of you eat. Both of you burn calories. But one of you is far more likely to struggle with obesity—and it’s not who you think. But when this indigenous woman moves into a mission compound a few miles away and dramatically changes her diet to a more Western style, she becomes obese.

A groundbreaking new study recently published in the prestigious journal PNAS by McGrosky and coinvestigators with lead author Herman Pontzer throws a wrench into a long-standing assumption: that obesity in wealthier societies is primarily caused by people moving less. According to their data, collected from over 4,200 adults across 34 populations worldwide—including hunter-gatherers in Tanzania, herders in Siberia, farmers in Bolivia, and office workers in the U.S.—the real culprit behind the global obesity crisis isn’t lack of regular physical exercise. It’s what’s on our plates.

The Global Obesity Puzzle

Obesity has exploded alongside economic growth, and it’s easy to see why we might blame sedentary lifestyles. We’ve gone from playing games outdoors to spending hours playing video games, walking to cars, from farming to keyboards, and from cooking to ordering in. But what if, despite all that, we’re actually burning more energy now than ever before?

That’s what the study findings suggest. Total daily energy expenditure (how much energy our bodies use each day) is actually higher in more economically developed countries. Even activity-related energy expenditure—calories burned through movement—didn’t decrease with modernization. In fact, people in high-income countries like the U.S. and Norway burned about the same amount of energy as active, traditional populations like the Hadza hunter-gatherers in Tanzania or Siberian herders.

So if we’re burning just as many, or even more, calories in industrialized societies, why are we gaining so much weight?

It’s Not What You Burn—It’s What You Eat

The researchers found that body fat and BMI (body mass index) were much higher in wealthier countries, even though total energy expenditure was also higher. After accounting for differences in age, sex, and body size, they discovered that the variation in energy expenditure explained only about 10% of the rise in obesity seen with economic development.

What explained the other 90%? Food.

People in richer countries are eating more, and more importantly, they’re eating very differently. Diets in industrialized nations are increasingly dominated by ultra-processed foods (UPFs)—think sugary drinks, packaged snacks, frozen dinners, and fast food, and a lack of plant derived complex carbohydrates (i.e. fiber) and polyphenols. In the 25 populations where dietary data were available, the percentage of daily calories from UPFs was closely linked to body fat levels. The more UPFs a population ate, the fatter they got—regardless of how much they moved.

These foods aren’t just convenient; they’re engineered to be irresistible. They’re often calorie-dense, low in fiber, and rapidly digested and absorbed in the upper small intestine starving the gut microbes living further down in the large intestine. This makes these foods not only easier to overeat but also ensures that the great majority of calories are absorbed and stored as fat.

More Calories In, More Fat On

One of the study’s key insights is that modern food systems have made it easier than ever to consume more energy than we expend. While traditional largely plant-based diets are generally rich in plant-derived fiber and polyphenols, and require more energy to digest, industrial diets streamline the absorption process and bypass our microbial ecosystem. This means that even if your calorie count stays the same, you might absorb more energy—and gain more weight—from modern foods.

The researchers also observed that as economies grow, people gain more lean mass (muscle and bone), which helps explain part of the increase in BMI. But the real concern is the sharp rise in body fat percentage, especially among women in high-income countries.

Rethinking the “Move More” Message

So does this mean we should stop promoting physical activity? Absolutely not! Exercise remains vital for cardiovascular and brain health, preventing sarcopenia, mental well-being, immune function, and overall quality of life. But when it comes to tackling the obesity epidemic, exercise alone isn’t enough. This study underscores the need to shift more of our focus toward what—and how—we eat.

Public health strategies must now prioritize reducing the consumption of ultra-processed foods and promoting whole, minimally processed alternatives. And importantly, we must recognize that not all calories are created equal.
A calorie from a candy bar doesn’t behave the same way in the body as a calorie from a sweet potato or from a delicious bowl of berries, even though the calories may be the same. Assuming the same amount of carbohydrates are ingested as simple sugars and absorbed rapidly in the small intestine, as compared to complex carbohydrates (i.e. fiber) which require the fermentation by the gut microbes in the colon, the net calories being absorbed into the systemic circulation is significantly lower in the second scenario.

The Takeaway: A New Paradigm for Global Health

This ambitious global study offers a powerful message: the obesity crisis isn’t simply about moving too little—it’s about eating too much of the wrong kinds of food (at the wrong times, like snacking in front of the TV). While economic development has improved access to food, it’s also ushered in an era of cheap, ultra-processed, hyper-palatable products that undermine our natural hunger and fullness cues.

Fighting obesity in the 21st century means rethinking food systems, not just encouraging people to exercise more. It means creating environments where healthy choices are taught at an early age, and are accessible, affordable and culturally supported. And it means shifting public health messaging away from “move more” to “move more and eat smarter”—without losing sight of the value of daily movement.

In short, if we want to solve the global obesity puzzle, we must stop blaming the treadmill and start looking at the supermarket aisle. Both are equally important.