In quiet kitchens and bustling supermarkets, in gym floors echoing with sneakers and in living rooms awash with the blue glow of late-night television, a silent war rages—a war over the shape and weight of human bodies.
Obesity. A word heavy not only with pounds and kilograms but with stigma, confusion, and fear. For decades, societies have pointed accusing fingers in one of two directions: too little exercise, or too much food. Move more, say some. Eat less, say others. But what if the reality is far more nuanced—and far more unsettling—than either camp has believed?
The Old Equation and the New Doubt
We’ve been raised on a simple arithmetic: calories in, calories out. Like money in a bank account, we are taught that what matters most is balance. Eat more calories than you burn, and the excess becomes stored fat, tucked beneath the skin, wrapping the internal organs, accumulating around the waist.
So the logic seems simple. If obesity rates are climbing, either people are eating too much or moving too little. But this stark division—diet or exercise—has proven deceptive. For though the equation itself is scientifically true, it is also incomplete. A new study, monumental in its ambition and scope, peels back the curtain on the forces shaping human weight, and the story it tells is a complex symphony of metabolism, evolution, and modern temptation.
Across the Continents, a Data Mosaic
Imagine researchers standing on the threshold of the world, clipboards in hand, gazing out at deserts and jungles, cities and savannahs. In this new study, led by Amanda McGrosky and colleagues, scientists ventured beyond lab walls to capture a portrait of humanity itself.
They collected data from 4,213 adults aged 18 to 60, spanning 34 diverse populations across six continents. Their subjects were a tapestry of lifestyles: hunter-gatherers chasing antelope through African brush, pastoralists herding goats across windswept plains, farmers tending rice paddies under sweltering suns, and urban dwellers navigating office cubicles and grocery store aisles.
This research was more than a clinical survey—it was a globe-spanning investigation into how humans live, move, and consume in a world transforming faster than biology can adapt.
Measuring the Fires Within
To understand why some bodies store more fat than others, the researchers scrutinized how humans burn energy. Inside each of us, an invisible fire flickers: the basal metabolic rate (BMR), which sustains the heartbeat, powers the lungs, fuels cellular repair. This is the bare minimum energy needed to keep a person alive, even if they lay in bed all day, unmoving.
But humans rarely sit idle. Above and beyond basal metabolism lies activity energy expenditure (AEE)—the calories burned by every step, every arm swing, every involuntary twitch of muscle. When basal and activity energy are added together, the result is total energy expenditure (TEE), a grand measure of how many calories a body burns each day.
This study measured all three: BMR, AEE, and TEE. They also measured body fat percentage and Body Mass Index (BMI)—the familiar (though imperfect) ratio of weight to height used to categorize people as underweight, healthy, overweight, or obese.
Initial Surprises in the Data
At first glance, the data delivered an unexpected twist. People in wealthier, industrialized nations—where obesity rates are highest—actually burned more total energy per day than their peers in poorer regions. Their TEE, AEE, and BMR were all higher, along with their body mass, BMI, and body fat.
This painted a paradoxical picture. How could populations battling obesity be burning more energy?
One reason is simple physics: bodies are bigger in developed nations. Height and body mass trend higher, and bigger bodies burn more calories at rest simply because they have more tissue to sustain. A tall, muscular frame requires a higher BMR than a small, lean body.
Yet that didn’t fully explain the differences. McGrosky’s team realized they needed to dig deeper, refining the numbers to account for body size, age, and sex—variables that shape energy needs.
After the Numbers Were Adjusted
When the data were corrected for these factors, a fascinating truth emerged.
TEE and BMR decreased modestly—by about 6-11%—with higher economic development. That is, once researchers accounted for body size and other differences, people in richer countries weren’t necessarily burning more total calories.
But AEE—the energy expended in physical activity—remained higher in wealthier populations. Even after adjusting for size and age, people in industrialized countries were moving more than those in less developed regions.
This revelation struck at the heart of an enduring myth. It suggested that, on average, people in modern societies do not necessarily suffer from a lack of physical activity as the primary driver of obesity. Indeed, in many cases, they move as much or even more than people in traditional lifestyles.
A Weak Link Between Calories Burned and Obesity
So why the relentless rise in obesity in industrialized nations?
Here, the researchers made one of their most crucial observations. Total energy expenditure is only weakly associated with obesity.
It accounted for about 10% of the increased incidence of obesity in wealthier nations. The real culprit, lurking in plain sight, was not so much how much energy people were burning—but rather what they were eating.
The human body is finely tuned by evolution to regulate appetite and metabolism in an environment of scarce resources. Our ancestors evolved with an intimate knowledge of hunger. Periods of famine sculpted our physiology, hardwiring us to seek calorie-dense foods when available.
Modern civilization has short-circuited these ancient signals. In grocery aisles and fast-food menus, ultra-processed foods beckon with colors, flavors, and textures engineered to overwhelm the body’s satiety cues.
Ultra Processed Foods and the Subtle Hijack of Biology
In the modern supermarket, a war is waged between evolution and innovation. Bright packages promise pleasure and convenience. Inside are ultra-processed foods (UPFs): packaged pastries, sweetened beverages, salty snacks, instant noodles, and ready-to-eat meals. These are not merely food—they are meticulously crafted chemical experiences.
The researchers found that the percentage of UPFs in the diet was positively correlated with body fat percentage. The more ultra-processed foods people consumed, the higher their body fat tended to be, regardless of how active they were.
UPFs are diabolically efficient at sabotaging natural hunger regulation. Their hyperpalatability—the intense burst of sweetness, salt, fat, and artificial flavors—triggers the brain’s reward systems in ways that natural foods rarely can. Their energy density means that even small portions pack enormous caloric loads.
Moreover, as the researchers noted, processing can increase the percentage of consumed calories that are absorbed into the body rather than excreted. Nutrient structures that might have been tough to digest in whole foods become easier for the gut to assimilate once processed. The net result? People end up absorbing more energy than their ancestors would have from similar quantities of raw ingredients.
Disrupted Satiety and Relentless Cravings
In healthy bodies, hormones like leptin and ghrelin keep appetite balanced. When we’ve eaten enough, signals of fullness travel from the gut to the brain, whispering that it’s time to stop. But UPFs seem to disrupt satiety signaling. They encourage continued consumption even when the body has received more than enough energy.
A bag of potato chips or a frosted pastry can vanish into the stomach almost unnoticed, leaving taste buds clamoring for more. In contrast, a plate of leafy greens or a bowl of lentils delivers bulk and nutrients, stretching the stomach and sending stronger signals of fullness.
The modern food environment turns an ancient survival instinct—seek calories whenever possible—into a curse. Our biology is not maladaptive; it’s simply outmatched.
Exercise Still Matters
One might read these findings and despair, imagining exercise as useless in the fight against obesity. But the researchers are careful to stress that physical activity remains vital. While exercise may not be the primary determinant of body fat, it carries profound benefits for metabolic health, cardiovascular fitness, insulin sensitivity, mental health, and longevity.
Regular movement, they note, reduces risk factors for chronic diseases, lowers inflammation, and improves psychological well-being. It may not erase the consequences of a diet overloaded with ultra-processed calories, but it can mitigate some damage.
The Road Ahead and Unanswered Questions
So where does this leave us?
The research underscores that obesity is driven less by how many calories people burn and more by how modern diets are reshaping biology. Yet this is not an indictment of individuals. It is a challenge to societies, to policy-makers, to the industries that manufacture food.
Understanding why UPFs have such powerful effects remains a crucial frontier. Is it the unique nutrient profile? The flavor engineering? The texture that dissolves rapidly in the mouth? The speed at which these foods spike blood sugar? Scientists suspect the answer may be all of the above.
Amanda McGrosky and her colleagues emphasize the need for continued research into how food processing changes the relationship between energy intake and fat accumulation. They call for public health interventions that focus not just on calories, but on the nature of the foods consumed.
A Subtle Revolution in How We See Obesity
In the end, the findings hint at a quiet revolution. Obesity may not stem primarily from laziness, sloth, or ignorance. Instead, it reflects a mismatch between ancient biology and modern abundance. It is less a personal failing and more a public health crisis crafted by technological progress and commercial interests.
The road forward is not simple. It demands rethinking how societies produce, market, and consume food. It demands compassion for those struggling with weight in a world engineered to seduce them into excess. And it demands continued scientific curiosity, the same spirit that drove McGrosky and her team to examine the footprints of humanity across six continents.
The war on obesity will not be won in gyms alone. It will be won—or lost—in the places where food is made, marketed, and consumed. In the end, the real battleground lies not only in calories burned, but in the mysterious ways modern foods infiltrate biology, bending age-old survival instincts toward modern harm.
More information: Amanda McGrosky et al, Energy expenditure and obesity across the economic spectrum, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2420902122