Obesity remains a growing public health crisis in the U.S. and other developed nations, according to both the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO). While poor diet and sedentary lifestyles are both major contributors, new research suggests that one factor is far more responsible than the other: overeating.
“It’s a mix of poor diet, high stress levels, lack of muscle, and lack of frequent movement,” said Lindsay Allen, a registered dietitian nutritionist and owner of Back in Balance Nutrition in Florida, in an interview with Fox News Digital.
A global study of over 4,000 adults across 34 populations—from traditional hunter-gatherers to modern office workers—looked at total energy expenditure (TEE), body fat percentage, and BMI. Surprisingly, once body size was accounted for, participants burned similar numbers of daily calories regardless of lifestyle or income level.
“The differences in body fat we see across populations likely aren’t caused by variations in activity level or total calories burned,” said study authors Amanda McGrosky (Elon University) and Amy Luke (Loyola University). “Instead, excess fat is more often the result of consuming more calories than the body uses.”
Overeating 10 Times More Impactful Than Lack of Exercise
Published in the journal Proceedings of the National Academy of Sciences (PNAS), the study found that increased caloric intake has played a much larger role in rising obesity rates than decreased physical activity.
“Our analyses suggest that increased energy intake has been roughly 10 times more important than declining activity rates in driving the modern obesity crisis,” the authors concluded.
Dr. Brett Osborn, a Florida neurosurgeon and longevity expert, agreed: “Exercise burns far fewer calories than people think,” he said. “We’re not gaining weight because we stopped moving—we’re gaining because we’re overfed.”
Why Muscle Mass and Stress Still Matter
Allen emphasized that other factors like stress and muscle mass shouldn’t be ignored.
“Having sufficient muscle significantly boosts metabolism and fat-burning ability,” she said. “And high stress levels can shift the body into ‘survival mode,’ changing metabolism and promoting fat storage.”
The Role of Ultraprocessed Foods
The study also pointed to the rise in consumption of ultraprocessed foods (UPFs)—items that are calorie-dense, shelf-stable, and engineered to be hyper-palatable—as a key driver of excessive calorie intake.
“As more regions adopt diets heavy in ultraprocessed foods, we expect obesity rates to rise in areas that currently have lower prevalence,” McGrosky and Luke noted.
These foods are often easy to overeat and fail to trigger the body’s natural feelings of fullness. Their flavor and texture, along with rapid digestion and absorption, make them ideal for fat gain and difficult for the body to regulate.
“Ultraprocessed foods override our satiety signals,” said Osborn. “They promote inflammation and push our biology into a fat-storing mode.”
Exercise Still Matters — Just Not for Weight Loss
Despite the findings, researchers and experts still stress the importance of physical activity for heart health, mental wellness, and longevity.
Allen urged people to shift focus away from calorie-burning and toward building strength and muscle.
“Exercise is one of the best things you can do for your body—but not because it burns tons of calories. It’s because it builds the muscle that helps you burn more fat long-term.”
Osborn echoed that sentiment: “Progressive strength training boosts metabolism and accelerates fat burning,” he said. “With proper nutrition and strength work, your body becomes a fat-burning machine.”
Bottom Line: Focus on Food Intake
Ultimately, the study points to overeating—especially of ultraprocessed, nutrient-poor foods—as the biggest driver of obesity.
“If you’re concerned about excess body fat, focus on ‘calories in,’” McGrosky and Luke advised. “It’s far easier to control the calories you consume than to significantly increase the calories you burn.”
Osborn added a warning:
“The problem isn’t laziness. It’s the relentless stream of ultraprocessed, nutrient-void products marketed as healthy or convenient. And it’s slowly killing us—predictably, and on a massive scale.”
