Obesity among adults continues to be a major public health issue in the United States and other economically developed countries, according to the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO). 

"It's a mix of poor diet, high stress levels, lack of muscle and lack of frequent movement," Lindsay Allen, a registered dietitian nutritionist and owner of Back in Balance Nutrition in Florida, told Fox News Digital.

While both overeating and lack of exercise are often to blame, a new global study suggests that one factor far outweighs the other.

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Scientists studied more than 4,000 adults from 34 populations, ranging from hunter-gatherer communities to office workers. 

They analyzed each person’s total energy expenditure (TEE), which is the number of calories burned daily, along with body fat percentage and BMI (body mass index).

When adjusted for body size, people across all lifestyles and income levels burned similar amounts of energy, even if their daily routines were drastically different.

"The differences in body fat that we see across populations likely aren't due to major differences in activity level or total daily energy burned," study authors Amanda McGrosky (from Elon University in North Carolina) and Amy Luke (Loyola University in Illinois) told Fox News Digital.

"Rather, excess body fat is likely primarily the product of too many 'calories in,' or eating more calories than are burned."

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People in more economically developed areas burned more calories, the researchers noted — but that’s primarily because they tend to weigh more, and heavier bodies naturally burn more energy.

Published in the journal Proceedings of the National Academy of Sciences (PNAS), the study concluded that how much people eat plays a far greater role than the amount of exercise when it comes to putting on weight.

"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 stated.

Dr. Brett Osborn, a Florida neurosurgeon and longevity expert, agrees with the adage that "you can’t out-train a bad diet."

"Exercise burns far fewer calories than people want to believe," Osborn, who was not involved in the study, told Fox News Digital.

"This latest data only confirms what I’ve seen in my clinic: We’re not gaining weight because we stopped moving. We’re gaining because we’re overfed."

Allen pointed out that muscle mass should also be considered.

"Having sufficient muscle is what drastically improves metabolism and fat-burning capabilities," she told Fox News Digital, also emphasizing the importance of stress levels. "People who live in high-stress societies tend to be fatter because stress hormones change our metabolism and shift us into 'survival mode.'"

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Another finding involves ultraprocessed foods (UPFs), which have long ingredient lists that are often calorie-dense, shelf-stable and "highly palatable."

"As more populations are exposed to increasing amounts of highly processed, highly palatable foods, we will likely see increases in obesity in regions with relatively low rates currently," the study authors predicted.

Ultraprocessed foods tend to be easier to overeat due to their flavor and texture, and they also don't keep people feeling full, the study found.

They are also more calorie-dense, meaning they pack more energy into smaller volumes, and are more easily absorbed, making it harder for the body to eliminate excess energy.

"Ultraprocessed foods are engineered to override our biology, namely our satiety signaling in the brain," Osborn told Fox News Digital. "They also promote inflammation and shift our physiology to a state that favors fat accumulation over fat burning." 

The researchers pointed out that physical activity is still important, particularly for cardiovascular health, mental well-being and longevity.

Allen noted that people "need to consider all the other benefits of exercise and shift the mindset away from how many calories they are burning (which actually doesn't work very well for weight loss) toward building as much strength and muscle as they can."

Osborn agreed, recommending "progressively intense and challenging workouts" — strength training in particular — to help boost the metabolic rate and facilitate fat-burning. 

"As I always say to my patients, "By virtue of proper nutrition and strength training, your body will not only burn fat, but it will also burn fat fast."

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However, Allen noted, when it comes to preventing obesity, the findings point to excess calorie consumption — particularly ultraprocessed foods — as the major contributing factor. 

"If you’re worried about excess body fat, focus on calories in," McGrosky and Luke advised. "Especially over the long term, it’s much harder to meaningfully change the total number of calories you burn than it is to change the number of calories you take in."

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Osborn added that the problem isn’t laziness or lack of movement.

"It’s the constant stream of ultraprocessed, nutrient-void products being shoveled into mouths under the guise of health or convenience," he said. "And it’s killing us — slowly, predictably and en masse."