A new study is shaking up the way we think about weight gain, and it starts with one bold claim: overeating is ten times more responsible for the global obesity crisis than a lack of exercise. 

If you’ve always believed that hitting the gym could undo those extra snacks, this research might change your outlook. The study, published in the Proceedings of the National Academy of Sciences, zeroes in on the primary cause of obesity. Sure enough, it’s not how many steps you took today, but how many calories you took in.

Researchers examined data from over 4,000 people across 34 global populations and found that daily energy burned (i.e., calories out) doesn’t vary much, regardless of lifestyle. 

That’s pretty interesting right there. Folks who live in active communities burn about the same adjusted energy as those in sedentary environments. The key difference, however, is what’s on their plates.

Study Finds Overeating, Not Inactivity, Drives Obesity

For years, we've blamed modern weight gain on people sitting too much and moving too little. But, as it turns out, you can’t outwalk, outrun, or outlift a consistently high-calorie diet. And it's not just the number of calories that matters. It’s where they come from. 

While other factors like stress and low muscle mass also play a role, it seems that overeating is the core issue. In short, obesity and overeating go hand in hand, and ignoring the food side of the equation is like trying to bail out a sinking boat with a thimble.

Ultraprocessed Foods and Weight Gain Go Hand in Hand

A major culprit in the calorie overload is ultraprocessed foods. These aren’t just your typical takeout burgers or vending machine snacks. We’re talking about items with long, chemical-sounding ingredient lists designed to last forever and taste irresistible. They’re convenient, sure. But they’re also calorie-packed, easy to overeat, and don’t keep you full.

According to this new obesity study of 2025, ultraprocessed foods are engineered to mess with your body’s natural “I’m full” signals. They spike inflammation, throw your metabolism into storage mode, and encourage your body to hang onto fat rather than burn it. 

And, because they’re so common – from frozen meals to packaged breads and energy bars – many people are unknowingly eating themselves into chronic weight gain.

Exercise vs. Diet for Weight Loss: What Actually Works?

Don’t worry, your morning walks and yoga classes aren’t in vain. Exercise is still hugely important for your heart, brain, stress levels, and longevity. But, when it comes to the scale, diet takes the lead.

The researchers behind this study estimate that food intake is ten times more important than the role of physical activity in the rise of global obesity rates. While that doesn’t mean you should cancel your gym membership, it does mean that focusing solely on workouts won’t get you very far.

Why Diets Fail, Long Term

So, if diet is king, why do so many people struggle to keep weight off? A big part of the problem is the food environment. Ultraprocessed items are everywhere, often marketed as "healthy," "low fat," or "all natural." But, they’re still hyper-palatable and energy-dense. That makes portion control tricky and satisfaction short-lived.

Another big issue is stress. In high-pressure societies, cortisol levels remain elevated, altering metabolism and encouraging fat storage. Pair that with poor sleep, nutrient deficiencies, and minimal muscle mass, and you’ve got the perfect storm for weight gain. 

So, it’s not that people don’t want to lose weight; it’s that everything around them, from food labels to work stress, makes it harder to do so. That’s exactly why diets fail, long term.

Reframing the Fight Against Obesity

It’s time to reframe the conversation around obesity. The focus shouldn’t be on blaming people for being inactive or lazy. That’s outdated and unhelpful. Instead, we need to take a hard look at our collective food habits. The primary cause of obesity isn’t movement, but the sheer volume and type of food we consume.

One of the main problems is the constant stream of nutrient-void, ultraprocessed products being funneled into mouths under the guise of health or convenience. In the end, it’s not just a personal choice issue; it’s a public health one.

Calories In, Not Just Calories Out

So, where do we go from here? The authors of the 2025 study have one clear recommendation: pay attention to what’s going in. Long term, it’s much easier to reduce your calorie intake than meaningfully increase the number you burn. 

Strength training can help boost your metabolism, and exercise remains essential for overall health. But, if fat loss is the overall goal, start with the kitchen.

The truth is, the primary cause of obesity lies in our modern diets – particularly the omnipresence of ultraprocessed foods that hijack hunger signals and pack a calorie punch. Exercise is still a key part of the puzzle, but it can’t solve the problem alone. For real change, we need to rethink the way we eat.