Obesity Redefined: Why It’s Now Recognized as a Chronic, Progressive Disease

Written by Dr. Ritva Mettänen, MD

Obesity: A Chronic, Progressive, and Multidimensional Disease

In recent years, our understanding of obesity has undergone a seismic shift. What was once dismissed as a failure of willpower or the simple result of overeating is now recognized for what it truly is: a chronic, progressive, and relapsing disease.

This was echoed across every keynote at the 2024 European Congress on Obesity (ECO) in Malaga, where speakers opened with a simple, unequivocal statement:

“Obesity is a chronic disease.”

While this definition has become widely accepted in clinical and scientific circles, many—including healthcare professionals — still struggle to fully grasp what it means in practice. If we, as clinicians, took years to understand and accept it, how can we expect the public to understand it overnight?

That’s why this article is here: to unpack what obesity really is, how it develops, and why treating it requires more than just telling someone to “eat less and move more.”

Obesity Is Not a Choice — It’s an Adaptive Response

The body doesn’t passively store fat. It adapts to it.

When a person gains weight, the brain recalibrates its internal energy regulation systems and raises the “set point” for body weight. This makes weight loss more difficult — and weight regain far more likely. From an evolutionary standpoint, this makes perfect sense. Fat stores once protected our ancestors from famine. But in today’s world, this survival mechanism has turned into a chronic health challenge.

When someone loses weight, the body doesn’t celebrate. It defends against it. Hunger hormones like ghrelin surge, while leptin, which signals fullness, plummets. These changes persist long after the diet ends, often for months or even years, driving a relentless biological push to regain lost fat.

Why “Eat Less, Move More” Doesn’t Cut It

The idea that weight loss is simply a matter of willpower fails to reflect the neurological and physiological changes that occur in obesity.

People living with obesity often describe food cravings as overwhelming—sometimes even painful. These aren’t imagined. They’re driven by changes in brain function.

Key neurons in the hypothalamus, such as melanocortin neurons, become dysregulated. Mitochondria lose metabolic flexibility. Neural networks start prioritizing external cues like advertisements and packaging over internal hunger and satiety signals.

Similar neural networks are processing food and other rewards, meaning that food can become just as compelling as other addictive stimuli.

Food, Reward, and the Brain

In obesity, the brain’s reward system becomes hypersensitive to food—especially high-calorie, hyperpalatable foods. Over time, this can lead to a paradox: food becomes less pleasurable, but the urge to eat it remains.

Food cue reactivity is influenced by nutritional state, appetitive gut hormones, obesity, eating behaviour, and bariatric surgery.

This altered reward processing also weakens the brain’s inhibitory systems, which normally help us make rational food choices. That’s why so many people find it difficult to “just eat less”—especially in moments of stress or fatigue.

Adding to the complexity, gut hormones modulate appetite and also food hedonics, cue reactivity, and taste responses. These hormones don’t just make us hungry—they shape our emotional and sensory relationship with food.

Genetics + Environment = The Perfect Storm

Genetic predisposition plays a major role in how we store fat, experience hunger, and respond to food. But our environment—rich in calories, low in movement, high in stress—amplifies that risk dramatically.

We now live in a world that makes weight gain almost inevitable for many, especially those with a genetic susceptibility.

Why Weight Regain Happens (and Why It’s Not Your Fault)

Every time you lose weight, your body doesn’t just shed fat—it loses some muscle too. When weight comes back, fat typically returns faster than muscle, leading to a higher body fat percentage and often a higher total body weight.

This cycle isn’t failure. It’s biology doing its job.

The Limits of BMI

Despite being a standard clinical measure, BMI tells us very little about a person’s metabolic health, fat distribution, or muscle mass. It’s a crude tool at best.

Today’s experts are calling for more nuanced diagnostic methods, including waist circumference, body composition analysis, and inflammatory and insulin resistance biomarkers.

Final Thoughts: You Are Not Broken

Let’s be clear:

Obesity is not your fault.
It’s a complex, chronic disease—like asthma or diabetes—and it deserves evidence-based care, not stigma.

Yes, you have a responsibility: to care for your health, seek support, and take the first step toward change. But you are not to blame for having the disease.

Your body is doing what it’s evolved to do. That doesn’t mean change is impossible—it means the path forward requires compassion, science, and support.

Because this isn’t about discipline. It’s about healing—physically, mentally, and emotionally. And you deserve nothing less.

Resources:
https://doi.org/10.1038/s41366-024-01582-8
https://doi.org/10.1016/j.bbr.2024.115401
https://doi.org/10.1016/j.neuron.2025.03.010
https://www.nature.com/articles/s41586-024-08165-7?utm