By Dr. Asnat Raziel, bariatric surgeon specializing in obesity treatment, metabolic health and healthy longevity; Director of the Multidisciplinary Center for Obesity Treatment, Assia Medical, Assuta Ramat HaChayal, Tel Aviv
Obesity as a chronic disease: what does it actually mean?
Seeing obesity as a chronic disease is now the approach that guides medicine around the world. For years, excess weight was treated as a matter of willpower. Today we know it is a complex biological disease that persists over time and tends to come back. That is why its treatment also needs to be ongoing, just as it is for diabetes or high blood pressure.
This shift changes the experience of patients. It lifts the blame, opens the door to effective treatment and moves the conversation from “try harder” to “let’s understand what is happening in your body.”
How many people live with obesity?
According to the World Health Organization, 2.6 billion adults worldwide were living with overweight in 2024, or 45% of all adults. Of these, 940 million were living with obesity, 16% of the world’s adults. The WHO explicitly defines obesity as a chronic disease.
In adults, the accepted definition is a BMI (body mass index) of 30 or higher. As we will see, however, this number alone does not tell the whole story. For more background, see the Wikipedia entry on obesity.
Why is obesity a chronic disease and not just a “habit”?
First, obesity disrupts the way the body works. Enlarged fat tissue releases hormones and inflammatory substances, and fat also builds up inside organs such as the liver. A position statement from the World Obesity Federation describes obesity as a “chronic, relapsing, progressive disease process” that damages the arteries, heart, liver, muscles and pancreas.
Second, body weight is regulated by the brain. A complex network of hormones from the gut and from fat tissue keeps the brain informed about hunger, fullness and energy stores. In people with obesity, this system is set to defend a higher weight. I cover this in more detail in my article on the physiology of satiety.
Third, obesity has many causes: genetics, environment, sleep, stress, medications and hormonal changes. So there is no single solution that fits everyone. For one woman, the change begins at menopause; for another person, it starts after a psychiatric medication or a stretch of poor sleep. Understanding the main driver points the way to the right treatment.
Obesity as a chronic disease: why does the weight come back?
Weight regain is one of the clearest signs that we are dealing with a chronic disease. In a study published in the NEJM, researchers followed 50 patients who lost an average of 13.5 kg on a low-calorie diet. A full year later, their levels of leptin and other satiety hormones were still low, while ghrelin, the hunger hormone, remained high. Not surprisingly, they still felt strong hunger.
In other words, the body “remembers” its previous weight and tries to get back to it. This is not weakness; it is biology. It is also why stopping treatment often leads to weight regain, and why patients often ask me whether obesity is a lifelong disease.
The new definition: clinical and preclinical obesity
In January 2025, an international commission of 58 experts published a new definition of obesity in The Lancet Diabetes & Endocrinology. It has been endorsed by 76 organizations worldwide. Under this definition, BMI is only a screening tool. Confirming excess body fat requires an additional measure, such as waist circumference or a direct measurement of body fat. With a BMI above 40, excess fat can be assumed without further testing.
The commission distinguishes between two conditions:
- Preclinical obesity: excess body fat, but the organs are still functioning normally. The risk of future illness is increased.
- Clinical obesity: a chronic, systemic disease in which excess fat is already impairing organ function or limiting everyday activities.
According to the commission, people with clinical obesity should receive timely, evidence-based treatment. In preclinical obesity, by contrast, the recommendation is counseling, monitoring and, in some cases, intervention to reduce risk.
The complications of obesity as a chronic disease
According to the WHO, obesity is linked to type 2 diabetes, cardiovascular disease, liver and kidney disease and several types of cancer. What’s more, in 2023 a higher-than-optimal BMI was responsible for about 3.7 million deaths worldwide. Obesity also takes a toll on the joints, sleep and mood.
The good news is that even modest weight loss improves many of these complications. Losing 5% of body weight, for example, often improves blood sugar levels and blood pressure.
How is obesity treated as a chronic disease?
Treatment is built in layers and tailored to each person:
- Lifestyle: a way of eating you can sustain, physical activity and sleep. This is the foundation of any treatment.
- Professional support: a clinical dietitian and emotional support when needed.
- Obesity medications: drugs that act on the hunger and satiety centers in the brain, used long term and under medical supervision.
- Bariatric surgery: an effective option in suitable cases, according to clinical guidelines.
As with any chronic disease, the goal is not a short-term “diet.” The goal is to keep the disease under control for years. Just as with high blood pressure, no one expects a single pill or one month of effort to solve the problem for good. That is also why it matters to talk about weight stigma, which keeps people away from treatment. For broader background, see my article on overweight and obesity.
The bottom line
Defining obesity as a chronic disease is grounded in science; it is not just a change of words. The body defends its weight through hormones and brain mechanisms, which is why willpower alone is not enough. Ultimately, an accurate diagnosis and ongoing treatment make it possible to bring the disease under control and prevent its complications. If you recognize yourself in this article, it is worth seeking a medical evaluation.
The information in this article is general and does not replace personal medical advice.
Frequently asked questions
Is everyone with a BMI over 30 ill?
Not necessarily. Under the new definition, excess body fat must be confirmed, and doctors need to check whether it is already affecting how the body functions.
If I’ve lost weight, is the disease gone?
Not always. The body tends to regain weight, so follow-up continues, and sometimes treatment does too.
Does treating obesity as a chronic disease mean lifelong medication?
For many people, treatment continues over the long term, as with any chronic disease. The decision is always individual and made together with your doctor.
Why does it matter to define obesity as a disease?
The definition reduces blame and opens access to evidence-based care and to funding for treatment.
Sources
Obesity and overweight – WHO fact sheet
Definition and diagnostic criteria of clinical obesity
Long-term persistence of hormonal adaptations to weight loss
