What is obesity?
Scientific support: Prof. Dr. Christina Holzapfel
The term obesity stands for severe overweight. Obesity is developing when fat accumulates excessively in the body due to an imbalance between energy intake and consumption.
Obesity is a chronic disease that requires lifelong care and treatment. In addition, obesity is associated with other disease risks such as type 2 diabetes and cardiovascular diseases and can lead to serious health problems.
Contents
1. Obesity: Disease and risk factor
The World Health Organization (WHO) classifies obesity as a chronic disease. Since summer 2020, obesity has also been recognized by the German Bundestag as a chronic, progressive disease.
Before that, obesity in Germany was long regarded only as a risk factor for non-communicable diseases such as type 2 diabetes and cardiovascular diseases and not as an independent disease. Obesity is associated with an increased risk of numerous concomitant and secondary diseases. It can also have a negative impact on well-being and quality of life. The health risks vary greatly from person to person.
As a chronic disease, obesity requires comprehensive and lifelong treatment.
Good to know:
At the beginning of 2025, an international commission proposed a new classification into two categories for the definition of obesity: “clinical obesity” and “preclinical obesity”. Clinical obesity is defined as a chronic disease characterized by obesity-related, persistent organ dysfunction. Preclinical obesity, on the other hand, is associated with an increased health risk, but is not yet a permanent disease.
2. What are the symptoms of obesity?
If more energy (kilocalories) is taken in than is consumed, this leads to an increase in body weight and, over time, to obesity. If the body weight continues to rise because the fat content is increasing, overweight develops into obesity. Overweight and obesity are defined by the body mass index (BMI) – the ratio of body weight to height:
- A BMI between 18.5 kg/m² and 24.9 kg/m² is considered normal weight.
- A BMI of 25 kg/m² or higher is considered overweight.
- A BMI of 30 kg/m² or higher is referred to as obesity.
In addition to the BMI, the distribution of fat tissue is also important. A distinction is often made between the “apple type” and “pear type.” In the apple type, fat is mainly deposited in the abdominal area, while in the pear type, the hips, buttocks and thighs are more likely to be affected. Abdominal fat (apple type) in particular is metabolically very active and is associated with an increased health risk. The waist circumference is a measure to determine abdominal fat. Abdominal obesity is defined as a waist circumference of 88 centimeters and higher in women and 102 centimeters and higher in men.
3. What are the possible consequences of obesity?
Obesity increases the risk of developing numerous concomitant and secondary diseases. Obesity is often associated with reduced physical performance and restricted mobility as well as breathing disorders during sleep (sleep apnea syndrome). In addition, obesity is one of the most important factors in the development of type 2 diabetes. Obesity also significantly increases the risk of cardiovascular diseases and certain cancers. The various consequences of obesity can affect the whole body.
Read more about the possible concomitant and secondary diseases of obesity here.
Good to know:
The longer obesity exists and the more pronounced it is, the higher the risk of developing concomitant and secondary diseases.
4. Who suffers from obesity?
Various factors have an effect on the individual risk of developing obesity. Genetics plays a role. People with a genetic predisposition find it difficult to maintain a normal body weight.
Your own lifestyle also has a significant impact on the risk of developing the disease. People who eat too many calories and do too little exercise have an increased risk of developing overweight and obesity. Persistent stress and poor sleep also have a negative effect on health and can contribute to weight gain.
5. How does obesity develop?
Obesity is developing when more energy (kilocalories) is taken in through food than the body consumes. In most cases, too much food intake is accompanied by too little physical activity. The body stores the excess energy in the form of fat cells. Once they have been created, fat deposits are not automatically broken down again as long as the body is not in a sustained negative energy balance – i.e., the body consumes more energy than it takes in. Therefore, even a slight imbalance between energy intake and consumption over a longer period can lead to overweight or even obesity.
In addition to lifestyle, genetic, biological, socio-economic (e.g., education, employment, income) and psychological factors also play an important role in the development of obesity. Genetic predisposition and epigenetic imprinting have an effect on metabolism and energy consumption. But gender, age, existing illnesses and birth weight also are influencing factors.
In addition, there are environmental factors and living conditions that no longer correspond to evolutionary behavior: In the past, it was essential for the human body to build up fat reserves in order to survive times of low food availability. Today, for example, the constant availability of energy-rich foods and drinks and large portions combined with a predominantly sedentary lifestyle contribute to the development of overweight and obesity. In this context, experts speak of an obesogenic environment.
Sources:
Blüher, M.: Obesity: global epidemiology and pathogenesis. In: Nat Rev Endocrinol, 2019, 15: 288-298
Deutsche Adipositas-Gesellschaft et al.: S3-Leitlinie Adipositas - Prävention und Therapie. Version 5.0. 2024
Deutsche Gesellschaft für Ernährung (Hrsg.): 15. DGE-Ernährungsbericht. Bonn. 2024
Guh, D. P. et al.: The incidence of co-morbidities related to obesity and overweight: A systematic review and meta-analysis. In: BMC Public Health, 2009, 9: 88
Rubino, F. et al.: Definition and diagnostic criteria of clinical obesity. In: Lancet Diabetes Endocrinol, 2025 (Online ahead of print)
World Health Organization: WHO European Regional Obesity Report 2022. 2022. Copenhagen
World Health Organization: Obesity: preventing and managing the global epidemic. Report of a WHO consultation. In: World Health Organ Tech Rep Ser, 2000, 894: 1-253
As of: 17.02.2025
