The prevalence of overweight is increasing dramatically in industrialized societies but also in developing countries.
Actually, 8% of the Swiss population is obese (BMI>30 kg/m²) and approximately 1% is morbidly obese (BMI>40 kg/m²). These rates are rather low compared to other countries.
The causes for the rise of obesity rates are determined by the contemporary environment and societal changes:
- unlimited availability of aliments (especially more energy-dense, nutrient poor foods with high level of sugar and saturated fats)
- and a general reduction of physical activity.
In combination, this means that more energy is taken in than can be spent by the body, resulting in a positive energy balance. The excessive energy is stored in the body as fat which leads finally to overweight and obesity.

In few cases obesity can be related to hormonal disorders (e.g. thyroid hypofunction, or increased release of the adrenal stress hormone cortisol, also known as cushing syndrome). Therefore, special examination is needed to exclude this possibility.

Although the environmental and societal changes are shared by the entire population, not all people develop obesity. Apparently, people who are not affected by overweight appear to have a very effective regulatory system, which continuously keeps the balance between energy intake and output. The brain is the most important regulatory centre which controls eating behaviour and energy output through integrating hormonal and metabolic signals from the body. In recent years, this complex regulatory system has been extensively studied worldwide, leading to highly increased knowledge of this disease.

Genes are important in determining a person's susceptibility to weight gain. This fact is scientifically proven but often neglected even by health care professionals.