What is overweight? What is obesity?

The causes of weight gain are as varied as the terms used to describe it: Overweight, obesity, adiposity. But what do these terms mean? How do they differ and what criteria are they based on?

Definitions for adults

Overweight and obesity are terms that are often used as synonyms. However, there are clear differences in definition and health consequences. Whether and to what extent a person is overweight is determined by a defined criterion - the body mass index (BMI). BMI is the ratio of body weight in kilograms to height in metres squared [Ogden, 2004] and is classified by the World Health Organization (WHO) as follows:

BMI	Weight status
<18,5	Underweight
18,5 - 24,9	Normal weight
25 - 29,9	Pre-obese
30 - 34,9	Grade 1 obesity
35 - 39,9	Grade 2 obesity
>40	Grade 3 obesity

Obesity is a more advanced form of overweight and is classified as a disease by the WHO. However, BMI should not be used as the sole measure of weight as it has some limitations. For example, it does not take into account age or gender, both of which can influence weight in adults. BMI also ignores the distribution of fat and muscle mass, as only total weight is used in the calculation [Wellens et al, 1996]. In addition, the way fat is distributed in the body can affect the severity of obesity. The distribution of body fat can be either subcutaneous or visceral. Subcutaneous fat is stored directly under the skin and is mainly found in the hips, thighs and buttocks. Visceral fat is stored in the abdominal cavity around the organs. This type of fat distribution is more closely associated with the risk of secondary diseases [World Health Organisation, 2022].

Definitions for children and adolescents

Classifying and defining weight status in children and adolescents is complicated by the fact that body size is still increasing and body composition is still changing. It is particularly important to consider additional factors such as age and gender when defining weight status. The „Arbeitsgemeinschaft Adipositas im Kindes- und Jugendalter” (AGA) therefore recommends the use of BMI reference curves (also known as BMI percentile curves) for children and adolescents [Deutsche Adipositas Gesellschaft, 2024].

What is metabolism?