What is obesity?

What are the official criteria for obesity?

Obesity is defined in medical terms and is not simply diagnosed based on a gut feeling. The World Health Organisation (WHO) and the Dutch General Practitioners’ Guidelines (NHG) use the Body Mass Index as their primary screening tool.

You calculate your BMI by dividing your weight in kilograms by your height in metres squared. The result is classified into categories:

• BMI below 18.5 — underweight
• BMI 18.5 to 25 — a healthy weight
• BMI 25 to 30 — being overweight
• BMI of 30 or above — obesity
• BMI of 40 or above — severe (morbid) obesity

A BMI of 30 or above is therefore medically classified as obesity. That sounds simple, but in practice, doctors look beyond this figure alone. For example, BMI does not take into account muscle mass, fat distribution or age. That is why waist circumference is often measured as well, as fat around the waist is more strongly linked to health risks such as cardiovascular disease than fat elsewhere on the body.

For men, a waist circumference of 102 centimetres or more is considered to indicate an increased risk; for women, 88 centimetres or more. When combined with an elevated BMI, this provides a more comprehensive picture of a person’s health risk than BMI alone.

Important to note: a diagnosis of obesity is ultimately always made by a doctor, who takes into account not only BMI and waist circumference but also medical history, lifestyle and any underlying conditions. A number on its own never tells the whole story.

Why is obesity more than just a number on the scales?

One of the biggest misconceptions about obesity is that it is purely a matter of “eating too much and not exercising enough”. That view is outdated and is no longer held by modern medicine. Since 2013, the American Medical Association, amongst others, has officially recognised obesity as a chronic disease, and the WHO also uses this classification.

What does that mean in practical terms? It means that obesity is caused by a combination of factors, some of which are beyond a person’s direct control:

Genetic predisposition. Research shows that body weight is largely determined by genetics. Some people naturally have a metabolism that stores fat more easily, or hormones that regulate feelings of hunger and fullness differently.

Hormonal regulation. Hormones such as leptin and ghrelin regulate hunger and satiety. In people with obesity, this system is often disrupted, causing the body to react more strongly to hunger and to take longer to signal a feeling of fullness. This is a physiological process, not a lack of willpower.

Environmental factors. We live in an environment where high-calorie food is widely available, whilst physical activity is often no longer a regular part of our daily lives. This “obesogenic environment” makes it harder for everyone to maintain a healthy weight, but it affects people with a predisposition to obesity particularly severely.

Psychological and social factors. Stress, lack of sleep, medication and life events can all affect eating habits and weight. This, too, is often beyond the direct control of someone who is struggling with their weight.

These factors also explain why traditional diets do not work in the long term for many people. People who lose weight on a strict diet often find that their body adapts and the weight returns over time. This is known as the yo-yo effect, and it is a direct consequence of the way the body is programmed to maintain a certain weight. We explore this in more detail in our blog post on the yo-yo effect after dieting.

Recognising obesity as a chronic condition is important because it changes the way we view it. Not as a personal failure, but as a medical problem that requires a medical approach — just like other chronic conditions such as diabetes or high blood pressure.