Is obesity a disease or a lifestyle issue?

Major medical organisations now classify obesity as a chronic disease. They include the WHO, the AMA and the Endocrine Society. In obesity, the hormone signals that control appetite and energy storage stop working as they should. It is not simply a matter of willpower or lifestyle choices.

What is BMI and how accurate is it?

BMI (Body Mass Index) is a ratio of weight to height. It is useful for screening large groups of people, but it has real limits. It does not tell muscle from fat. It does not account for where fat sits on the body. And it may underestimate risk in some ethnic groups. Waist-to-height ratio is increasingly used alongside BMI.

At what BMI is someone considered obese?

A BMI of 30 or above is classified as obesity. Class I is 30–34.9, Class II is 35–39.9, and Class III (severe) is 40 and above. These cut-offs may vary for certain populations. Asian populations, for example, may face risk to their metabolic health at lower cut-offs.

How common is obesity?

Over 1 billion adults worldwide are classified as obese as of 2022. In the United States, about 40% of adults have obesity. Global rates have tripled since 1975.

What is Obesity?

Adults with obesity globally

1B+

2022 data

Of US adults have obesity

40%

CDC 2021–2023

Rates tripled since 1975

3×

WHO data

Obesity is now classified as a chronic disease by major medical organizations. It is driven by biology, not personal failure.

How obesity is defined

In clinical settings, obesity is most commonly defined using Body Mass Index (BMI), a ratio of weight to height. Across large groups it tracks health risk broadly.

BMI classifications

World Health Organization standard categories

UnderweightBMI Below 18.5

Normal weightBMI 18.5 – 24.9

OverweightBMI 25.0 – 29.9

Obesity (Class I)BMI 30.0 – 34.9

Obesity (Class II)BMI 35.0 – 39.9

Severe obesity (Class III)BMI 40.0+

BMI is a screening tool, not a perfect individual measure. See limitations below.

Limitations of BMI

Muscle vs fat

BMI does not tell fat from muscle. A heavily muscular person may have a high BMI without excess body fat.

Fat distribution

Fat deep in the belly, called visceral fat, carries more risk than the fat under the skin. BMI cannot tell the two apart.

Ethnic variation

Asian populations may have risk to their metabolic health at lower BMI levels. Some clinical guidelines apply lower cut-offs for this group.

Better alternatives exist

Waist measurement and waist-to-height ratio are increasingly used alongside BMI. They capture risk to the heart and metabolism better.

Obesity as a chronic disease

Major medical organisations now classify obesity as a chronic, relapsing disease. They include the World Health Organization, the American Medical Association and the European Association for the Study of Obesity. Relapsing means it tends to come back.

This classification reflects the biological reality. In obesity, the hormone signals that control appetite and energy storage stop working as they should. That means willpower alone cannot permanently resolve it. The same is true of other long-term diseases such as high blood pressure or type 2 diabetes. They are managed, not cured.

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Last reviewed: September 2026

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