What is Obesity?
Obesity is a long-term medical condition involving excess body fat that affects health. It is not simply a lifestyle issue — it involves complex interactions between genetics, hormones, environment, and behaviour.
Quick read · 4 min
- •Over 1 billion adults worldwide are classified as obese
- •Obesity is defined by BMI — a ratio of weight to height — but BMI has real limitations
- •Major medical organisations now classify obesity as a chronic disease, not a personal failure
- •It is driven by biology (genetics, hormones, environment) — not just choices
Based on clinical trials · No rankings · Every claim linked to source
Last reviewed: March 2026
Scale of the problem
Sources: Global Obesity Federation 2022 [1] · CDC NCHS Data Brief 2024 [2]
How obesity is defined
In clinical settings, obesity is most commonly defined using Body Mass Index (BMI) — a ratio of weight to height. It is used because it is easy to measure and correlates broadly with health risk at the population level.
| BMI range | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (Class I) |
| 35.0 – 39.9 | Obesity (Class II) |
| 40.0 and above | Severe Obesity (Class III) |
Limitations of BMI
BMI is a population-level screening tool, not a perfect individual measure. It has several important limitations:
- ⚠BMI does not distinguish between fat mass and lean muscle mass — a heavily muscular person may have a high BMI without excess body fat.
- ⚠It does not account for fat distribution. Visceral fat (around the abdomen) carries higher metabolic risk than subcutaneous fat, but BMI cannot tell them apart.
- ⚠Asian populations may have metabolic risk at lower BMI thresholds. Some clinical guidelines apply lower cut-offs (e.g. BMI ≥27 or ≥25) for this group.
- ⚠Waist circumference and waist-to-hip ratio are increasingly used alongside BMI to better capture cardiometabolic risk.
Obesity as a chronic disease
Major medical organisations — including the World Health Organization, American Medical Association, and European Association for the Study of Obesity — now classify obesity as a chronic, relapsing disease.
This classification reflects the biological reality: obesity involves dysregulation of hormonal signals that control appetite and energy storage, which means it cannot be permanently resolved by willpower alone. The same is true of other chronic diseases like hypertension or type 2 diabetes — they are managed, not cured.