Obesity is a chronic disease in which excess accumulated fat negatively affects health and shortens life expectancy. In 2024, 1.1 billion people were living with it — one in seven worldwide, according to WHO. Below — what BMI shows, where it doesn't work, and which risks are confirmed.
BMI: definition and thresholds
BMI = body weight (kg) / height² (m²). For adults, WHO defines overweight at BMI ≥ 25 and obesity at ≥ 30; for people of South, East, and Near Eastern and African origin, the thresholds are lower — 23–27.4 and 27.5. In children aged 5–19, obesity is defined by deviation from the WHO reference median: more than 2 standard deviations. For children under 5, the criterion is different — a weight-for-height ratio more than 3 standard deviations above the median of the growth standards.
What limits BMI
The BMI calculator does not distinguish muscle from fat: with a large muscle mass, it may show overweight without excess fat. Additionally, the waist-to-height ratio is used: at a BMI up to 35, less than half of height is considered normal. Fat distribution is also assessed by the waist-to-hip ratio. Thresholds vary by country: in Japan, obesity is defined at a BMI above 25, in China — above 28.
Risks that are confirmed
The strongest link is with type 2 diabetes: excess fat underlies 64% of cases in men and 77% in women. The list of associated conditions includes heart disease, stroke, high blood pressure, some types of cancer (breast, uterine, bowel), osteoarthritis, and fatty liver disease. Everyday consequences include shortness of breath, difficulty moving, fatigue, joint pain, snoring, and depression. Life expectancy with obesity is 2–10 years shorter; at a BMI above 40 — by about 10. In 2023, a BMI higher than optimal caused about 3.7 million deaths (WHO).
Causes beyond "willpower"
Obesity is the result of the interaction between environment and biological predisposition, not a single habit. Environments that promote weight gain limit access to healthy and affordable food and to safe physical activity. Genetic variation influences individual susceptibility; the condition itself is accompanied by disruption of the neurobiological and metabolic pathways that regulate appetite, energy balance, and adipose tissue function. Some cases arise against a background of endocrine disorders, medication use, or psychiatric disorders. The "slow metabolism" version is weakly supported by evidence: people with obesity usually burn more calories than lean people, because a heavier body requires more energy.
Scale
In 2024, 45% of adults worldwide (2.6 billion) had overweight, of whom 940 million were living with obesity — 16% of all adults. Since 1990, the prevalence of obesity among adults has doubled, and among adolescents it has quadrupled. Among children aged 5–19, 400 million had overweight, of whom 170 million had obesity. In Ukraine, according to WHO data, as of 2022, 23.6% of adults had obesity versus 14% in 1991.
Diagnosis and who to contact
The diagnosis is made by measuring weight and height with BMI calculation; waist circumference is used additionally. In children, assessment is based on age and sex. A family doctor assesses associated conditions; referral to a weight management program is possible.
You should see a family doctor routinely if your BMI is in the overweight or obesity category and:
- you want help with weight loss;
- your waist circumference is greater than half your height;
- you have prediabetes, diabetes, high blood pressure, joint pain, high cholesterol, sleep apnea, or cardiovascular disease;
- there is a family history of obesity or an eating behavior disorder, such as binge eating.
Call 103 or 112 immediately if there are signs of stroke or heart attack: these are emergencies where every minute counts.
At the population level, WHO insists on changing environments: affordable healthy food, safe places for activity, regulatory and fiscal measures. This material is for reference: it does not replace consultation with a doctor who sees a specific person, not a sample average.
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This content is for reference and does not replace a doctor's consultation. Medical disclaimer