Obesity is a single condition; your BMI says whether you have it
The thing itself didn't change; its official category did.
TaughtObesity is one thing: a BMI of 30 or above. You either cross that line or you don't, and crossing it means you have the disease of obesity.
NowIn January 2025 a Lancet Diabetes & Endocrinology Commission of 58 experts split the concept in two ('clinical obesity,' an actual disease with organ dysfunction caused by excess body fat, and 'preclinical obesity,' excess fat with raised future risk but no current illness) and moved diagnosis beyond BMI alone.
What actually happened
The Commission, led by Francesco Rubino of King's College London and published on 14 January 2025, started from a blunt problem: BMI is a measure of size, not of health, and treating 'BMI ≥ 30' as a disease diagnosis both over-labels healthy people and misses sick ones. Their fix reframes obesity as a spectrum rather than a single yes/no box.
Under the new framework, excess body fat should be confirmed by more than a BMI number: for example a DXA body-fat scan, or waist circumference or waist-to-height ratio in addition to BMI. Only when that excess adiposity is actually damaging organs or limiting daily function is it 'clinical obesity,' a disease to be treated as one. Excess fat without that damage is 'preclinical obesity': a risk state to monitor, not a diagnosis of current illness.
This is a reclassification, not a debunking; obesity is still real and still serious. What changed is the definition: the flat old equation of 'BMI 30 equals the disease' has been replaced by a two-tier diagnosis that asks whether the fat is harming the body, and uses body-composition measures rather than BMI alone to decide.
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Who was taught this
Still standard through 2025, so anyone who finished school between 1950 and 2025 learned the earlier version.