The formula used to decide whether you have obesity was invented in the 1830s by a Belgian astronomer. Adolphe Quetelet was studying populations, not patients — he wanted a mathematical description of the "average man," and dividing weight by height squared gave him a tidy way to do it. He never meant it to measure any individual's health, and it didn't even get the name "body mass index" until 1972. Yet nearly two centuries later, that same ratio is still the number on the chart that sorts people into "normal," "overweight," and "obese."
Starting there tells you something important: a lot of what we assume about obesity is built on tools and ideas that were never designed for the job. Understanding what obesity actually is — how it's diagnosed, why medicine now calls it a disease, and why "chronic" is the key word — means untangling the science from a century of moral baggage.
From moral failing to medical diagnosis
For most of the twentieth century, obesity wasn't treated as a medical condition at all. It was treated as a character problem: eat less, move more, and if you couldn't, that was a failure of willpower. Medicine largely went along with it, which is why "treatment" for decades meant being told to try harder.
That framing began to shift as the biology accumulated. In 2013 the American Medical Association formally recognized obesity as a disease — a contested vote at the time, but a turning point. It said, in effect, that obesity is a medical condition with biological drivers, not simply the sum of a person's choices.
The most rigorous step came in 2025, when a global commission of 58 experts — published in The Lancet Diabetes & Endocrinology and endorsed by 76 medical organizations worldwide — proposed a complete overhaul of how obesity is defined and diagnosed.
How obesity is actually diagnosed
Here's where the Quetelet problem comes back. BMI is cheap, fast, and genuinely useful across large populations — but at the level of one human being it's crude. It can't tell muscle from fat, it doesn't see where fat sits (the visceral fat around your organs is the dangerous kind), and its cutoffs don't translate cleanly across age, sex, or ethnicity. A muscular athlete and a sedentary person can share a BMI; so can two people in very different health.
The 2025 Lancet commission's answer was to stop using BMI as a diagnosis on its own. It recommends BMI be treated as a screening signal, with actual excess body fat confirmed by a direct measurement or at least one additional measure such as waist circumference. More importantly, it splits obesity into two states: