How is obesity defined, and why do people argue about BMI?
The number used to define obesity began with an astronomer who never meant it for medicine.
Guess first
?
Which one is it?
In practice, obesity is defined by a number: body mass index, or BMI, which is a person's mass in kilograms divided by the square of their height in metres. For adults, a BMI of 25 to 29.9 counts as overweight and 30 or more counts as obese. People argue about it because BMI cannot tell muscle from fat or say where the fat sits, so it works better as a rough rule of thumb for groups than as a verdict on one person's health.
The story of the number is odd. A Belgian astronomer and statistician, Adolphe Quetelet, devised the basis of it between 1830 and 1850 while studying what he called "social physics", and he never intended it as a way to assess anyone's health. The name "body mass index" arrived in 1972, coined by Ancel Keys and colleagues. Keys judged it appropriate for population studies and inappropriate for individual evaluation.
1830–1850
Quetelet devises the basis of the index
1972
Ancel Keys and colleagues coin 'body mass index'
1998
US overweight line lowered to 25
2010
Study finds a better risk measure
That caveat is exactly where the arguments live. Lean male athletes often have a BMI that is misleadingly high relative to their . A tape measure can do better in some cases: a 2010 study that followed 11,000 people for up to eight years found that was a better measure of the risk of heart attack, stroke or death. And the lines can move. In 1998 the US National Institutes of Health lowered its overweight cut-off to a BMI of 25 to match WHO guidelines, which redefined about 25 million Americans as overweight.
The line matters in medicine too. The FDA and the European Medicines Agency have approved weight-loss medications for adults with a BMI of at least 30, or at least 27 with a weight-related . So an old statistician's ratio now sits inside drug rules, which is why the argument about it keeps coming up.
Quiz me
0/3
Recap
BMI is a rough ratio that works for describing groups but misses muscle, fat distribution and differences between populations.
💡 A trick to remember it · BMI is a bathroom-scale shadow: it shows how big, never what the body is made of.
Surprising fact · A 1998 change of one cut-off redefined about 25 million Americans as overweight without any of their bodies changing.
Sources (3)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.