How BMI was created

It's Sunday, August sixteenth. My BMI has always told me I'm overweight. And look, I can stand to lose a few pounds, but I've always thought it was kind of insane that you can measure someone's health based on just their height and weight. No bone density, no muscle mass, no body composition, just two numbers and a formula that spits out a verdict. I've questioned it for as long as I can remember, but I never actually looked into where the formula came from until this week, and what I found validated every instinct I've had about it. The formula was invented in the 1830s by a Belgian astronomer who had no medical training, was studying population statistics rather than individual health, used data exclusively from white European men, and explicitly said it should never be applied to individuals. That formula sat mostly unused for over a century until American insurance companies adopted it because it was cheap and easy to calculate. And in 1998, a single decision lowered the "overweight" threshold and made 29 million Americans overweight overnight without a single one of them gaining a pound. Adolphe Quetelet (keh-TLAY) was a Belgian mathematician, astronomer, and statistician who was fascinated by the idea of "l'homme moyen," the average man. In the 1830s, he was trying to describe the statistical distribution of physical characteristics across populations, essentially looking for patterns in how human bodies are built at scale. He noticed that in well-nourished adults, weight tended to increase in proportion to the square of height, and he published that observation as a formula: weight in kilograms divided by height in meters squared. He called it the Quetelet Index. It was a statistical tool for studying groups of people, not a diagnostic tool for evaluating any one person's health, and Quetelet said so explicitly.

His data came entirely from white European men. No women, no children, no one from Asia, Africa, or the Americas. The "average man" he was defining was average only within a narrow slice of humanity, and the formula he built reflected the body proportions of that specific population. For over a century, the Quetelet Index remained a niche academic tool that almost nobody in medicine used.

In the early 1900s, American life insurance companies started noticing that their heavier policyholders were dying sooner and filing more claims. A statistician at Metropolitan Life Insurance named Louis Dublin, who was also not a doctor, developed height-weight tables that insurers used to set premiums. These tables were essentially the Quetelet Index applied commercially: a quick, cheap way to sort people into risk categories without doing any actual medical evaluation. In 1972, an American physiologist named Ancel Keys formally renamed the Quetelet Index the "Body Mass Index" and recommended it as the best simple proxy for body fat in research. But Keys also warned, echoing Quetelet 140 years earlier, that it was designed for population studies and should not be used to diagnose individuals. Medicine adopted it anyway. In 1985, the NIH (National Institutes of Health) formally adopted BMI as the standard for defining obesity in the United States. The thresholds they set were a BMI of 27.8 for men and 27.3 for women to qualify as "overweight." Those numbers stayed in place for 13 years.

In 1998, the NIH revised the guidelines and lowered the overweight cutoff to 25 for all adults, bringing the US in line with World Health Organization standards. The change was based on a 1996 report by the International Obesity Task Force, and this is the part worth knowing: the primary funders of that report were Hoffmann-La Roche and Abbott Laboratories, two pharmaceutical companies that manufacture weight-loss drugs. The organizations that funded the research recommending a lower threshold had a direct financial interest in more people being classified as overweight, because a larger "overweight" population means a larger market for their products.

The effect was immediate. Roughly 29 million Americans who went to sleep on a Monday at a "normal" weight woke up on Tuesday classified as "overweight" by federal health guidelines. Nothing about their bodies changed. Nothing about their health changed. A committee moved a number, and millions of people crossed a threshold that didn't exist the day before.

Whether the lower threshold is medically justified is a legitimate debate, and many researchers believe that health risks do increase above a BMI of 25. But the process by which that threshold was set, funded by companies that profit from the outcome, applied universally to a population it was never designed to evaluate individually, using a formula built on data from a single demographic almost 200 years ago, raises questions about how much weight that number should carry in your doctor's office. The most basic problem with BMI is that it can't distinguish between muscle and fat. A pound of muscle and a pound of fat weigh the same but have completely different health implications, and BMI treats them identically. A professional athlete with 8 percent body fat and heavy muscle mass can register as "overweight" or even "obese" by BMI standards, while a sedentary person with low muscle mass and high visceral fat can register as "normal." The number tells you something about weight relative to height and almost nothing about body composition, metabolic health, or actual disease risk.

This connects directly to our diabetes episode from a few weeks ago. We covered how South Asians develop Type 2 diabetes at a BMI of 22, which BMI classifies as perfectly "normal," because South Asian bodies tend to carry more visceral fat at lower overall weights. A South Asian person at BMI 22 can have the same metabolic risk profile as a European person at BMI 30, but the BMI chart says one is healthy and the other is obese. The formula was built on European bodies, and when it's applied to populations with different body compositions, it systematically misses risk in some groups and overstates it in others.

Doctors still use BMI because it's free, it takes five seconds to calculate, and there isn't a universally adopted replacement that's equally convenient. Waist-to-hip ratio, body fat percentage, and metabolic blood panels are all better indicators of health, but they require more time, more equipment, and more cost. BMI persists not because it's the best tool but because it's the easiest one, which is the same reason the QWERTY keyboard persists and the same reason we still change our clocks for daylight saving time: the system we have is good enough to avoid the cost of switching to something better, even when something better exists. The number your doctor uses to tell you whether you're a healthy weight was invented by an astronomer who explicitly said not to use it on individuals, adopted by insurance companies because it was cheap, lowered by a committee funded by weight-loss drug manufacturers, and built on data that excluded most of the world's population. I'm not saying BMI is useless, but I am saying it deserves a lot less authority than most of us give it.

Stay informed, stay curious, and we'll see you Monday.