Gradually, though, the popularity of BMI spread from epidemiologists who used it for studies of population health to doctors who wanted a quick way to measure body fat in individual patients. By 1985, the NIH started defining obesity according to body mass index, on the theory that official cutoffs could be used by doctors to warn patients who were at especially high risk for obesity-related illness. At first, the thresholds were established at the 85th percentile of BMI for each sex: 27.8 for men and 27.3 for women. (Those numbers now represent something more like the 50th percentile for Americans.) Then, in 1998, the NIH changed the rules: They consolidated the threshold for men and women, even though the relationship between BMI and body fat is different for each sex, and added another category, "overweight." The new cutoffs—25 for overweight, 30 for obesity—were nice, round numbers that could be easily remembered by doctors and patients.
Keys had never intended for the BMI to be used in this way. His original paper warned against using the body mass index for individual diagnoses, since the equation ignores variables like a patient's gender or age, which affect how BMI relates to health. It's one thing to estimate the average percent body fat for large groups with diverse builds, Keys argued, but quite another to slap a number and label on someone without regard for these factors.
Now Keys' misgivings are gaining traction across the world of medicine: BMI simply doesn't work when it comes to individual measurements. Whether that's a problem worth worrying about is another question. Some researchers say BMI's inaccuracies in individual measurements result in little actual harm, since an attentive doctor can spot outliers and adjust her diagnosis accordingly. But this begs the question: If a doctor's eye is better than BMI at determining a patient's healthy weight, then why use BMI for individuals at all?
No matter how attentive they might be, health professionals have increasingly used body mass index to justify lifestyle recommendations for their patients. And online BMI calculators—there's even one hosted by the NIH—invite people to diagnose themselves without any medical supervision whatsoever. Faulty readings could promote a negative self-image among healthy people and lead them to pursue unnecessary diets. Or the opposite problem: People with a little too much body fat might be lulled into a false sense of complacency by a misleading BMI.
A recent critique (PDF) of the body mass index in the journal Circulation suggests that BMI's imprecision and publicity-friendly cutoffs may distort even the large epidemiological studies. (There's no definitive count of how many people are misclassified by BMI, but several studies have suggested that the error rate is significant for people of certain ages and ethnicities.) It's impossible to know which studies have been affected and in what direction they might have been skewed.
Our continuing reliance on BMI is especially grating given there's a very reasonable alternative. It turns out that the circumference around a person's waist provides a muchmore accurate reading of his or her abdominal fat and risk for disease than BMI. And wrapping a tape measure around your gut is no more expensive than hopping on a scale and standing in front of a ruler. That's why the American Society for Nutrition, the American Diabetes Association, and other prominent medical groups have lately promoted waist circumference measurements as a supplement to, or replacement for, the body mass index.
Yet few doctors have made the switch. The waist measurements require slightly more time and training than it takes to record a BMI reading, and they don't come with any official cutoffs that can be used to make easy assessments. The sensitivity of doctors to these slight inconveniences signals just how difficult it will be to unseat Quetelet's equation. The body mass index is cheap and easy, and it has the incumbent advantage. In short, BMI is here to stay—despite, but also because of, its flaws.