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It wasn’t so long ago that fat people were considered healthy. Doctors were far more worried about underweight Americans, many of whom were too poor to afford enough calories. But as farms industrialized and food became cheaper, the tables began to turn. Shortly after World War II, it became clear that eating too much food led to just as many problems as not eating enough. Insurance companies noticed that their fattest policyholders were significantly more likely to die early than those of average weight. They searched for a way to measure excess fat and hit upon a simple formula developed in 1832 by a Belgian statistician, mathematician, and astronomer named Adolphe Quetelet: Simply divide a person’s weight by the square of his height. This formula, known as body mass index (BMI), spread from insurers to health researchers and finally, in the 1980s, entered the clinical realm.
Today, BMI is still one of the most commonly used measures of health. When primary-care physicians record your height and weight, as required by law, the electronic record they see displays your BMI. Insurance companies use it to set premiums: A 2013 report from the provider eHealthInsurance found that people with BMIs in the “obese” category paid 22 percent more, on average, than those in the “normal” range. Doctors typically use BMI to advise their patients: If you’re below 18.5, you’re underweight; 18.5-24.9 is normal; 25-29.9 is overweight; and 30-plus is obese.
There’s just one problem: A higher BMI doesn’t necessarily mean you’re less healthy. In fact, patients with heart disease and metabolic disorders whose BMIs classify them as overweight or mildly obese survive longer than their normal and underweight peers. A 2013 meta-analysis by the National Center for Health Statistics looked at 97 studies covering nearly 3 million people and concluded that those with overweight BMIs were 6 percent less likely to die in a given year than those in the normal range. These results were even more pronounced for middle-aged and elderly people. This is known as the obesity paradox. “The World Health Organization calls BMIs of 25 to 29.9 overweight,” says Paul McAuley, an exercise researcher at Winston-Salem State University. “That is actually what is healthiest for middle-aged Americans.”