It's no secret that Body Mass Index (BMI) is a somewhat controversial measure of health. Calculated using only your height and weight, it classifies us as underweight, normal weight, overweight, and obese. But it's not always an accurate prediction of how healthy you are---since BMI doesn't touch on cardiovascular fitness or body composition, it's not unheard of for a professional athlete (especially one with a ton of muscle weight) or someone who can crush a triathlon to technically qualify as "overweight" or even "obese." (This Eighth Grader Perfectly Described How Outdated BMI Is for Measuring Health.)
But what your BMI actually means is changing, according to a new study published in the Journal of The American Medical Association. Researchers from the University of Copenhagen in Denmark analyzed the heights, weights, and health outcomes—AKA mortality rates—of over 100,000 individuals using study data from the 1970s to 2013. And they found a few confusing things.
Firstly, they found that obesity isn't as dangerous as it used to be. The risk of premature death as well as the overall mortality rate for obese individuals (with a BMI of 30 or above) is no longer higher than it is for normal weight (those with a BMI between 18.5 and 25) individuals. (Still, There Is a Serious Global Obesity Problem.)
Secondly, they found the BMI that is currently associated with the lowest risk of death has increased from 23.7 in the 1970s to 27, which falls in the overweight category. What gives?
According to the study authors, this may have more to do with medical advances than individual waistlines—at least when it comes to addressing the first finding. As a society, we've gotten a lot better at treating the dangerous diseases and health complications that come with obesity and lowered the risk of mortality associated with those complications. (In fact, there's a New Pee Test Can Predict Your Risk for Obesity.)
And as far as the second finding, take that with a grain of salt before you try to put on a few pounds to up your BMI to 27, says Niket Sonpal, M.D., assistant clinical professor at Touro College of Medicine in New York City. Being in the obese category is never healthy, he says.
"Overall, we should shift away from crude numbers as the sole measure of health but take into account dynamic measures like exercise tolerance, total body fat percentage, and other bio markers collectively to assess health," says Sonpal. "BMI is just one component of the overall global health index." (Are You Skinny Fat?)