Anyone who's had a physical or participated in high school gym class is probably familiar with BMI. Those three little letters, which stand for body mass index, would have you believe that a simple math problem is all you need to calculate and rate your overall health.
If this sounds simplistic, it is! Many experts -- including the person who invented it -- say BMI fails to fully capture the state of a person's health. Yet it persists as a go-to indicator of well-being, even though there are more effective, relevant stats that can paint a more complete picture.
What is BMI, exactly?
Body mass index measures a person's height in relation to their weight. It's calculated by taking your weight in kilograms divided by your height in meters squared. If you'd prefer to keep the math stateside, your BMI is your weight in pounds multiplied by 703, all divided by your height in inches squared. Whatever number you come out with is your BMI, and if it falls between 18.5 and 24.9, you're normal. Congrats! Any number below means you're underweight, and above the range means you’re overweight, possibly even obese.
What weighs more: a pound of muscle or a pound of feathers?
But just because you fall into a range of BMI -- underweight, normal, overweight, or obese -- doesn't mean that’s what you indeed are. BMI doesn’t recognize that you could be lean because of unhealthy behaviors like crazy diets or illegal substances, or even that you might be super tall. At the same time, BMI doesn't differentiate between the muscle builder in the gym and the couch potato who have the same height and weight. Ultimately, the BMI labels millions of Americans as obese or underweight, that indeed may not be at all.
"BMI is a great overall measure of a population’s obesity status but it should never be used for an individual," said Dr. Robert Davidson, director of the Nutrition and Human Performance program at Logan University. "This is because it is overly simplistic as a weight-to-height ratio and has no predictive power to predict the body composition of an individual due to the many factors that contribute to body composition, none of which are caused by either height or weight."
Why does BMI exist if it's such an ineffective stat?
Even the guy who developed BMI in the first place raised his hand with a caveat on his healthy math problem. Back in the 19th century, Belgian statistician Lambert Adolphe Jacques Quetelet (yikes!) created the Quetelet Index of Obesity, which is the current BMI formula but with a way cooler name. Since Quetelet was a mathematician and not a doctor, he said at the time that the index shouldn't be used to indicate overall health. Nobody listened, apparently. BMI stuck.
Now, doctors have used it to compare patients, while insurance companies have used it to evaluate members.
It turns out that Quetelet may have been right all along, which, again, makes you wonder why no one listened to him. Around 54 million Americans who are categorized as obese or overweight according to BMI are actually neither, and BMI isn't the best predictor of cardiovascular disease.