Body mass index (BMI) has been used for years as a measure of healthy weight. However, a recent joint study by researchers at the University of California, Santa Barbara and UCLA found the measure to be flawed.
According to the findings published in the International Journal of Obesity, as many as 54 million Americans categorized as obese or overweight by the index may really be perfectly healthy. Jeffrey Hunger, the co-author of the paper, says the index does not take into account all the factors that make someone healthy or unhealthy.
“In the overweight BMI category, 47 percent are perfectly healthy. So to be using BMI as a health proxy, particularly for everyone within that category, is simply incorrect. Our study should be the final nail in the coffin for BMI.”
When you look at other “underlying clinical indicators,” some people who have a healthy weight as indicated by the BMI may actually be unhealthy. When you add these people, the number misclassified by the BMI jumps to 74.9 million.
Hunger says several health indicators were evaluated for the study. Using data from the 2005-2012 National Health and Nutrition Examination Survey, researchers looked at the link between BMI and other health factors like blood pressure, blood sugar, and cholesterol.
The results revealed that approximately two million people who had a BMI of 35 or higher, tagging them as “very obese,” were actually healthy. While, more than 30 percent of those considered “normal weight” by the index, roughly 20.7 million people were really unhealthy when other factors were assessed.
To calculate BMI, a person’s weight in kilograms is divided by the square of the person’s height in meters. A “healthy” BMI is between 18.5 and 24.9, overweight BMI is anywhere from 25 to 29.9 and obese is 30 or higher, as defined by the Centers for Disease Control and Prevention.
The calculation was designed as another way to evaluate fitness than just using weight alone. However, this study shines a light on how the BMI has been used inaccurately to evaluate a person’s overall health in relation to a healthy weight.
Under rules recently proposed by the U.S. Equal Employment Opportunity Commission, employers would be authorized to charge employees up to 30 percent more for their health insurance costs if 24 health standards, including BMI, were not met. Employees can potentially be asked to pay higher health costs based on a high BMI number.
“The public is used to hearing ‘obesity,’ and they mistakenly see it as a death sentence,” said co-author A. Janet Tomiyama, a psychologist at UCLA. “But obesity is just a number based on BMI, and we think BMI is just a really crude and terrible indicator of someone’s health.”
Past research has also shown problems using BMI as a measure of healthy weight. In 2010, another study found that waist size was a more accurate predictor than BMI of a child’s future risk for heart disease. Additionally, a 2014 study published in the journal Pediatric Obesity revealed that one-quarter of kids considered obese based on body fat content were not identified as obese using their BMI number.
With the obesity rate in the U.S. skyrocketing, the study authors emphasized that the BMI alone should not be used as a forecaster of future health. People who have a healthy weight as indicated by the BMI could still be at risk for disease.
“Policymakers should consider the unintended consequences of relying solely on BMI,” the authors wrote in the study, “and researchers should seek to improve diagnostic tools related to weight and cardiometabolic health.”
The study suggests people should focus on a well-balanced diet and regular exercise rather than stressing about a healthy weight as calculated by the body mass index.
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