BMI Misclassifies Both the Healthy and the Overweight

By John Henry Dreyfuss, MDalert.com.

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  • Nearly 75 million American adults are misclassified as cardiometabolically unhealthy or cardiometabolically healthy when body mass index (BMI) categories are used as the main indicator of health.
  • Using blood pressure, triglyceride, cholesterol, glucose, insulin resistance, and C-reactive protein data is more accurate than BMI.
  • BMI misclassified nearly half of overweight individuals, 29% of obese individuals, and even 16% of individuals with type II/III were obesity as metabolically healthy.
  • Conversely, nearly 30% of normal-weight individuals were misclassified by BMI as unhealthy.

Body mass index (BMI) (Figure 1) is one of the most widely used indicators of cardiometabolic health in the United States. Many employers use BMI as a basis for discounting employees’ health insurance rates, or to penalize employees who are classified as overweight or obese based on BMI. The U.S. Equal Employment Opportunity Commission has proposed rules allowing employers to penalize employees by as much as 30% of health insurance costs if they fail to meet a specified BMI.

 

Figure 1. The widely used BMI chart has been shown
to be an inaccurate indicator of cardiometabolic health.

 

A study published February 4, 2016, in the International Journal of Obesity  suggests that BMI routinely misclassifies healthy individuals as unhealthy, and unhealthy individuals as healthy. Nearly 75 million U.S. adults may be misclassified based on BMI.

The Analysis

The study in the International Journal of Obesity was designed to examine cardiometabolic health misclassifications based on the standard BMI categories. The trial included 40,420 individuals aged ≥18 years who had participated in the 2005-2012 National Health and Nutrition Examination Survey (NHANES).

The researchers used data on blood pressure, triglyceride, cholesterol, glucose, insulin resistance, and C-reactive protein to establish a comparison group against which to judge the accuracy of BMI. Population frequencies and percentages of metabolically healthy versus unhealthy individuals were also stratified based on BMI.

The results of the comparison revealed that nearly half of overweight individuals, 29% of obese individuals (Figure 2), and 16% of participants with type II/III obesity were classified as metabolically healthy based on BMI. Perhaps more importantly, more than 30% of normal weight individuals were misclassified as cardiometabolically unhealthy based on BMI. The researchers also reported a significant gender x BMI interaction (F[4,64]=3.812, P=0.008) which was associated with misclassification.

 

 

Figure 2. The medical complications associated with obesity.

 

Using BMI categories as the main indicator of health, the authors estimated that nearly 75 million U.S. adults would be misclassified as cardiometabolically unhealthy or cardiometabolically healthy. “Policymakers should consider the unintended consequences of relying solely on BMI, and researchers should seek to improve diagnostic tools related to weight and cardiometabolic health,” the authors advised.

Why Might BMI be Inaccurate?

According to an article on NPR.org, there are a number of reasons that could explain the inaccuracy of BMI for gauging the health of an individual. Here are a few examples from the NPR article:

  • The person who conceived of BMI, a 19th Century Belgian mathematician named Lambert Adolphe Jacques Quetelet was a mathematician and not a physician. He said explicitly that BMI was not an accurate indicator of adiposity.
  • There is no physiologic reason to square a person's height in figuring the percent body fat. Also, BMI ignores waist size, which is a clearer indicator of obesity and diabetes risk in many people.
  • BMI does not allow for the relative proportions of bone, muscle, and fat in the body. For instance, because bone is denser than muscle and fat, a person with dense bones, high muscle tone, and a low percentage of body fat will likely still have a high BMI. As a result, many athletes and fit, health-conscious people who exercise regularly are often misclassified as overweight or obese. A high BMI does not mean that an individual is overweight, let alone obese. It could mean the person is fit and healthy, and muscular, with very little fat.
  • The BMI formula is generally more accurate in people who lead fairly sedentary lives. The formula assumes low muscle mass and high relative fat content. It applies moderately well when applied to this sector of the population, but is frequently inaccurate in people who are lean, fit, and healthy.
  • BMI calculations assume that there are distinct categories of underweight, ideal weight, overweight, and obese that are separated precisely by a decimal place. This is simply not the case in the real world.

Examples of BMI Inaccuracies

In an interview with WebMD, Michael Roizen, MD, offered the following example of BMI inaccurately classifying an athlete as overweight: During his prime years, the basketball player Michael Jordan had a BMI that ranged from 27 to 29. According to BMI Mr. Jordan would have been classified as overweight, yet his waist size was <30 inches. Dr. Roizen is Chair of the Wellness Institute at Cleveland Clinic, Cleveland, Ohio, and is a member of the Advisory Board for MDalert.com.

“Fat around your waist [Figure 3, Figure 4] is more biologically active and can do more damage to your body than weight around your hips,” Dr. Roizen noted. “The data show that waist circumference is more reliable and more closely correlated with diseases associated with obesity.”

 

Figure 3. Central obesity is more closely associated with a variety of health risks than is BMI.

 

 

Figure 4. Central obesity and high waist circumference
are accurate predictors of cardiometabolic health risks.

 An article on MedicalNewsToday.com offers additional examples of problems with BMI as a measure of cardiometabolic health:

  • BMI exaggerates thinness in short people and fatness in tall people.
  • Waist size is closely tied to diabetes risk, regardless of BMI.
  • Waist-to-height ratio is a better determinant of cardiometabolic health than is BMI.
  • BMI does not take into account a person's body fat content versus muscle (lean tissue) content.

Despite accumulating evidence of its inaccuracy, physicians and governmental agencies continue to promote BMI as a physiologically useful measure:


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