BMI: The Limitation in Assessing a Healthy Weight

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Body Mass Index (BMI) is a measurement that is calculated using your weight and height squared (BMI = kg/m2). This number is then assessed by placing it into one of four categories:

underweight  <18.5kg/m2
normal weight 18.5-24.9kg/m2
overweight 25.5-29.9kg/m2
obese >30.0kg/m2

Sounds easy right?

As much as calculating your BMI can be easy, it doesn’t tell you the whole picture. The BMI calculation does not take into consideration the composition of your body weight.

For example, individuals who may be very muscular, like bodybuilders, will have a calculated BMI that is extremely high. What BMI fails to consider is that this individual’s weight is mostly made up of muscle, not fat.  Thus, this individual may be inaccurately classified as being overweight or obese when they are in fact very healthy and active.

Take Dwayne “The Rock” Johnson who stands at 1.96m and weighs about 118kg. When you calculate his BMI it equals 30.73kg/m2. With this BMI, The Rock would be placed in the “obese” category despite being a healthy individual.

In practice, the BMI is used as a tool to stratify health risks – for example, having a very high or a very low BMI can be associated with undesirable health risks. However, this is not always the case. Healthy individuals can present at any size.

Often health professionals will take multiple measurements in order to get a more accurate measurement, like hip-to-waist ratio or skin fold measurements. As obesity is recognized as a chronic health condition, health professionals are now using obesity-related health conditions such as diabetes, high blood pressure, heart disease and mobility as factors in assessing weight and health. By collecting more data, health professionals can make an appropriate interpretation of a person’s body composition and health status.

Blog post contributed by Alyssa Ramuscak, Nutrition & Dietetics Student, University of Western Ontario