BMI, or body mass index, is a calculation based on the height and weight of a person that is supposed to be a general measure of overall health and body composition. For many years this was the standard to determine whether a person was within an acceptable weight range for their height to deem them “healthy”, overweight or obese.

Over the past few years, there has been many speaking out against the use of this measure within the health and fitness industry. This is because the calculation does not take into account any factors other than height and weight. It is an inaccurate measure of body fat content and does not take into account factors like muscle mass, bone density, overall body composition, racial and differences between sexes.

Traditionally BMI has been used as a tool to predict the risk of disease and mortality by measuring a correlation between obesity and things like heart disease, stroke, heart failure and diabetes. However, there have been a number of studies that have indicated that some people considered “obese” on the BMI scale, in fact, have a lower cardiovascular risk and an improved metabolic profile compared to some individuals with a “normal BMI” who are more metabolically unhealthy and at a higher risk for disease than their obese counterparts.

It doesn’t take a scientist to understand why BMI might not give an accurate or useful body composition profile. Consider an athlete, someone who competes as either a bodybuilder or something like an MMA fighter. These athletes are not only muscular, but typically quite lean. Not to mention their fitness levels are often well above the average Joe. Someone with this body type who is 5’10” and 190 lbs would have a BMI of 27.3. This BMI would put this individual in the category of overweight, yet no one looking at this athlete would think that could be possible. The calculation wrongly assumes lower muscle mass and high relative fat content.

BMI also does not consider localized body fat, which is potentially more dangerous than overall fat as it relates to body composition. Abdominal fat has been found to have severe health risks including cardiovascular disease, insulin resistance and higher mortality rates. Additionally, thinner people might also have an excess of visceral fat, which is linked to higher risk of disease when compared to lean muscle mass.

Unfortunately, despite its obvious flaws in accurately capturing the overall fat content and health of an individual, BMI is sort of all we have. There are more efficient and accurate methods to measure body fat percentage, but these come with a hefty price tag that is simply not practical for most people. Measuring height and weight is convenient and practical during a quick doctor’s visit. Until there is a better method available, BMI will continue to be used by healthcare professionals, but it should come with a disclaimer that the results should be taken with a grain of salt.

Kristen Hansen, BA, CSEP-CPT, PFT-NAIT, NASM-CES, FRCms
SVPT Fitness & Athletics