American Council on Exercise by American Council on Exercise

The American Medical Association (AMA) recently adopted a new policy encouraging physicians not to rely on body mass index (BMI) as the sole means of assessing an individual’s weight status. Veterans of the fitness industry are likely aware of the decades-long debate about the use of BMI, with criticisms ranging from the fact that the overweight and obese categories were established using an entirely European male study population to the fact that it does not take into account the person’s ratio of body fat versus lean tissue, including muscle and bone mass.

While Cedric X. Bryant, PhD, ACE President and Chief Science Officer, agrees with many of these criticisms, he says, “The problem with BMI is really more fundamental, in that it is a flawed and crude estimate of one’s weight status. And I’m using the term ‘weight status’ with intention, as opposed to body-fat level or obesity.”

This raises another commonly cited problem with BMI: It is often misidentified as a body-composition assessment, when it is simply a ratio of a person’s height and weight.

BMI = Height (m)/Weight2 (kg)

A lot of this probably feels like old news to experienced health coaches and exercise professionals who have seen these issues debated countless times. The scientific community has long understood the limitations of BMI, but BMI has been able to withstand these criticisms because it’s so widely used and provides a good rough estimate at the population health level.

The truth is, BMI is probably not going away any time soon. It’s free, efficient and provides a means of assessing large groups of people. It’s long been used to track disease risk factors in large populations—from diabetes, stroke and heart failure to the risk of hospitalization and death from COVID-19—and is firmly entrenched in the research in these areas and clinical practice.

So, why has this suddenly become a hot topic, from The New York Times to countless Twitter feeds?

This is the first time that an association like the AMA has come out and said that, from a clinical perspective, physicians need to look at BMI alongside other measures, as it’s not the most reliable indicator of health status on its own (more on those other measures below).

“That’s a huge step forward,” says Dr. Bryant.

Another issue with the use of BMI centers on the classification categories, which are underweight, normal, overweight, obesity and extreme obesity. It is true that as a person’s BMI goes up, they tend to show a higher risk of certain health conditions. For example, if a person’s BMI increases from 29.0 kg/m2 to 30.5 kg/m2 [the equivalent of a 5’10” (1.8 m) person going from 202 pounds (92 kg) to 212 pounds (96 kg)], there is a reasonable probability that their health profile is going to be less favorable. But, there is nothing magical about having crossed over from the overweight to the obesity category, which begins at 30.0 kg/m2, as this individual would have done.

Imagine a client coming to you in tears because they were told by their doctor that they have obesity because their BMI crossed a somewhat arbitrary threshold established by a mathematician more than 50 years ago. The emotional and psychological toll that may have on an individual because of the weight stigma and lack of size inclusivity in modern society was one of the concerns that drove the AMA to discourage physicians from telling patients they have overweight or obesity based on this single value.

Lost in that doctor–patient conversation is any discussion of the individual’s health habits, including what they eat, how they move, how they manage stress and how well they sleep, all of which have a tremendous impact on their overall health. Many people who are categorized as having overweight or obesity may show very healthy profiles in terms of having normal blood sugars or normal blood pressure and may not have the conditions frequently associated with overweight and obesity.

All of this begs the question…

Should health coaches and exercise professionals be using BMI with their clients? And if so, how?

The first consideration in an exercise facility is whether measuring weight is even necessary or appropriate for the individual client. If a client does not have weight-related goals or expresses any reservations about this initial assessment, you should instead focus on baseline measurements that better align with their objectives and personal values.

If you and the client deem weight to be an important consideration, determining a person’s BMI should be only the first step in the assessment of their weight status. Once you have that number, the follow-up question is, does this person have a weight status that merits further investigation?

If the answer is yes, the next step is to evaluate their body-fat distribution (e.g., using waist circumference or waist-to-height ratio), which is a more reliable marker of health risk than BMI. Abdominal fat is the more metabolically concerning fat, as it is associated with metabolic syndrome and the associated risk factors. Again, communicate with your client before taking any circumference measures, as some may be hesitant to have those numbers collected and documented.

Then, discuss the client’s personal and family history and ask about any history of chronic disorders that have a strong association with an unhealthy body weight. This conversation can reveal vital information about a person’s health risk associated with their weight.

“Another way to determine whether a person’s weight status is problematic is by looking at their functional capabilities,” explains Dr. Bryant. Does their weight prevent them from doing those things that they need and want to do?

Final Thoughts

The takeaway message, for physicians working in clinical settings and health coaches and exercise professionals working in fitness settings, is that you ought to take a more holistic view of weight status and health in order to provide people with the most helpful guidance.

An important consideration that is sometimes lost in the discussion of BMI, body composition and body-fat distribution centers on the client's history with body-image concerns or eating disorders. The last thing you want to do as a health coach or exercise professional is introduce assessments or measurements that may be triggers for that individual. Never lose sight of the need for rapport and empathy during these conversations and let the client take the lead when deciding which assessments are most important to them and most applicable to their goals.

For health coaches and exercise professionals, BMI can serve an initial, non-invasive way to get some indication that weight status may be an issue. From there, you can move onto the additional assessments and conversations that make your role so valuable and impactful for your clients. In combination, these indicators will give you a more complete picture of a client’s risk level associated with their weight status.