Body mass index (BMI) is a simple tool for assessing whether an individual falls into a normal weight range for their height. In order to assess your BMI, you take your weight in kilograms and divide it by the square of your height in meters. Those with a BMI of 25 to 29.9 score are considered overweight, while those above 30 fall into the obese category.
However, simple may not mean best. There has been intense scrutiny around using BMI as an indicator of health over the years, with many pushing back on its usefulness, as well as suggesting that overemphasis on BMI may cause more harm than good.
That now includes the American Medical Association (AMA). Here’s what you need to know about BMI.
On June 14, the AMA announced its recommendation for doctors to no longer use BMI exclusively as a way to assess wellness.
The decision to move away from relying on the controversial height-to-weight ratio stems partially from the fact that the original data collected was based only on previous generations of non-Hispanic white populations.
In the statement, the AMA said that it "recognizes that relative body shape and composition differences across race/ethnic groups, sexes, genders, and age-span is essential to consider when applying BMI as a measure of adiposity and that BMI should not be used as a sole criterion to deny appropriate insurance reimbursement.”
Instead, the AMA suggests that BMI should be used in conjunction with other tools to measure obesity, such as “measurements of visceral fat, body adiposity index, body composition, relative fat mass, waist circumference and genetic/metabolic factors.”
Why there’s debate
On an individual level, BMI has several drawbacks. But it may be a good tool to track rising or falling obesity rates in a population.
One reason why people like BMI is its simplicity. However, it’s the simplicity of this formula that also makes it problematic when assessing individuals. BMI does not account for muscle vs. fat. For example, an athlete may have a low body fat percentage, but a high BMI, due to muscle.
When it comes to health, it also may matter more where you carry fat, which is something BMI can’t tell you. Abdominal fat, for example, has been linked to high blood pressure, Type 2 diabetes and heart disease.
It’s also possible to have a higher than average BMI and be overall healthy, a condition called metabolically benign obesity. This may be due, at least partially, to how different bodies respond to fat. Different racial and ethnic groups may also carry and hold weight differently.
There’s also an important economic reason why BMI should not be the marker of weight and health. Insurance companies may not cover the costs of treatments for people who do not fall into the appropriate category under BMI. For example, in May 2021, the Washington Post reported that a Black woman suffering from an eating disorder was told her BMI was too high for her insurance to cover treatment. She had to pay $800 out of pocket per month.
The tool may also underestimate the number of people who have obesity. A new study presented the annual meeting of the Endocrine Society suggests that BMI misses cases of obesity when obesity is determined by fat vs. muscle percentage.
A better measure than BMI may already exist
A study published on Sept. 20 analyzed 387,000 adult participants from the United Kingdom and found that waist-to-hip ratio (WHR) — the waist circumference measurement divided by the hip circumference measurement, according to the National Institutes of Health — had “the strongest and most consistent association with mortality irrespective of BMI.” The study authors suggested that the clinical recommendations should “consider focusing on adiposity distribution compared with mass.”
Waist-to-hip ratio can assess where more dangerous fat is better than BMI
Dr. Fatima Cody Stanford, an obesity medicine physician, told VeryWell Health that WHR is likely the better indicator of health. "When we carry adipose (body fat) in our midsection, it increases our risk for cardiometabolic diseases like heart disease and type 2 diabetes," she explained.
BMI is an imperfect tool, but one of the few screening options we have
“Right now, the best tool we have that’s readily available in any clinic is going to be screening based on BMI. We need better things," obesity medicine doctor Carolynn Francavilla Brown told STAT News. "[The AMA policy] is a push in the right direction, but we also have to accept the reality of what we have right now.”
Focus on BMI can harm the relationship between doctors and patients
“Clinicians' focus on BMI can lead to unproductive weight-related conversations that fracture the doctor-patient relationship and may introduce mistrust. This can lead to patients opting not to follow physician advice, even when that advice is not weight-focused, and not pursuing follow-up care due to faltering trust, a vital element of effective doctor-patient relationships. Additionally, misplaced BMI assessments can unnecessarily divert clinician focus to weight, an easy default but often misguided explanation for various signs and symptoms, and can result in missed diagnoses, sometimes with grave consequences.” — S. Bryn Austin, professor of social and behavioral sciences at Harvard T.H. Chan School of Public Health, and Dr. Tracy K. Richmond, assistant professor of pediatrics at Harvard Medical School, MedPage Today
BMI may be more accurate than we assume in finding revealing excess body fat
“Despite its limitations and notorious counter-examples, BMI is highly related to body fat and correctly categorizes people as having excess body fat more than 80 percent of the time. Additional simple measurements such as waist circumference may be even more informative because they provide information about where fat is distributed in the body.” — Kevin D. Hall, senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, Washington Post
Relative fat mass (RFM) index may be better measurement, without the need for a scale
"The team of researchers behind RFM say it's more accurate than BMI, and it can also be worked out with just a tape measure — so you don't need a set of scales to calculate it, as you do with BMI. In the case of RFM, it's the distance around your waist in relation to your height that counts, rather than your weight. The researchers say that gives a better idea of whether someone's body fat is at a healthy level or not. 'We wanted to identify a more reliable, simple and inexpensive method to assess body fat percentage without using sophisticated equipment,’ says lead researcher Orison Woolcott, from the Cedars-Sinai Medical Center in Calif. ‘Our results confirmed the value of our new formula in a large number of subjects. Relative fat mass is a better measure of body fatness than many indices currently used in medicine and science, including the BMI.’” — ScienceAlert
Weight is still something to discuss with your health care provider, even if BMI isn’t used
“The thing I don’t want to happen as a result of this is that people of color and Black people in particular ignore BMI and discussing excess body weight with a health care professional because they misunderstand the intent here. The goal is to personalize how BMI is used in medical decision-making and to move away from blanket generalizations that can lead to stigma and bias.” — Dr. Jamy Ard, professor of epidemiology and prevention at the Wake Forest University School of Medicine, CNN