You may know your body mass index, or BMI, without being aware of the checkered history behind it. In the last decade or so, critics have picked BMI apart for its racist past. 

In the 19th century, a mathematician used data from white Western Europeans to develop BMI, a simple division of a person’s height by the square of their weight. While he created the index for statistical use in populations, it’s now widely used by healthcare providers to screen individual patients for weight-related health problems. As for what BMI actually measures — it’s just a proxy for body fat. BMI doesn’t account for the differences between fat and muscle mass in a person’s body composition, which can limit its usefulness. In one 2016 study, the index incorrectly classified 75 million Americans as overweight or obese.

So, when UK researchers gathered data from 1.5 million people across numerous racial and ethnic groups to investigate BMI as a predictor of obesity-related complications like type 2 diabetes, they expected to find flaws. But they didn’t expect their research to reveal such stark differences in the link between BMI and diabetes risk for white and non-white populations, particularly South Asians, said Dr. Rishi Caleyachetty, lead author on the study published in The Lancet Diabetes and Endocrinology earlier this month.

Today, the WHO says anyone with a BMI of 30 or higher is obese and at a risk of developing certain health issues, including type 2 diabetes, as a result. The study suggests this rule bears out for some — but not all — groups of people. For South Asian populations, heightened diabetes risk started with a BMI of 23.9, while the number for Black participants checked out at 28. 

For years, clinicians have been using the tip of the BMI index to predict people’s risk of developing obesity-related complications, when in reality, some populations face those risks within the “normal” or “overweight” weight ranges. That means there’s likely a large swath of Black, South Asian and other people who are vulnerable to type 2 diabetes, but have slipped through cracks in our current diagnostic guidelines.  

Studies like this expose the dangers of creating global health standards based on data from solely white populations  — and spur questions about the future of BMI use in the doctor’s office. Although many experts see BMI as a helpful, easy way to measure the health of populations, others, including Caleyachetty, think new BMI frameworks should be created for clinicians to use when they’re assessing a patient’s risk of developing obesity-related complications.

“There are a lot of people from these communities that wouldn’t have had advice, or referrals for services, or even a blood test for diagnosing type 2 diabetes, because we’re using outdated BMI thresholds,” says Caleyachetty. 

Caleyachetty and his colleagues hope to convince UK health agencies to change national standards, so that people from Black, South Asian and other immigrant populations are considered for various diagnostic tests earlier, and aren’t excluded from weight management programs that usually require a BMI of 30 or higher for admission. 

Dr. Sandra Albrecht, a US social epidemiologist and assistant professor at Columbia University, warns that although changing BMI thresholds might sound great in theory, race is a social construct — and she doesn’t want people to mistakenly believe it can be a “genetic marker for higher risk.” 

She explained other factors, like patients’ activity levels and diets, likely contributed to some of the numbers researchers came across in this study.

BMI is a small puzzle piece in the larger scheme of a patient’s health. When treating new patients, Dr. Fatima Stanford, an acclaimed obesity medicine physician scientist and educator at Massachusetts General Hospital and Harvard Medical School, says she looks at social factors, like physical activity, among other tangible factors, like waist circumference. In some studies, waist circumference has shown to be a slightly more accurate marker than BMI in predicting obesity-related chronic illnesses like diabetes. 

Stanford does see some value in tweaking BMI thresholds for different populations. But she also understands why doctors and patients still use it as a measure — it’s easy to calculate, cheap and tangible. Changing BMI thresholds is also a step towards undoing “the structural systemic racism of how we conduct research and take care of patients,” Stanford says. 

In fact, in 2019, Stanford proposed her own new standards for the US. She also defined lower BMI cutoffs for populations of color, but based her conclusions on the connection to a broader range of metabolic diseases than just type 2 diabetes.

Overall, while BMI probably isn’t going anywhere right now, we’ll likely see a shift away from it in the future. 

“I think bringing to light any nuances of ways we can advance the outcomes for patients from diverse backgrounds will only be beneficial,” Stanford says. “Historically, there are deficits in the way we do things that need to change.” 

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