BMI is calculated by taking your weight in kilograms and dividing it by your height in meters squared. If your result is 18.5 or below, you are considered underweight; 18.5–24.9 means you are at a “healthy weight”; 25.0–29.9 is overweight; and 30.0 and above means you have obesity.
Why BMI Is Less Accurate for Some People
Why? The reasons are complex, but Stanford says, “When we look at BMI charts, we haven’t taken into account differences in body structure.” For instance, on average, people of different ethnicities and races tend to accumulate body fat differently, though individuals within each group can differ greatly from each other. Black people tend to have more subcutaneous fat, which lies just below the surface of the skin, in the hip, buttock and thigh regions, while white people tend to carry more fat around the organs, known as visceral fat, Stanford explains.
Further, Black people may have less body fat and more lean muscle mass than white people at the same BMI.
But remember that BMI is a number used to diagnose obesity alone, and to identify potential risk factors for health conditions. And that “it is a population-based tool. It was not designed to be used for individuals,” says Sylvia Gonsahn-Bollie, MD, a Black obesity medicine doctor in the Washington, DC, metro area and the author of Embrace You: Your Guide to Transforming Weight-Loss Misconceptions into Lifelong Wellness. “It is very challenging at the individual level for a clinician to really say this is your healthy weight, because we’re using all that bias from the population-based BMI and trying to apply it to the individual.”
How We Came to Rely on BMI to Determine Disease Risk
So, how did the medical establishment come to rely so much on standards that cause some people to be misclassified as having an unhealthy weight?
BMI, Stigma, and the BIPOC Community
The problem is worse when you overlay race bias, she adds. “So if you have a Black woman with obesity, you know we have a lot of different potential things that are negative that are coming at you.”
Adjusting BMI and Using Other Measures to Assess Health Risks
Gonsahn-Bollie uses adjusted BMI charts such as Stanford’s as one of several tools to help determine a person’s health status. “Your healthy weight should be individualized to you. We can use these population-based tools as a starting point. So we will look at your specific, adjusted BMI based on your age, race, ethnicity, musculature, and your biological sex. We would also use your waist circumference and your body fat percentage, and then see if you have any metabolic health or obesity-related diseases.”
Reframing How You Consider BMI as an Individual
None of this means people should be overly concerned by what the standards are, says Gonsahn-Bollie. Rather than focusing on getting to a “normal” or so-called “healthy” standard BMI or weight, she likes to work with her patients on achieving a “happy weight,” where you have reached your health goals and are able to maintain that weight happily. “That’s about self-perception and self-acceptance. It’s based on the individual. No one can tell you your happy weight.”
Helenica Yusuf, a Brooklyn, New York–based certified dietitian-nutritionist, says the BMIs of the patients in Brookdale Hospital’s diabetes prevention program are recorded because they must reach a certain threshold to qualify. But she usually discusses the measurement with patients — many of whom are Black — only if they ask about it. Otherwise, the focus is on exercising more, eating a healthier diet, and reducing their average blood sugar levels to help prevent type 2 diabetes. “In the process they lose weight, their BMI drops — not significantly — but it drops enough to say that instead of having obesity, now they’re overweight. That’s all right. They’re healthy. Because you can have someone who looks solid but still is very healthy.”
Gonsahn-Bollie has some advice for Black people, particularly women, who worry that their healthcare provider is making inaccurate or unfair assessments of them or their health habits based on their BMIs. “First, make sure that your clinician is aware of these adjusted BMI charts. Yes, some doctors will argue that they’re still investigational, but I think it speaks to the fact that we need to at least look at them. Second, have them check your waist circumference. Last, remember that you’re not making excuses when you investigate. You’re empowering yourself.”