The problem with using BMI to assess health

In the 1830s, Belgian astronomer Adolphe Quetelet created a calculation to determine the properties of an "average" man. This calculation, which was originally called Quetelet's index, was later renamed BMI by American physiologist Ancel Keys as a way to roughly measure obesity.

BMI and other similar metrics were also used to develop height-weight tables that life insurance companies used to estimate risk. These tables formed "recommendations for the general population going from 'average' to 'ideal' weights," said Katherine Flegal, an epidemiologist.

Currently, the World Health Organization classifies a BMI between 25 and 29 as overweight while a BMI of 30 and over is considered obese. However, a person's BMI may not accurately reflect a person's weight or health.

According to Tayag, BMI does not account for where fat is distributed on a person's body. For example, visceral fat, which is fat that builds up around organs and tissues, has been linked to serious health problems, but fat under the skin is typically less harmful. Muscularity is also not considered, meaning that some athletes could be considered overweight.

There are people with high BMIs who are perfectly healthy, and "there are people with normal BMIs that are actually sick because they have too much body fat," said Angela Fitch, an assistant professor at Harvard Medical School and the president of the Obesity Medicine Association.

Although BMI can be a helpful measure when it is considered alongside other health factors, "BMI becomes troubling when it is all that doctors see," Tayag writes.

"The default is to hyper-focus on the weight number, and I just don't think that that's helpful," said Tracy Richmond, a pediatrics professor at Harvard Medical School.

Currently, BMI is a highly influential aspect of healthcare. For example, FDA requires people to have a BMI of 30 or higher, or 27 or higher with a weight-related health issue to access new weight-loss drugs like Wegovy and Zepbound. Fertility treatments, as well as orthopedic and gender-affirming surgeries, may also be withheld from patients until they reach certain BMIs.

"The problem with BMI's continued dominance is that it makes it even harder to move away from simply associating a number on a scale with overall health, with all the downstream consequences that come along with a weight-obsessed culture," Tayag writes.

Efforts to move away from BMI

Recently, the medical community has begun to gradually move away from its reliance on BMI to determine a person's health. For example, the American Medical Association (AMA) in June voted to move away from using BMI alone to assess health and weight.

Going forward, AMA recommends physicians use BMI in combination with several other factors when assessing patients' health and weight, such as visceral fat levels; body adiposity index; fat, bone, and muscle percentages; and genetic and metabolic factors. The organization also noted that BMI should not be used as a sole criterion for denying insurance reimbursement.

There is also an ongoing effort by health experts to establish rigorous biological criteria for obesity, which would eliminate BMI-based definitions of the condition.

According to Caroline Apovian, a professor at Harvard Medical School, it will likely be "at least 10 years" before a comparable replacement for BMI emerges — though it will take longer to become as common as BMI among the general public.

Overall, health experts say that it will likely be difficult to replace BMI and that more research is needed to show other measures can be similarly effective.

"I think the trend is moving away from BMI, but these entrenched issues don't fade away easily," said Loren Schechter, a plastic surgeon at Rush University. "Unfortunately, oftentimes you need a lot of literature and experience to do away with some of these things that have been ingrained in the system for a while." (Tayag, The Atlantic, 11/29)

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