The good, bad and ugly about BMI

By Sandee LaMotte, CNN

(CNN) — It’s a dirty word for some: BMI.

Short for body mass Index, BMI is nothing more than a crude measurement of a person’s body fat based on height and weight. It was invented for researchers to use on masses of people to see how weight affects the development of disease and chronic health conditions. For those studies, separating populations of people into weight categories by BMI works well.

Research has found that as BMI climbs, so does the risk for chronic conditions, including heart disease, high blood pressure, high cholesterol, type 2 diabetes, respiratory problems, stroke, mental illness, sleep apnea, osteoarthritis, physical pain and at least 13 types of cancer, according to the US Centers for Disease Control and Prevention.

Yet critics say the term BMI has become a societal judgment by lumping individuals into arbitrary categories that perpetuate misconceptions about body weight. Even with all the backlash against “fat shaming,” weight stigma against the seriously overweight or obese remains deep-rooted, according to research.

“Our society and social media say, ‘You have to be thinner; you are not good enough unless you are thin.’ Nor can you be healthy if you’re not thin, whereas you can be healthy and large, just as you can be unhealthy and thin,” said registered nurse Joann Hendelman, clinical director of the National Alliance for Eating Disorders.

“I put ‘obesity’ in quotes because I personally find it to be a slur,” said anti-weight discrimination advocate Susan Vibbert, who serves on Project HEAL’s board of directors. Project HEAL is a nonprofit advocacy organization dedicated to helping people with disordered eating.

“Yet it has become completely medicalized that health equals weight, weight equals health based on BMI. And it’s not true,” Vibbert said.

A BMI measurement can be flat wrong in some cases, said Thomas Wadden, professor of psychology at the Perelman School of Medicine at the University of Pennsylvania.

“Consider a young woman who is 5 feet 5 inches and 150 pounds. She would be on the cusp of being overweight with a BMI of 25,” said Wadden, the former director of Perelman’s Center for Weight and Eating Disorders.

“But she may be incredibly muscular, and she may have most of her weight in her lower body, where it’s not as damaging to her health as weight in the upper body,” he said. “She could easily say, ‘I am in perfectly good health, so just take your BMI of 25 and shove it.’”

How to determine adult BMI

To calculate adult BMI, weight is divided by the square root of a person’s height. (For the mathematically challenged, the National Institutes of Health has a free calculator to do the work for you.)

As currently defined, a BMI between 18.5 and 24.9 is a healthy weight, between 25 and 29.5 is overweight, between 30 and 34.9 is obese, between 35 and 39.5 is class 2 obesity, and anything over 40 is “severe” or class 3 obesity, which used to be called morbid obesity. People are considered underweight if their BMI is lower than 18.5.

Muscle and bone weigh more than fat, so BMI measurements can overestimate body fat in athletes and people with a muscular build or a larger body frame. Conversely, BMI can underestimate body fat in older adults and anyone who has lost muscle, according to the Harvard T.H. Chan School of Public Health in Boston.

More conundrums: Women naturally have more body fat and less muscle mass than men, while some racial and ethnic groups are genetically predisposed to carry more or less lean muscle mass and body fat.

The use of BMI for children is also problematic, according to the CDC, because a child’s healthy weight range is based on a BMI between the 5th and 85th percentile on the CDC growth charts.

“It is difficult to provide healthy weight ranges for children and teens because the interpretation of BMI depends on weight, height, age, and sex,” the CDC noted.

As a result, parents should never use an adult BMI calculator to determine their child’s weight status, the CDC said.

The need for ‘a bigger, broader picture’

Still, there is a role for the use of BMI in the doctor’s office, said Dr. Justin Ryder, associate professor of surgery and pediatrics at Northwestern University’s Feinberg School of Medicine in Chicago.

“There’s enough data to support that if you carry enough weight over a long enough period of time, chronic disease follows,” Ryder said. “It may not be in the form of hypertension or diabetes or heart disease, it could be musculoskeletal problems, depression or sleep issues that cripple quality of life.”

Researchers have found that fat cells emit inflammatory signals to the tissues they surround, including the same type of cytokines that created the “cytokine storms” in Covid-19 that put many in intensive care.

“Fat is not just there as a storage vessel,” Ryder said. “It is an active endocrine tissue. So, the longer you carry excess adiposity or fat, the longer it has to excrete some of the inflammatory cytokines and other agents that are associated with chronic disease.”

What should a person do? Get a doctor who considers the whole patient, Ryder advised.

“Doctors have to take a bigger, broader picture,” he said. “They should look at their adult patient and not just say, ‘OK, your BMI is 31, you need to lose weight,’ as that’s not necessarily the answer all the time.”

Are there other options?

If BMI is so cumbersome, why don’t doctors use other measurement tools?

Waist circumference, for example, is another way to measure body fat, especially the type of fat most dangerous to health: visceral or “hidden” fat.

You can’t pinch this type of fat, as it hides beneath abdominal muscles. While even thin people can have it — a condition called TOFI, or “thin outside, fat inside” — visceral fat typically grows along with an expanding tummy.

Visceral fat wraps itself in and around the liver, heart, kidneys and intestines, excreting inflammatory proteins that heighten the risk of high cholesterol, heart disease, stroke, type 2 diabetes, insulin resistance, and Alzheimer’s and other dementias.

“Men should have a waist circumference of less than 40, and women should have less than 35,” Wadden said. “Anything over those numbers may lead to intra-abdominal fat in the upper body where it’s more likely to be associated with health complications.”

Other measurement tools the medical field is considering instead of BMI include the waist-to-height ratio, or WHtR, method, which is calculated by dividing waist circumference by height. Some researchers tout this tool as an excellent predictor of future cardiovascular health. It’s easy to use and less age-dependent but should not be used in children younger than age 6, according to a 2022 review of literature.

However, at this point, none of those alternatives appears to be a viable solution, according to Ryder.

“The other tools that we could use to measure body fatness are just not practical in the clinical sense,” he said. “They’re good tools from a research perspective but doing them in the clinic would just add extra cost for the patient. Nor are they really more informative than what we’re currently using.”

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