BMI, Obesity and the Problem With How We Talk About Weight

For decades, one number has played an outsized role in determining whether your weight is considered healthy or not: your body mass index, or BMI.

The calculation is simple, and the resulting number places most adults into one of four categories: underweight, healthy weight, overweight or obese. But bodies – and overall health – aren’t usually so simple.

While BMI can help identify potential health risks, it doesn’t distinguish fat from muscle, show where fat is stored or account for differences in body composition and metabolism. As a result, two people with the same BMI can have very different health profiles. It’s a distinction that has fueled a broader shift in how we think and talk about obesity, and is why “obesity medicine specialists no longer only look at your BMI,” says Dr. Caroline Apovian, a professor of medicine at Harvard Medical School and co-director of the Center for Weight Management and Wellness at Brigham and Women’s Hospital.

Here’s what BMI is, when it’s useful, where it falls short, and what else to pay attention to.

What Is BMI and How Is It Calculated?

BMI is a person’s weight relative to their height. To calculate it using pounds and inches, divide your weight by your height in inches, then divide that number by your height in inches again, then multiply that result by 703. You can alternatively use the official adult BMI calculator from the Centers for Disease Control and Prevention.

For adults, a BMI below 18.5 is categorized as underweight, 18.5 to 24.9 as healthy weight, 25 to 29.9 as overweight and 30 or higher as obese, explains Apovian.

She says BMI’s appeal is its simplicity, since it requires knowing only your height and weight, making it inexpensive, noninvasive and easy to track across large populations.

Limitations of Knowing Your BMI

Problems can arise when a broad screening tool is treated as a precise measurement of individual health. One reason is that “BMI doesn’t account for different body types,” says Dr. Joseph Bass, chief of the division of endocrinology, metabolism and molecular medicine at Northwestern University.

A muscular athlete, for example, can have a high BMI despite relatively little body fat. Bass points to Arnold Schwarzenegger at the peak of his bodybuilding career as an example of an exceptionally fit individual who would have had a seemingly problematic BMI.

BMI also doesn’t account for differences associated with age, sex or race, which can affect body composition, and the relationship between BMI and certain health risks. Such limitations are among the reasons BMI “falls short of being accurate for everyone,” says Apovian.

BMI also “doesn’t address the question of body fat distribution,” Bass says, even though where fat accumulates can profoundly affect health. Waist circumference, for instance, can provide information BMI misses, particularly about abdominal fat.

This matters because a waist circumference greater than 35 inches in women or 40 inches in men is associated with greater health risks, Apovian explains, and is why clinicians look at “where the fat is distributed” and whether a patient has related conditions such as prediabetes, diabetes or high blood pressure.

Genetics complicate the picture further, “as there is clear genetic evidence that among people living with obesity, there are tremendous differences in certain susceptibilities to comorbidities,” says Bass.

Because of these limitations, physicians usually consider waist circumference, body composition, fitness, blood pressure, blood sugar and cholesterol levels alongside BMI. These indicators “all matter,” Bass says, and can help provide a more complete picture of a person’s health, including both strengths and potential concerns.

How the Definition of Obesity Is Changing

Perhaps more consequential than any change in measurement is the evolving scientific understanding of obesity itself and how we talk about it.

Where weight was once viewed largely by the number on the scale, researchers now recognize that muscle mass, fat distribution, body composition and metabolic health can provide information that BMI alone cannot.

And Bass calls the discovery of the hormone leptin a “watershed moment” because it helped reveal some of the molecular and hormonal mechanisms involved in weight regulation. The body has physiological systems that defend its weight, he explains, likely an evolutionary adaptation that helped humans survive periods of food scarcity. Recognizing those mechanisms in the wake of this discovery has helped move the conversation away from treating obesity solely as a matter of “will” or “motivation,” he says.

Apovian agrees and also points to the country’s abundance of ultra-processed, calorie-dense foods and products high in refined carbohydrates, added sugars and unhealthy fats as another factor that can interfere with appetite regulation and contribute to weight gain.

This environment can contribute to “a defect in the signaling pathways to fullness in the brain” in many people, Apovian explains. “These are the signals that you have had enough food,” she says, and when those signals don’t function as they should, some people may continue eating.

Given so many biological, genetic and environmental influences, simply labeling someone as obese does little to explain that person’s overall health.

Why Does the Way We Talk About Weight Matter?

Understanding the biology of weight regulation matters because assumptions about personal responsibility or “willpower” can create shame — feelings that may prevent people from seeking treatment and guidance that could help them most.

“Weight stigma is real,” Apovian says. “But once the stigma is gone, we can really treat obesity and give patients the treatments and recommendations they need.” As we do so, she explains, “we can be more focused on overall health and not the number on the scale.”

That doesn’t mean weight – or your BMI – is irrelevant, she stresses. Instead, she recommends considering it alongside additional measurements such as waist circumference, blood pressure, muscle mass and fitness levels. Even assessing something as simple as “how many times can you sit and stand from a chair in 30 seconds” can provide useful information about physical function, she says.

For people trying to manage their weight and improve their health, Bass recommends a Mediterranean-style diet emphasizing fruits and vegetables, whole grains and unsaturated fats, plus getting enough sleep, managing stress and maintaining regular preventive care. “There isn’t ‘one’ thing,” he says.

Apovian agrees and also emphasizes minimizing ultra-processed foods and added sugars, plus getting the recommended amount of physical activity of at least 150 minutes of moderate-intensity aerobic activity per week and two days of muscle-strengthening activity.

BMI can help start a conversation about health, she says, but given all the other factors that shape well-being, “it shouldn’t be allowed to end it.”

This article originally appeared on USA TODAY.

Reporting by Daryl Austin, USA TODAY / USA TODAY

USA TODAY Network via Reuters Connect