If you’ve been to a doctor’s office or fitness center in recent years, you’ve likely had your Body Mass Index calculated. The BMI formula uses a person’s weight in kilograms divided by the square of their height in meters. The resulting number is classified into one of four categories: underweight, normal weight, overweight, or obese. These labels are prolific in the U.S. medical system. At one point, BMI classification was even used to establish eligibility for COVID vaccines.
Earlier this month, the American Medical Association voted to move away from using BMI as a measurement to assess weight and health. The calculation is problematic for many reasons. To understand why, you must first understand its history.
BMI was invented in the 1830s by a Belgian statistician who wanted to describe the “average man.” It became more widely used in the 1970s when physiologist Ancel Keys was looking for a better way for U.S. life insurance companies to estimate people’s body fat, and correlate that with their risk of dying. Keys demonstrated that BMI was a better and simpler calculation than the insurance industry’s methods at the time.
While BMI is useful in evaluating large populations, it is much less helpful in evaluating the health of a single person. The measure provides no way to quantify a person’s bone or muscle mass. As a result, athletes often have higher BMI because they have more muscle mass. At the same time, people lose muscle mass and gain abdominal fat as they age; this unhealthy change is not be picked up by measuring BMI.
Another major problem is that BMI was developed and tested primarily in white men, but body composition varies widely in different genders, races, and ethnicities. A healthy weight and body composition for a black woman may appear to be “obese” or “overweight” on a BMI scale.
Evidence demonstrates these problems. A study published in the International Journal of Obesity in 2016 compared BMI with other measures of health including blood pressure, triglycerides, cholesterol, glucose, and insulin resistance in more than 40,000 adults. Researchers found nearly half of overweight individuals and nearly 30 percent of obese individuals were metabolically healthy. Conversely, more than 30 percent of people classified as “normal weight” in the study were metabolically unhealthy. Extrapolating this data, they estimate that more than 74 million U.S. adults are misclassified as healthy or unhealthy by BMI measurements.
The new policy adopted by the American Medical Association doesn’t do away with BMI completely. Instead, it recommends that doctors use other factors to evaluate health and weight including abdominal fat, blood sugar levels, and thyroid tests.
“There are numerous concerns with the way BMI has been used to measure body fat and diagnose obesity, yet some physicians find it to be a helpful measure in certain scenarios,” said AMA Immediate Past President Dr. Jack Resneck, Jr. “It is important for physicians to understand the benefits and limitations of using BMI in clinical settings to determine the best care for their patients.”
The take-home message: BMI is not an accurate predictor of health in individuals.
