BMI Is Actually Pretty Accurate at Measuring American Obesity, Study Finds

Body mass index might be a more accurate measurement than commonly thought. New research shows that a vast majority of people with a BMI signifying obesity have substantial excess body fat.

Scientists at Johns Hopkins University conducted the study, published this month in JAMA. In a nationally representative sample of Americans, they found that adults with an obese BMI almost always met the criteria for obesity in terms of waist circumstance or body fat percentage, too. The findings suggest that BMI remains an important population-level tool for assessing obesity, the researchers say, even as many people are trying to phase it out.

The researchers analyzed data from the National Health and Nutrition Examination Survey (NHANES), a survey of Americans’ dieting and lifestyle habits regularly run by the Centers for Disease Control and Prevention. As part of the NHANES, some people are given more extensive examinations, which include tests measuring their body fat or waist circumstance.

The researchers looked at the most recent NHANES data (2017-2018) that included people who received these other tests in addition to having their BMI checked. Obesity is typically defined as having a BMI 30 and over (27 for people of Asian descent). But it can also be defined by having a body fat percentage of 25% for men and 35% for women, or a waist circumstance of 40 inches for men and 35 inches for women.

More than 98% of people who were considered obese using BMI alone also fit the bill when either waist circumstance or body fat percentage were taken into account, the researchers found.

“Although certain patient populations (eg, athletes) may warrant further evaluation, our results suggest that these individuals make up a very small portion of the population,” the researchers wrote.

The findings are especially relevant given recent developments in the field of obesity medicine. Earlier this January, a large group of experts called for a sizable shift in how obesity is diagnosed.

They’ve pushed for doctors to stop using BMI as the sole criteria for measuring obesity. Instead, they say, doctors should either use two measurements of body size (one of which can include BMI) or a direct measurement of body fat to diagnose obesity. They further called for obesity to be grouped into two broad categories, depending on whether a person’s obesity is actively causing related health problems: preclinical and clinical obesity.

Other researchers and advocates in the body positivity and fat acceptance movement have long called for BMI to be phased out, and the group’s conclusions were strongly backed by many public health groups, including the American Heart Association and World Obesity Federation.

The study researchers note, however, that direct body fat tests require specialized equipment and could be more expensive for patients in terms of out-of-pocket costs, and given their findings, many people might not benefit from undergoing these other tests. Either way, it seems likely that the scientific debate over the usefulness of BMI isn’t quite settled yet.

“For nearly all U.S. adults with elevated BMI, there may be limited utility to confirming excess adiposity,” the authors wrote. Currently, under the current BMI criteria, around 40% of adults in the U.S. are considered obese, though the rate may be starting to finally decline, thanks in part to the arrival of newer, more effective weight loss medications like Wegovy.