Body mass index may be measured more accurately than is commonly thought. New research shows that the vast majority of people with BMI say obesity has too much body fat.
Scientists at Johns Hopkins University conducted the study, which was published this month in JAMA. In a nationally representative sample of Americans, they found that adults with obese BMI almost always meet the obesity criteria for waist circumference or percentage of body fat. The researchers say the results show that BMI remains an important population-level tool for assessing obesity, even if many people are trying to gradually disappear.
The researchers analyzed data from the National Health and Nutrition Examination Survey (NHANES), a survey of Americans’ dieting and lifestyle habits often run by the Centers for Disease Control and Prevention. As part of NHANES, some people have undergone a wider range of tests, including tests that measure their body fat or waist condition.
The researchers studied the latest NHANES data (2017-2018), which included people who received other tests, in addition to checking for BMI. Obesity is usually defined as having a BMI of 30 and above (persons of Asian descent 27). However, it can also be done by a male body fat percentage of 25%, a female body fat percentage of 25%, a male waist of 40 inches and a female waist of 40 inches.
The researchers found that more than 98% of people who are obese with BMI are also suitable for the bill when considering waist conditions or percentage of body fat.
“Although some patient populations, such as athletes, may require further evaluation, our results suggest that these people account for a small population,” the researchers wrote.
These findings are particularly important given the latest developments in the field of obesity medicine. In early January this year, a large number of experts called for a considerable change in the diagnosis of obesity.
They urged doctors to stop using BMI as the only criterion for measuring obesity. Instead, doctors should use two measurements of body size, one of which can include BMI, or directly measure body fat to diagnose obesity. They further call for the division of obesity into two broad categories, depending on whether one’s obesity is actively causing related health problems: preclinical and clinical obesity.
Other researchers and advocates of human motivation and fat acceptance exercise have long called for the phasing out of BMI, and the group's conclusions have been strongly supported by many public health groups, including the American Heart Association and the World Federation of Obesity.
However, the researchers noted that direct body fat tests require specialized equipment and may be more expensive for patients out of pocket, and given their findings, many people may not benefit from taking these other tests. Either way, the scientific debate on the usefulness of BMI seems to be unresolved.
“For almost all American adults with elevated BMI, there may be limited utility to confirm excessive obesity,” the authors write. Currently, approximately 40% of adults in the United States are considered obese under current BMI standards, although this rate may eventually start to decline, thanks in part to newer, more effective weight loss pills (such as Wegovy).
