Using the Body Mass Index (BMI) to diagnose obesity should go the way of blood-letting, an international panel of 58 scientists argued in the British medical journal The Lancet.
Doctors should instead measure how excess body fat affects the body — a measurement called adiposity. Physicians have used BMI in part because it’s a simple calculation that compares weight relative to health.
The Lancet Diabetes & Endocrinology Commission argues that BMI is more simplistic than simple, because it can both underestimate and overestimate obesity. In doing so, it can exclude or obscure better measures of good health.
“Current BMI-based measures of obesity can both underestimate and overestimate adiposity and provide inadequate information about health at the individual level, which undermines medically-sound approaches to health care and policy,” according to the report.
Rethinking Weight Vs. Health
The panel proposes distinguishing between what they call preclinical obesity, when someone carries excess fat but has properly functioning organs, and clinical obesity, when excess fats affect organs and tissues.
The American Medical Association issued a complementary report two years ago. That report was less concerned with using adiposity as a tool and more focused on differences between categories of people.
“The report also outlined the harms and benefits of using BMI and pointed to BMI as an imperfect way to measure body fat in multiple groups given that it does not account for differences across race/ethnic groups, sexes, genders, and age-span,” an AMA press release stated then. “Given the report’s findings, the new policy supports AMA in educating physicians on the issues with BMI and alternative measures for diagnosing obesity.”
Alison Brown, a program director at the U.S. National Heart, Lung, and Blood Institute agrees that the new stance aligns with the one the AMA issued in 2023. She says BMI may still have some uses — for instance for measuring health risks at a general population level.
Read More: The Rise Of The Obesity Epidemic
The Failure of BMI
Perhaps the biggest cause of contention with BMI is the ways it fails to distinguish weight from muscles versus fat.
“One prime example is the case of athletes that might present with being classified as being overweight or obese due to the weight of muscle mass,” says Brown.
Alternatively, an individual labeled obese based primarily on BMI could be considered relatively healthy if they don’t have type 2 diabetes, high blood pressure, or high cholesterol levels, among other medical indicators. Meanwhile, someone with a healthy BMI may be considered pre-diabetic or at risk for developing type 2 diabetes.
She also agrees that changes in what is considered healthy can sometimes confuse the general public.
“Evolving health recommendations can be challenging for the average consumer, but it is important that science communication reiterates that nutrition science is not static and ever-evolving as more research is conducted,” she says. “This is no different than scientific research leading to changes in prescription medications to better treat diseases and conditions."
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
