Spending yesterday with some of the top experts in obesity, health policy, and health systems reminded us of something essential that often escapes attention. The static measure of BMI cannot possibly be adequate for assessing the chronic disease of obesity.
What brought this thought to mind was an expert in health benefit design who said:
“We have to make sure that they have a BMI that makes them an appropriate candidate for therapy. Because believe me, we have many seeking it who are not.”
This is a more sophisticated version of a friend telling us, “Look at you. You don’t have to worry about your weight.” It is well-intended and it happens all the time. But it misses a basic fact. Size and appearance do not tell us much about the complex, chronic disease of obesity.
With a new paper in Clinical Obesity, Bruno Halpern and colleagues offer a different way of looking at BMI. They propose classifying obesity based on weight history.
A Missing Perspective: Health Over Time
Obesity isn’t a snapshot — it’s a chronic, dynamic disease process. That simple truth is exactly what Body Mass Index (BMI), born in the 19th century, misses when clinicians lean on a single reading. BMI distills a person’s height and weight into a number, but it does not measure adiposity, fat distribution, or the biological impact of excess fat on tissues, organs, and metabolism. As recent expert analysis concludes, using BMI alone can both underestimate and overestimate true adiposity and health risk, and thus fails to capture the complexity of obesity as disease.
Consider two individuals with the same BMI: one might have higher muscle mass and relatively little visceral fat; the other could have abundant abdominal fat with serious metabolic dysfunction. BMI treats them as identical, even though their cardiometabolic risk is very different. It’s not just the quantity of weight that matters, but the quality and distribution of fat – especially visceral fat that drives insulin resistance, inflammation, and organ dysfunction.
Better Classifying Obesity
Emerging frameworks, including those reflected in Halpern and colleagues’ work on better classifying obesity, underscore that excess adiposity should be confirmed and characterized (e.g., with waist circumference or other anthropometrics) before diagnosing obesity. And beyond that, clinicians should distinguish between pre-clinical obesity – excess fat without health impairment – and clinical obesity – excess fat with demonstrable impact on health and physiology.
A static BMI reading can’t show whether a patient’s fat is causing liver steatosis, dysglycemia, hypertension, or joint disease. Nor can it reveal the trajectory of weight change, metabolic adaptation, or how long someone has lived with adiposity. These are all factors that determine disease progression, prognosis, and the need for intervention. In other words, BMI is a starting point. It is nothing more than a screening tool that serves as a signal of risk. But it cannot, by itself, diagnose a chronic disease that evolves over time and across tissues.
Click here for the new paper by Halpern et al and here for further perspective on a path forward from reliance on BMI.
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