For decades, the diagnosis of obesity relied heavily on a single, straightforward equation—one you may hear us mention constantly throughout your process: Weight in kilograms divided by height in meters squared (kg/m2). Known as Body Mass Index (BMI), if such result reached 30 or higher, you were classified as having obesity, and, by extension, supposed ill.
While BMI serves a purpose in population-level screening and epidemiologic tracking, its use as an individual diagnostic tool has become virtually obsolete. In early 2025, the Lancet Diabetes & Endocrinology Commission published a landmark framework redefining obesity and its diagnosis, shifting the paradigm from a simple weight metric to a nuanced, organ-function-based clinical condition. Not all fat bodies are ill—a topic we’ll discuss in future posts.
Here is a deep dive into what this shift means and why it matters.
Not everything with BMI is wrong, classifying obesity only through BMI served four main functions (Hong & Park, 2025):
Despite these practical benefits, we now know that using BMI as the sole diagnostic criterion is fundamentally flawed. BMI fails to distinguish between lean muscle mass, bone density, fat mass, and total body water, and it ignores critical demographic variables such as age, sex, and ethnicity. Crucially, BMI cannot differentiate between subcutaneous fat and visceral fat—the latter known to be the most dangerous metabolically (Hong & Park, 2025).
Furthermore, a rigid focus on BMI creates diagnostic blind spots. There are individuals with "normal" BMIs (18.5 - 24.9 kg/m2) who nevertheless exhibit insulin resistance, dyslipidemia, systemic inflammation, and other metabolic complications—a phenotype known as Metabolically Obese Normal Weight (MONW) (see Pluta et al., 2022). These are routinely missed in clinical screenings, due to their “apparent health”, which in turn keeps this population unfortunately underdiagnosed. Conversely, muscular individuals or those with benign subcutaneous fat are frequently mislabeled as obese-ill.
To address these limitations, the Lancet Commission introduced a refined diagnostic model that distinguishes between general obesity (or, high adiposity), preclinical obesity, and true clinical obesity (Fourman et al., 2025; Rubino et al., 2025).
Under this updated framework:
This paradigm shift carries major implications for clinical and surgical practice, in as much as it changes—for the better—the amount and kinds of patients we receive:
The current redefinition of obesityrepresents a vital step toward precision medicine. By moving beyond BMI andevaluating how adipose tissue actually impacts organ function and physicalcapability, clinicians can better target intensive therapies toward those in immediateneed while promoting preventive lifestyle strategies for those in preclinicalstages.
Bariatric Nutrition | Behavioral Neuropsychology
Specialistin Bariatric Nutrition, Metabolic Syndrome, and Eating Disorders
Al-Roub, N. M. et al. (2025). Body Mass Index and Anthropometric Criteria to Assess Obesity. JAMA Network Open, 8(12), e2549124.
Fourman, L.T. et al. (2025). Implications of a New Obesity Definition Among the All of Us Cohort. JAMA Network Open, 8(10), e2537619.
Hong, S. et al. (2025). From Old to New: A Comprehensive Review of Obesity Diagnostic Criteria and Their Implications. Endocrinology and Metabolism, 40(4), 517–522.
Pluta, W. et al. (2022). Metabolic Obesity in People with Normal Body Weight (MONW)—Review of Diagnostic Criteria. International Journal of Environmental Research and Public Health, 19(2), 624.
Rubino, F. et al. (2025). Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes &Endocrinology, 13(3), 221–262.
