In what situations could obesity be considered a disease in its own right? The 2025 Lancet Commission on the Definition and Diagnosis Clinical Obesity (1) defined clinical obesity as a chronic, systemic illness characterised by alterations in the function of tissues, organs, the entire individual, or a combination thereof, due to excess adiposity.
The Commission recommended a move away from a sole reliance on BMI to define obesity. Instead, excess body fat should be confirmed by either direct measurement of body fat, or at least one anthropometric criterion (e.g. waist circumference, waist-to-height ratio) in addition to BMI, using validated methods and appropriate cutoff points. People with confirmed obesity status should be assessed for clinical obesity. The diagnosis of clinical obesity requires one or both of the following criteria: evidence of reduced organ or tissue function due to obesity (i.e. signs, symptoms, or diagnostic tests showing abnormalities in the function of one or more tissue or organ system; 18 criteria for adults and 13 for children/ adolescents); or substantial, age-adjusted limitations of daily activities reflecting the specific effect of obesity on mobility, other basic activities of daily living, or both. Pre-clinical obesity was defined as being present when there is excess body fat but no associated health problems. Note that the definition of obesity as a standalone disease does not depend upon the presence of another disease.
The new definition means a move away from a strict epidemiological definition to one that is more clinically nuanced. People with clinical obesity should receive timely, evidence-based treatment, aiming to improve clinical manifestations of obesity and prevent progression to end-organ damage. People with preclinical obesity should receive evidence-based health advice, monitoring over time, and, where appropriate, more intensive therapy depending upon individual risk. All people with obesity (clinical or pre-clinical) should have equitable access to therapies.