Rethinking obesity: comparison of the new clinical obesity framework with traditional BMI-based classifications in midlife UK adults

Abstract Body mass index (BMI) is widely used to identify obesity but does not distinguish between fat and muscle mass nor reflect the clinical consequences of excess adiposity. We applied the Lancet clinical obesity framework - incorporating excess adiposity, functional limitations, and organ dysfunction - to age 46 data from the 1970 British Cohort Study, an age-homogenous birth cohort ( n = 6978). We compared prevalence and classification discordance with standard BMI categories and examined associations with demographic, behavioural, and health indicators and five-year mortality. Using the clinical framework, 29.8% had no obesity, 52.5% pre-clinical, and 17.7% clinical obesity compared to 0.7%, 29.1%, 37.8% and 32.4% classified as underweight (BMI < 18.5 kg/m 2 ), healthy (18.5–24.9 kg/m 2 ), overweight (25–30 kg/m²) and having obesity (≥30 kg/m²), respectively. Of those with BMI-defined obesity, only 31.4% met criteria clinical obesity, with 68.6% considered pre-clinical. The BMI overweight group also included individuals with no obesity and clinical obesity, indicating bidirectional discordance between definitions. In unadjusted models, pre-clinical and clinical obesity were associated with higher five-year mortality (Odds ratios: 2.82 (95% CI: 0.93, 12.2); 5.17 (1.54, 23.3)), whereas BMI categories were not. Findings suggest the clinical framework may offer a more meaningful and risk-relevant population-level classification of obesity. Findings have implications for clinical care, epidemiological research, and public health prevention strategies.