Classification and Severity Assessment of Obesity in Clinical Risk Prediction of Multimorbidity Trajectories

Obesity is a key risk factor for chronic disease and multimorbidity, yet variation in how obesity is defined may influence risk estimation in clinical practise. Waist-to-hip ratio (WHR) and waist-to-height ratio (WHtR) are better at capturing fat distribution than body mass index (BMI) and may have a role in obesity related multimorbidity prediction. To examine multimorbidity risk across obesity groups classified using BMI in combination with WHR/WHtR. A UK Biobank cohort free of comorbidity at baseline (n = 179 876) was followed for incident first long-term condition and multimorbidity (≥ 2 of 38 conditions) over a mean of 9.9 years. Six adiposity categories combining BMI with WHR/WHtR were analysed using Cox proportional hazards models. Subgroup analyses evaluated whether high central obesity (WHtR ≥ 0.6) provided additional risk stratification within BMI groups. Combined general with central obesity showed highest risk for the first condition (HR 1.11; 95% CI 1.087-1.138) and multimorbidity (HR 1.13; 95% CI 1.093-1.172). However, BMI-stratified subgroup analyses, central obesity provided additional risk discrimination only among individuals in the overweight category (BMI 25-29.9 kg/m²), but not among those with BMI ≥ 30 kg/m². Waist-based measures improved risk identification in adults with overweight BMI, supporting the potential role of combined BMI-waist approaches for earlier multimorbidity prevention.