Same Burden, Younger Age: Obesity Severity and Age-Equivalent Multimorbidity in Türkiye

Objective: Obesity is associated with greater chronic disease burden, but whether comparable multimorbidity is observed at younger ages with increasing obesity severity is less clear. We examined the association between body mass index (BMI) and somatic multimorbidity and quantified cross-sectional age-equivalent differences across BMI categories. Material and Methods: We analyzed data from 20,116 adults aged ≥18 years in the 2025 Türkiye Health Survey. Somatic multimorbidity was defined as at least two of 15 chronic disease domains. Associations with BMI category were estimated using weighted modified Poisson regression with household-clustered robust standard errors. Age-specific probabilities were estimated using adjusted logistic regression and marginal standardization. Results: Somatic multimorbidity prevalence increased from 20.95% in adults with normal weight to 55.90% in obesity class II/III. In the fully adjusted model, prevalence ratios were 1.15 (95% CI 1.09–1.21) for overweight, 1.37 (1.28–1.45) for obesity class I, and 1.60 (1.49–1.72) for obesity class II/III versus normal weight. A 50% predicted model prevalence was observed at 68.3, 63.2, 57.0, and 50.3 years, respectively. Relative to normal weight, this corresponded to cross-sectional age-equivalent differences of 11.2 years for obesity class I and 18.0 years for obesity class II/III. Sensitivity analyses were consistent. Conclusion: Increasing BMI severity was associated with progressively greater somatic multimorbidity and with the same model-predicted prevalence being observed at younger ages in this cross-sectional population. These age-equivalent differences should not be interpreted as individual-level acceleration of disease onset. The findings support earlier assessment of cumulative chronic disease burden in adults with more severe obesity.