Comorbidity and Proportional Life-Years Lost: A Dose-Dependent, Age-Varying Relationship

Background.— Chronic comorbidity reduces life expectancy, but the absolute life-years lost (LYL) varies across the adult age range, as the risk exposure decreases and disease burden increases over time. Absolute LYL and fixed hazard ratios do not reveal how the relative impact of a comorbidity burden changes in relation to the remaining life expectancy available at the index age. Aim.— To introduce proportional life-years lost (Prop-LYL) as a life-course measure of comorbidity-related survival loss and to examine its relation to physiological reserve and remaining time at risk. Methods.— Using the Gompertz–Makeham law of mortality from age 40 onward, chronic comorbidity burden was represented as a transition to a higher mortality-risk pathway. Prop-LYL was defined as the difference between baseline and affected remaining life expectancy, expressed as a proportion of baseline remaining life expectancy. An illustrative age-dependent reserve function was incorporated to represent changing capacity to withstand comorbidity-related physiological stress across adult life. Age- and risk-tier-specific trajectories were examined using published cardiometabolic multimorbidity life-years-lost data. Results.— Prop-LYL showed a nonlinear age pattern. In the sex-averaged illustrative trajectory, proportional loss was 45.9% at age 40, increased to 57.5% near age 60, and declined to 24.9% at age 85. The early-to-mid-life increase was consistent with the progressive expression of comorbidity burden as modeled reserve declined while substantial life expectancy remained. In later life, proportional loss contracted as the remaining survival horizon shortened. Higher comorbidity-risk tiers showed greater proportional loss, with peak discounts of 28.2%, 51.9%, and 62.3% in low-, intermediate-, and high-risk groups, respectively, followed by convergence at advanced ages. Conclusion.— Prop-LYL makes apparent the age-dependent interactions between comorbidity burden, physiological reserve, and remaining time at risk that are not apparent from absolute LYL alone. It provides a structured supplementary framework for interpreting age-specific survival loss in clinical, actuarial, and medicolegal settings; its application to individual assessment requires appropriate clinical and population-specific calibration.