Multimorbidity and functioning disparities in retired older adults: longitudinal trajectories, disease clusters, and mediating mechanisms

Multimorbidity is a major determinant of older adults’ functioning, yet its long-term pathways and underlying mechanisms remain insufficiently understood. Retired older adults face unique vulnerabilities due to declining occupational activity, reduced socioeconomic buffers, and increased dependence on functional capacity. This study examines how multimorbidity shapes functioning over sixteen years and whether Body Mass Index (BMI) mediates these association functioning and well-being in functioning and well-being in . Data from older adults aged 60 and over, in the Health and Retirement Study (2004–2020) were analyzed using growth curve models to estimate baseline levels and rates of change in functioning (N = 1,238), mixed-effects models to assess disease cluster, sex and race effects (N = 1,238), and conditional process analyses to test BMI mediation (N = 8,654). Multimorbidity is associated with reduced baseline functioning in the physical functioning and disability domains, but did not accelerate decline across all domains, indicating that the principal disadvantage emerges early and persists over time. Mixed-effects models showed strong negative and domain-specific associations, with musculoskeletal conditions most strongly associated with impairing physical functioning and respiratory conditions exerting the greatest association with disability. Significant sex and racial disparities were observed, with females and minority groups exhibiting consistently poorer functioning. BMI partially mediated the effects of multimorbidity on physical and cognitive functioning, with indirect effects strengthening modestly over the baseline and terminal periods. However, the disability domain was not mediated, suggesting that disability reflects more entrenched functional losses. Multimorbidity shapes functioning primarily through early deficits and disease cluster-specific pathways. BMI represents a modifiable mechanism that can enhance physical and cognitive functioning, supporting interventions focused on weight management, tailored chronic-disease care, and equity-centered strategies to preserve functioning and promote healthy aging among retired older adults.