Multimorbidity and memory-related disorders in older adults: A cross-national study based on three large longitudinal cohorts.

BACKGROUND: Memory-related disorders pose a growing burden on ageing populations, yet the global understanding of how multimorbidity links to such cognitive decline remains limited. This study aimed to examine the exposure-response relationship between multimorbidity-including its specific patterns-and memory impairment in older adults, and identify key mediating mechanisms. METHODS: A total of 20,242 participants aged 50 years old and above from three prospective and representative cohorts were included. Memory-related disorders were assessed via self-reported physician diagnoses, and multimorbidity was defined as the coexistence of two or more chronic diseases in the same individual. Time-dependent Cox proportional hazards regression models evaluated the association between multimorbidity and memory-related disorders, and identified high-risk multimorbidity patterns. Random forest-based double machine learning was used to identify mediating mechanisms. RESULTS: Multimorbidity was associated with a 111.8 % higher risk of memory-related disorders. Mental multimorbidity (HR = 2.664) and cardiometabolic multimorbidity (HR = 1.656) conferred the highest risk, while pulmonary multimorbidity showed moderate risk (HR = 1.227). Transition to multimorbidity correlated with increased vision loss (risk increment = 0.183), hearing loss (risk increment = 0.199), and worse BADL and IADL conditions. Vulnerable subgroups included older adults aged 50-64 years (HR = 2.620), females (HR = 2.114), those with lower educational attainment (HR = 2.216), non-retirees (HR = 2.557), smokers (HR = 2.296), drinkers (HR = 2.178), and those without investments (HR = 2.137). CONCLUSION: Multimorbidity elevates memory-related disorder risk in older adults, with sensory and functional impairment as key mediators. Health policies should integrate standardized cognitive check-ups, sensory and functional support, community anti-isolation interventions, and optimized electronic health records into national ageing frameworks.