Population burden and individual risk in transitions to somatic-cognitive involvement among middle-aged and older adults: a multinational longitudinal health-state transition study

BACKGROUND: Multimorbidity in ageing populations spans somatic, psychological and cognitive domains, but longitudinal transitions and the distinction between population burden and individual risk remain unclear. We used multinational cohort data to characterize these patterns. METHODS: We harmonized five waves (2010-2020) from CHARLS, HRS and ELSA. Among adults aged ≥45 years who were healthy or had single-domain involvement at baseline, health states combined somatic disease, depressive symptoms and cohort-standardized cognitive impairment. We estimated baseline-to-Wave 5 transitions and contributions to Wave 5 somatic-cognitive involvement, using modified Poisson, Fine-Gray, illness-death and cross-cohort heterogeneity models. RESULTS: Among 43,010 participants, 18,363 were observed at Wave 5. Cognitive-only status had the highest individual risk of Wave 5 somatic-cognitive involvement (18.9%; RR 9.68, 95% CI 6.4-14.6 vs healthy) but contributed only 2.8% of cases. Somatic-only status had lower individual risk but accounted for most cases (78.9%) because of its high prevalence. Cognitive-only status also had the highest subdistribution hazard (sHR 4.51). Mortality intensity was about four-fold higher after involvement. Among somatic-only participants, risk was higher with age ≥65 years (RR 4.03) and modestly higher with baseline ADL limitation (RR 1.55); associations were consistent across cohorts. CONCLUSIONS: Individual risk and population burden diverged: cognitive-only status identified a small high-risk group, whereas somatic-only status accounted for most cases. Findings support functional and cognitive monitoring among middle-aged and older adults with somatic morbidity, without implying causal effects.