Associations of social isolation and loneliness with physical and mental health in middle-aged and older adults: an 18-country multicohort study

Social isolation and loneliness are common in later life and associated with adverse health outcomes. However, their longitudinal, cumulative, joint, and mediating associations across diverse national settings remain insufficiently understood. We examined these associations among middle-aged and older adults across 18 countries. We used harmonised data from five longitudinal cohorts (HRS, ELSA, SHARE, CHARLS, and MHAS). Adults aged ≥50 years were followed for 2–10 years during 2010–2020. Social isolation was measured using a multidimensional index of objective social contact and participation; loneliness was assessed using short forms of the Revised UCLA Loneliness Scale. Outcomes included physical diseases, mental disorders, and self-rated health. Country-specific mixed-effects models estimated adjusted average marginal effects (AMEs) of baseline social isolation and loneliness on repeated post-baseline health outcomes, pooled using random-effects meta-analysis. We also conducted cumulative-exposure, subgroup, interaction, joint-exposure, mediation, and sensitivity analyses. Among 94,122 participants, baseline proportions of social isolation ranged from 17.94% in England to 38.01% in Mexico, and those of loneliness from 9.11% in Switzerland to 35.23% in Greece. Social isolation was associated with a higher probability of physical diseases (pooled AME 0.007; 95% CI 0.003 to 0.011) and mental disorders (0.026; 0.014 to 0.037), and poorer self-rated health ( − 0.048; −0.069 to −0.028). Corresponding estimates for loneliness were 0.011 (0.006 to 0.016), 0.243 (0.214 to 0.271), and −0.233 ( − 0.280 to −0.186), respectively. Exposure to social isolation or loneliness at more of the first two study waves was associated with poorer health at subsequent waves. Associations were stronger among participants with lower educational attainment, functional limitations, or no partner. Those experiencing both social isolation and loneliness had the poorest health outcomes. Mediation analyses estimated that loneliness accounted for 9.52%–31.63% of the associations between social isolation and subsequent health outcomes, compared with 1.77%–2.56% through social isolation in reverse models. Social isolation and loneliness were each associated with poorer physical, mental, and self-rated health. Repeated and joint exposures were linked to poorer subsequent outcomes. Public health strategies should assess both dimensions of social disconnection, particularly among people with repeated or joint exposure and socioeconomically or functionally disadvantaged groups.