Sociodemographic heterogeneity in the association between social isolation and all-cause mortality among Japanese older adults: JAGES longitudinal panel study

Background: Social isolation has been linked to higher mortality; however, it remains unknown how this association varies across individuals and social groups. Methods: We used a nationwide cohort study of ∼20,000 older adults with a 9·4-year follow-up period (Japan Gerontological Evaluation Study, 2013-2022), adjusting for 60 pre-baseline covariates. We also used a national insurance database to ascertain 4299 all-cause deaths. Using a causal machine-learning method, we estimated the heterogeneous association between social isolation and mortality. Findings: Social isolation was more prevalent among those with lower education and income and associated on average with 69·5-day shorter survival time (restricted mean survival time RMST difference; 95% CI: -111, -28·4). There was evidence of heterogeneity where social isolation was associated with even shorter survival days in some subgroups (e.g., -205 days among the bottom quintile of the estimated RMST differences 95% CI: -321, -87·8). These subgroups tended to be older, men, and have lower education. The adverse association of social isolation and mortality was particularly stronger when lower education was combined with higher income, especially among women. Individuals with lower levels of education accounted for a larger share of deaths attributable to social isolation (58·5% of excess deaths). Interpretation: Social isolation may be more harmful among those from socially disadvantaged backgrounds, and a combination of some characteristics can synergistically amplify its effects on mortality. Proper population-level interventions informed with the evidence of effect heterogeneity might not only improve health on average but also mitigate existing health disparities. Funding: Japan Society for the Promotion of Science, Health Labour Sciences Research Grants, Research Funding for Longevity Sciences from the National Center for Geriatrics and Gerontology, Research Institute of Science and Technology from the Japan Science and Technology, Japan Health Promotion & Fitness Foundation, Department of Active Ageing, Niigata University Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University Priority Research Areas Grant and National Research Institute for Earth Science and Disaster Resilience.