Bidirectional associations between loneliness and physical multimorbidity and the mediating role of social inactivity.

Physical multimorbidity and loneliness are increasingly recognised as public health concerns, but the nature of their potential relationships remains unclear. Here we report bidirectional associations between physical multimorbidity and loneliness in middle-aged and older adults and the mediating role of social inactivity. Longitudinal data from adults aged ≥45 years are analysed across 19 countries from five prospective cohorts: US Health and Retirement Study; English Longitudinal Study on Ageing; Survey of Health, Ageing and Retirement in Europe; China Health and Retirement Longitudinal Study; and Korean Longitudinal Study of Aging. Cox regression models suggest longitudinal associations between physical multimorbidity and loneliness in both directions (from physical multimorbidity to loneliness: adjusted hazard ratio (HR) = 1.25, 95% confidence interval (CI) = 1.20 to 1.30; from loneliness to physical multimorbidity: adjusted HR = 1.10, 95% CI = 1.05 to 1.17). Random-intercept cross-lagged panel models confirm significant association between T1 physical multimorbidity and T3 loneliness, with T2 social inactivity serving as a mediator. These findings suggest that physical multimorbidity and loneliness may create a vicious cycle where each condition exacerbates the other, with social inactivity mediating the direction from physical multimorbidity and loneliness.