Social isolation, loneliness, disability progression, and mortality among older adults with cardiac disease: a longitudinal NHATS study.

Background: Cardiac disease is the leading cause of morbidity and mortality among older adults, yet the psychosocial pathways through which it accelerates functional decline remain poorly characterized. Social isolation and loneliness are plausible mediators, but their longitudinal roles in the cardiac disease-disability-mortality cascade have not been examined in an integrated framework. Methods: We analyzed 5,654 US Medicare beneficiaries aged ≥65 from the National Health and Aging Trends Study (NHATS) followed across Rounds 7-14 (2017-2024). Cardiac disease was ascertained at baseline via self-reported physician diagnosis. Social isolation was measured via a validated composite index; loneliness was assessed with a single-item measure. Disability outcomes included Nagi physical function, mobility limitation, activities of daily living (ADL) impairment, and instrumental activities of daily living (IADL) impairment. We estimated group differences using generalized estimating equations (GEE), mortality using Kaplan-Meier and Cox proportional-hazards models, and mediation using the counterfactual framework. Results: At baseline, 1,361 participants (24.1%) had cardiac disease. The CD group exhibited higher social isolation prevalence (19.2% vs 16.8%, p=0.048), lower Nagi scores (7.1 vs 8.5, p<0.001), and greater comorbidity burden (2.9 vs 2.1 conditions, p<0.001). Over follow-up, CD was associated with persistently worse physical function (Round 14 Nagi: 7.1 vs 8.1, p<0.001) and higher mobility limitation (25.2% vs 15.0%). Cumulative mortality reached 49.0% in the CD group vs 33.4% in the No CD group (log-rank χ²=121.97, p<0.001). Mediation analysis showed a negligible indirect effect through social isolation, corresponding to -1.4% of the CD→disability effect (path a: β=-0.0114, p=0.801; path b: β=-0.7002, p<0.001; NIE β=0.0080), indicating no meaningful mediation by baseline social isolation. Conclusions: Among older adults, cardiac disease was associated with disability progression and mortality, with little evidence that baseline social isolation explained the functional decline association. Although social isolation was more common among participants with cardiac disease, its contribution to functional decline was minimal, indicating that clinical and biological factors may play a larger role. Interventions targeting disability in cardiac disease should include biomedical and rehabilitative strategies while continuing to address social health as a parallel concern.