Social Isolation and Health Outcomes in Japan: A Nationwide Cross-Sectional Study.

Introduction: Social isolation is an important social determinant of health, but its relationship with health-related outcomes remains incompletely understood. This study examined whether structural social isolation is associated with subjective health, symptom burden, chronic disease diagnoses, and healthcare-seeking behaviors in a nationwide Japanese population. Methods: We conducted a nationwide cross-sectional study using data from the 2024 Japan coronavirus disease 2019 and Society Internet Survey, including adults aged 18-79 years. Social isolation was assessed using the Lubben Social Network Scale-6. Primary outcomes were poor self-rated health and high symptom burden, defined by the Somatic Symptom Scale-8. Secondary outcomes included self-reported chronic diseases and healthcare-seeking behaviors. Results: Among 25,014 participants (mean age: 50.0 years; 49.9% men), 59.3% were classified as socially isolated. Social isolation was associated with higher odds of poor self-rated health (adjusted odds ratio [AOR]: 1.54, 95% confidence interval [CI]: 1.32-1.81) and high symptom burden (AOR: 1.23, 95% CI: 1.09-1.38). In contrast, social isolation was not associated with a higher prevalence of most chronic diseases. Socially isolated individuals were also less likely to have a regular doctor or receive preventive services. Conclusions: Structural social isolation was associated with poorer subjective health and greater symptom burden but not with a higher prevalence of diagnosed chronic diseases. These findings suggest that reliance on diagnosis-based indicators alone may underestimate the health needs of socially isolated individuals, even in healthcare systems with broad accessibility.