Social network size and mental health among older adults in Japan: A nationwide cross-sectional analysis of the JACSIS 2024 study

INTRODUCTION: Social relationships are crucial for healthy ageing, but the role of social network size in older adults' mental health remains underexplored in nationwide studies. We examined associations between social network size and mental health in a national sample of older Japanese adults. METHODS: We analysed cross-sectional data from the 2024 Japan COVID-19 and Society Internet Survey (JACSIS), a nationwide internet survey, including 6786 respondents aged 65-84 years. Mental health outcomes were poor mental health days in the past 30 days (any vs none; ≥7 vs < 7 days) and psychological distress (moderate: K6 ≥5; severe: K6 ≥13). Social network size was measured using the Lubben Social Network Scale (LSNS-6: family/friends; LSNS-8: including professional contacts) and categorized into tertiles. Logistic regression estimated odds ratios (ORs) adjusted for demographic, socioeconomic, health, lifestyle, and work-related factors. RESULTS: Overall, 11.8% reported any poor mental health day, 16.3% had moderate distress, and 1.9% had severe distress. In fully adjusted models, smaller social network size was consistently associated with poorer mental health in graded patterns. Compared with the highest LSNS-6 tertile, the lowest tertile had lower odds of no poor mental health days (OR = 0.80, 95% CI: 0.70-0.92) and higher odds of moderate (OR = 2.74, 95% CI: 2.28-3.29) and severe distress (OR = 6.54, 95% CI: 3.42-12.51). Similar patterns were observed for LSNS-8. CONCLUSIONS: Larger social networks were associated with fewer poor mental health days and lower psychological distress among older Japanese adults.