Association between intra-household isolation and frailty among community-dwelling older adults in Japan: A cross-sectional analysis of the Wako Cohort Study

BACKGROUND: Frailty is a growing public health concern, with emerging evidence highlighting the importance of social determinants, including external and intra-household isolation. This study examined the association between intra-household isolation and frailty among community-dwelling older adults in Japan and explored whether external social connections modified this association. METHODS: Data were obtained from the 2023 baseline survey of the Wako Cohort Study, including 6,429 participants aged ≥65 years in Wako City, Saitama Prefecture, without long-term care certification levels 3-5 (men: 45.0%; mean age: 76.5 years). Intra-household isolation was defined as living with others but having minimal interaction (talking <15 min/day and spending most time alone at home). Frailty was assessed using the 25-item Kihon Checklist. Modified Poisson regression analysis estimated the associations, adjusting for sociodemographic factors, health behaviors, health conditions, and external social connections. RESULTS: Intra-household isolation was observed in 5.3% of participants, while 21.0% lived alone. Frailty was observed in 28.3% of participants. Individuals with intra-household isolation were more likely to be frail than non-isolated individuals (prevalence ratio [95% confidence interval]: 1.42 [1.26-1.59]). This association remained significant when compared with individuals living alone (1.34 [1.16-1.54]) and was stronger among those aged 65-74 years than those aged ≥75 years. No significant interaction was observed between intra-household isolation and external social connections. CONCLUSION: Intra-household isolation among cohabiting older adults was independently associated and showed a stronger association with frailty than living alone. These findings suggest that living with others does not necessarily have protective effects unless meaningful interactions exist.