Homogeneity and heterogeneity in social networks and brain aging in community-dwelling older adults in Japan: The NEIGE Study

BACKGROUND: As the global population ages, understanding factors influencing structural brain aging is crucial for dementia prevention. Social networks have emerged as important social factors; however, longitudinal evidence linking specific structural characteristics of social networks to brain volume change remains limited. This study examined whether social network homogeneity and heterogeneity were related to longitudinal changes in brain volume (hippocampus, gray matter, and prefrontal cortex) among community-dwelling older Japanese adults. METHODS: Data were obtained from the Neuron to Environmental Impact Across Generations (NEIGE) study. Participants aged 65-84 years were randomly selected from Tokamachi City, Japan. Of the 527 older adults surveyed at baseline, 279 who underwent magnetic resonance imaging at both baseline (2017) and follow-up (2021) were included. Social relationship (dis)similarities were assessed using ten items, and network homogeneity and heterogeneity domains were derived through factor analysis. Covariates included sociodemographic factors, health behaviors, health conditions, and apolipoprotein E genotype. RESULTS: Participants were 47.6% male, with a mean age of 73.2 years (SD = 5.1). Multiple linear regression analyses with inverse probability weighting indicated no association between homogeneous social networks and brain volume changes. However, lower network heterogeneity was significantly associated with a greater decline in hippocampal volume. This was particularly pronounced in the right hippocampus. CONCLUSION: Greater social network heterogeneity may attenuate age-related hippocampal volume loss in older adults, highlighting potential benefits of social network heterogeneity for structural brain aging and cognitive aging in later life. Further studies are warranted to clarify the underlying mechanisms.