Background: Social connections are important for brain health. We explored the associations between positive and negative social connections and the rate of decline in brain health with ageing.

Methods: We analysed UK Biobank data from 5704 adults aged 40+ (Wave 1 Mage = 54.12, 51.19% female) with brain scans at waves 2 and 3. Predictor variables were positive (current household size, visits to friends/family, community engagement, having a confidante) and negative (loneliness, violence in romantic relationships, or belittling in romantic relationships) social connections at baseline. Outcome variables were brain volumes (total grey & white matter, hippocampus, amygdala) and white matter health (DDF- white matter integrity, white matter hyperintensities, PSMD- a marker of microstructural white matter changes) at waves 2 and 3. We ran linear mixed models controlling for age, sex, intracranial volumes (for brain volume analyses), physical activity, depression, alcohol use, smoking, education, pollution, hearing loss, BMI andhypertension.

Results: Having a greater household size was associated with a slower rate of decline in volumes of total white matter (3160.08, 95% CI: 418.75, 5888.47), right amygdala (43.18, 95% CI: 14.70, 71.02), left hippocampus (62.96, 95% CI: 18.20, 108.04), and right hippocampus (61.02, 95% CI: 15.39, 108.36), and white matter integrity/DDF (0.0014, 95% CI: 0.00047, 0.0022). Loneliness was associated with a slower rate of decline in the left amygdala (81.48, 95% CI: 19.59, 145.91).

Conclusions: In a large UK based sample, we found that living with more people was associated with slower decline in white matter volumes and integrity, and hippocampal volumes. Living with others may promote brain reserve and memory function. Loneliness was associated with slower decline in the left amygdala volume, which is associated with processing of aversive and fearful stimuli. This echoes previous findings indicating that people with depression and anxiety may experience less shrinkage in the amygdala. Our results highlight the importance of encouraging people to live with others, such as in intergenerational households. We also need to address loneliness at a population level to promote healthy brain ageing.