Outdoor physical activity, residential green spaces and the risk of dementia in the UK Biobank cohort.

Background: How the environment in which physical activity takes place influences brain health remains insufficiently studied. We aimed to investigate the association between outdoor physical activity and dementia in older adults, and to examine whether residential green space (GS) modifies this association.

Methods: This prospective longitudinal study used UK Biobank data from 187,724 adults aged 60-73 years at baseline (2006-2010) and 36,854 with brain Magnetic Resonance Imaging (2014-2021). Outdoor activity (via Metabolic Equivalent of Task) and GS (within 300 m of homes) were assessed, with dementia incidence tracked through electronic records until December 2022. Neuroimaging markers included hippocampal and total gray-matter volumes, and white-matter hyperintensity volume. Cox proportional hazards and linear regression models were applied.

Results: During a mean follow-up of 13.01 years, 7218 participants (47.1% female) developed dementia. After adjusting for covariates, higher outdoor activity is associated with a lower risk of all-cause dementia (HR_{top vs bottom quartile} = 0.84, 95% Confidence Interval [CI] 0.78-0.90). This association is most pronounced for vascular dementia (HR:0.72, 95%CI 0.63-0.83) and is stronger among those living in areas with high, compared to low, residential GS (0.73, 95%CI 0.63-0.85 vs 0.86, 95%CI 0.79-0.93, p_interaction = 0.04). Higher outdoor activity is also associated with higher hippocampal and total gray-matter volumes and fewer white-matter hyperintensities. A combined effect of outdoor activity and GS on hippocampal volume is observed.

Conclusions: Outdoor activity is linked to lower dementia risk, particularly in those living in more accessible GS. These findings underscore the importance of urban planning that prioritizes accessible GS to promote brain health in the aging population.