Association of greenspace and natural environment with brain volumes mediated by lifestyle and biomarkers among urban residents

Objectives: To examine the association between environmental measures (greenspace and natural environment) and brain volumes and potential mediators. Study design: Prospective study. Methods: Analysis included 34,454 UK Biobank participants (53.4% female) aged 40–73 years at baseline (2006–2010). Brain volumes were measured by MRI between 2014 and 2019. Results: Greater proximity to greenspace (1000 m buffer) at baseline was associated with larger total brain (standardized β per 10% increment: 0.013; 95% CI: 0.005–0.020), grey matter (0.013; 95% CI: 0.006–0.020), and white matter (0.011; 95% CI: 0.004–0.017) volumes after adjustment for covariates and air pollution. Corresponding estimates for natural environment (1000 m) were 0.010 (95% CI: 0.004–0.017), 0.009 (95% CI: 0.004–0.015), and 0.010 (95% CI: 0.004–0.016). Similar associations were observed for 300 m buffers. Smoking and physical activity mediated associations between greenspace and brain volumes. Among biomarkers, vitamin D, red blood cell indices, and creatinine were important mediators. Significant mediators combined explained 18.5% (95% CI: 13.2–25.3%) of the association with total brain volume and 32.9% (95% CI: 22.3–45.7%) with grey matter. For natural environment, mediators combined explained 20.6% (95% CI: 14.7–28.1%) of the association with total brain volume. Conclusions: Higher coverage of greenspace and natural environment may benefit brain health by promoting healthier lifestyles and improving biomarkers, including vitamin D and red blood cell indices.