Why might urban tree canopy reduce dementia risk? A causal mediation analysis of 109,688 adults with 11 years of hospital and mortality records

Dementia affects over 44 million people globally (Nichols et al., 2019) and currently has no effective treatment or known cure (Gauthier et al., 2016). Meta-analyses indicate many modifiable risk factors for dementia including physical inactivity (Xu et al., 2017), loneliness (Lara et al., 2019), insufficient sleep (Shi et al., 2018), depression (Cherbuin et al., 2015), and cardiometabolic diseases including heart disease (Wolters et al., 2018) and diabetes (Xue et al., 2019). Mechanisms under investigation often focus on β-amyloid, of which the abnormal build-up over decades is concomitant with neurodegeneration consistent with a diagnosis of dementia (Golde et al., 2018). For example, findings indicate higher levels of physical inactivity may increase β-amyloid over time (Brown et al., 2013; Rabin et al., 2019; Stillman et al., 2017). Increased β-amyloid is also associated with short or poor sleeps (Brown et al., 2016; Spira et al., 2013; Sprecher et al., 2015), diabetes and hypertension (Rodrigue et al., 2013; van Arendonk et al., 2023). Actions to reduce these risk factors may therefore help to reduce the burden of dementia potentially via preventing or delaying abnormal accumulation of β-amyloid.

Growing evidence links investments in environmental characteristics such as green space with reduced risks of dementia in Australia (Astell-Burt et al., 2020a), Canada (Paul et al., 2020; Yuchi et al., 2020) and the US (Aitken et al., 2021; Slawsky et al., 2022; Wu and Jackson, 2021). Recent evidence points to tree canopy as having a potentially key role for cognitive health (Astell-Burt and Feng, 2020b; Astell-Burt et al., 2020a), as well as for potential cardiometabolic (Astell-Burt and Feng, 2020c; Feng et al., 2022a; Moreira et al., 2020; Donovan et al., 2015), psychosocial (Astell-Burt and Feng, 2019; Astell-Burt et al., 2022c; Feng et al., 2022b; Jiang et al., 2020, Kardan et al., 2015), and behavioural (Astell-Burt and Feng, 2020a; Feng et al., 2021; Shin et al., 2020) pathways, relative to other forms of green space (e.g. open grass). This operates through domains of pathways that restore depleted cognitive capacities and relieving chronic stress, strengthening capacities for health-related behaviours, and mitigating harms from excess heat and air pollution (Astell-Burt et al., 2022b; Markevych et al., 2017). For example, systematic reviews report links between green space with reduced risks of cardiometabolic diseases including diabetes and cardiovascular diseases (Liu et al., 2022; Twohig-Bennett and Jones, 2018), loneliness (Astell-Burt et al., 2022b), physical inactivity (Remme et al., 2021), and some forms of mental illness (Kotera et al., 2021; Roberts et al., 2019).

The aim of our study was to strengthen the findings pertaining to tree canopy by conducting a causal mediation analysis of the multiple putative pathways outlined in a published conceptual model (Astell-Burt and Feng, 2020b), building upon our earlier work that reported lower dementia risk with higher levels of urban tree canopy (Astell-Burt et al., 2020a).

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