Abstract
Mental health problems are associated with lower quality of life, increased unscheduled care, high economic and social cost, and increased mortality. Nature-based interventions (NBIs) that support people to engage with nature in a structured way are asset-based solutions to improve mental health for community based adults. However, it is unclear which NBIs are most effective, or what format and dose is most efficacious. We systematically reviewed the controlled and uncontrolled evidence for outdoor NBIs. The protocol was registered at PROSPERO (CRD42020163103). Studies that included adults (aged ≥18 years) in community-based settings with or without mental and/or physical health problems were eligible for inclusion. Eligible interventions were structured outdoor activities in green and/or blue space for health and wellbeing. We searched ASSIA, CENTRAL, Embase, Greenfile, MEDLINE, PsycINFO, and Web of Science in October 2019; the search was updated in September 2020. We screened 14,321 records and included 50 studies. Sixteen studies were randomised controlled trials (RCTs); 18 were controlled studies; and 16 were uncontrolled before and after studies. Risk of bias for RCTs was low to moderate; and moderate to high for controlled and uncontrolled studies. Random effects meta-analysis of RCTs showed that NBIs were effective for improving depressive mood −0.64 (95% CI: 1.05 to −0.23), reducing anxiety −0.94 (95% CI: 0.94 to −0.01), improving positive affect 0.95 (95% CI: 0.59 to 1.31), and reducing negative affect −0.52 (95% CI: 0.77 to −0.26). Results from controlled and uncontrolled studies largely reflected findings from RCTs. There was less evidence that NBIs improved physical health. The most effective interventions were offered for between 8 and 12 weeks, and the optimal dose ranged from 20 to 90 min. NBIs, specifically gardening, green exercise and nature-based therapy, are effective for improving mental health outcomes in adults, including those with pre-existing mental health problems.
Highlights
Mental health disorders are the third leading cause of years lived with disability, with a global prevalence of greater than 10% (James et al, 2018)
When analysed by intervention sub-group we showed that green exercise interventions in mixed populations, including older adults with long-term conditions, were associated with large and significant effects (k=5; n=886; standardised mean difference (SMD)=−0.65; −1.16 to −0.13; I2=88.4%, p=0.000)
There was mixed evidence from two small randomised controlled trials (RCTs) that gardening interventions are effective at reducing depressive mood, but the effects were non-significant (k=2; n=96; SMD=−0.51; −1.92 to 0.90; I2=84.2%, p=0.012)
Summary
Mental health disorders are the third leading cause of years lived with disability, with a global prevalence of greater than 10% (James et al, 2018). The lifetime prevalence for major depressive disorders in the general population in the United States has increased during the past 25 years and is estimated to be 20.6% (Hasin et al, 2018). Generalised anxiety disorder is a relatively common mental health problem with a lifetime prevalence of 5.7% (Kessler et al, 2005). In England less severe and common mental health problems affect about 1 in 6 people (Bebbington et al, 2020). Lost productivity associated with common mental health problems such as depression and anxiety is estimated at US $1 trillion each year (Chisholm et al, 2016). In the UK, mental health problems are among the main reasons for sickness absence (Office for National Statistics, 2021b) and the social and economic cost of mental health in England has grown in the last decade to £119 billion a year (Centre for Mental Health, 2020b)
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