Nature prescriptions: a scoping review with a nested meta-analysis

Background: "Nature prescriptions" are gaining popularity as a form of social prescribing and in response to calls for sustainable healthcare. Our review and meta-analysis appraised evidence of effectiveness of nature prescriptions on various health outcomes. In doing so, we sought to determine the factors that are critical for the success of nature prescriptions, based on Social Cognitive Theory. Methods This is a scoping review with a nested meta-analysis for a subset of outcomes. Five databases were searched up to July 25, 2021. Randomised and non-randomised controlled studies featuring a nature prescription (i.e. an instruction or organised programme, by a health or social provider, to promote spending time in nature) are included. All health outcomes are eligible, but only key pre-specified outcomes are qualified for meta-analysis. Two reviewers independently conducted all steps of study selection; one reviewer conducted data collection and risk of bias assessment. Summary data was extracted from published reports for analysis. Random-effect models for meta-analysis were conducted using Review Manager 5.4.1. Findings We identified 86 unique studies (116 reports), of which 26 studies contributed data to meta-analysis. Compared to control, nature prescription programmes resulted in a greater reduction in systolic blood pressure (MD = -4.9mmHg [-9.6 to -0.1], I2=65%) and diastolic blood pressure (MD = -3.6mmHg [-7.4 to 0.1], I2=67%). They also had a moderate-to-large effect on depression scores (SMD=0.5 [0.2 to 0.8], I2=79%) and anxiety score (SMD=0.6 [0.1 to 1.2], I2=90%). Lastly, they resulted in a greater increase in daily step counts (MD = 900 steps [790-1010], I2=0%), but did not improve weekly time of moderate physical activities (MD = 25.9 minutes [-10.3 to 62.1], I2=53%). Most studies have moderate to high risk of bias, principally due to non-blinding nature of the interventions, small sample size and lack of analysis plan to rule out risks of bias. Interpretation Nature prescription programmes may provide cardiometabolic and mental health benefits and increase physical activity. Effective nature prescription programmes can select from a range of natural settings, activities and might be implemented via social and community channels, besides health providers. The Social Cognition Theory is useful in designing future nature prescription programmes.