Evaluating seven nature-based therapy interventions to reduce stress in at-risk adults: protocol for a multicentre experimental study across four European countries (GreenME Project)

Background Mental health conditions are widespread over the society and stigma exacerbates mental health impact on the individual. Nature-based therapies (NBTs) show potential to improve mental health and could have a role in modifying the links between stigma and mental health, but robust evidence remains limited. This paper describes a multi-country experimental study protocol designed to evaluate whether diverse NBTs are superior to usual care in reducing stress among adults at risk of developing mental health conditions. Secondary objectives include examining superiority in improving quality of life and subjective wellbeing, the durability of effects, exploring whether NBT participation modifies the relationship between stigma and mental health, assessing cost-effectiveness, and investigating nature connectedness as a potential mechanism. Methods Seven interventions are included: two in Spain, two in the United Kingdom, two in Italy, and one in Sweden. Participants are adults experiencing moderate-to-high perceived stress. Participants are randomized to NBTs plus unrestricted usual care or unrestricted usual care alone. Interventions involve non-clinical activities with direct contact with nature (gardens, forests, or the ocean), typically lasting 120 minutes over 12 consecutive weeks and facilitated by trained practitioners. Usual care varies by context. Data is collected at screening, baseline, post-intervention, 1-month and 3-months follow-ups. Measures include stress – as the primary outcome-, quality of life and subjective wellbeing – as the secondary outcomes-, exposure to green care, nature connectedness, internalized stigma and covariates. Primary analyses will follow the intention-to-treat principle, with intervention-level analyses followed by individual-level meta-analyses. Secondary analyses will also examine mechanisms, effect modification, and cost-utility and cost-effectiveness analyses. Discussion This study will generate robust evidence on health effects and cost-effectiveness of NBTs. Findings can support scaling up NBTs effectively for mental health.