OUTSIDE 2: Outdoor swimming as a nature-based intervention, offered in addition to usual care, compared to usual care alone, in people with depression: A study protocol for a randomised controlled trial and economic evaluation

Background: Major depression affects at least 10% of adults, yet evidence-based treatments have modest clinical effectiveness and acceptability. In recent years, outdoor swimming has grown increasingly popular and emerging quantitative and qualitative evidence suggests potential as an intervention for depression, however, there is yet to be a full-scale randomised controlled trial (RCT). This is a protocol to assess the safety and test the clinical- and cost-effectiveness of an 8-session outdoor swimming course (in addition to usual care) on depression symptom severity in adults experiencing major depression, in comparison to usual care alone.

Methods: This study is a pragmatic, parallel group, superiority RCT with 1:1 allocation comparing the outdoor swimming intervention (in addition to treatment as usual) with treatment as usual, aiming to recruit 480 adult participants meeting diagnostic criteria for major depression. Recruitment will take place across 21 sites with blind post treatment and follow up assessments. The primary outcome is depression symptom severity at T1 post-intervention, 12 weeks post-randomisation (the primary end point) using the Patient Health Questionnaire 9 (PHQ-9). Secondary clinical outcomes are anxiety (Generalised Anxiety Disorder 7 at T1 and T2 [38 weeks post-randomisation] and PHQ-9 at T2), mindfulness is measured as a potential mechanism at all timepoints (Five Facet Mindfulness Questionnaire 15). Health economic measures at all time points are: EQ-5D-5L, Recovering Quality of Life (ReQoL), Client Service Receipt Inventory and the Productivity Cost Questionnaire. A qualitative study will examine the experience of participants during and after the swim course.

Discussion: If the 8-session outdoor swim course is safe, clinically- and cost-effective, findings will support national implementation, offering an evidence-based intervention to those affected by depression, while potentially reducing healthcare costs. Furthermore, this may pave the way for other outdoor activities to support people with poor mental health to be developed and evaluated as interventions.

Trial registration: Controlled trial registration number is ISRCTN registration number 24759023. Registered on 21 February 2024 (https://www.isrctn.com/ISRCTN24759023).