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

Abstract 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 ). Administrative information Note: Numbers enclosed in braces within this protocol correspond to SPIRIT checklist item numbers. The sequence of items has been adjusted to group related content together. ( https://www.consort-spirit.org/ ). Plain Language Summary Background Depression is very common,with at least 1 in 10 people having an episode of depression during their lifetime. Many people believe that outdoor swimming can improve depression. There is some research that suggests outdoor swimming is helpful for depression, but we do not yet have enough evidence to be sure. OUTSIDE 2 is a large research study that aims to find out whether outdoor swimming can help people with depression. Aim The aim of this study is to find out if taking part in an 8-session outdoor swimming course, along with usual care, is safe, helps improve depression, and is good value for money. We will compare people who do the swimming course (alongside their usual care) with people who only receive their usual care. In our study we will also talk to participants about their experiences. We want to understand how the outdoor swimming course affects their depression, and how the activity itself might help them feel better. Describe your research plan, including People who are interested in taking part in the study can visit our website (outside2.co.uk) to read more information and sign up. Once someone agrees to join, they will be randomly placed into one of two groups available at their location to keep the study fair. One group will take part in eight one-hour outdoor swimming sessions in a small group. They will continue with their usual care for depression, which may include talking therapies, antidepressant medication, or community activities. The other group will continue with usual care only during the study. After the study ends, they will be offered the same outdoor swimming course, so no one misses out. The swimming sessions will be led by experienced swimming coaches who will help participants build confidence in the water and learn important water safety skills. During the study, all participants will complete questionnaires about their symptoms of depression, overall mental health, and use of health services. They will do these before they are put in a group, right after the swimming course or usual care period and six months later. Participants in the swimming group will also be asked to keep a diary about their experiences during the course. Researchers may visit some sessions to ask participants about how they are finding the swimming and how it affects their mood. Swim coaches will record attendance at each session and describe what activities were included. This helps the research team understand exactly what took place during each class. Knowledge mobilisation We want to make sure that the information we learn from this study reaches many different people, not just scientists. To do this, we will share our findings in several ways. We will create a Podcast mini-series, an animated video as well as research papers. Using these different methods helps us share our findings with many groups, including adults in the community, mental health professionals, people working in the swimming industry, scientists, and policymakers. By sharing the results in several ways, more people can understand if and how outdoor swimming might support recovery from depression and mental health more broadly, and it increases the chance that the study will make a real difference, whatever it finds.