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

This study protocol outlines a pragmatic, multi-site randomized controlled trial designed to evaluate the safety, clinical effectiveness, and cost-effectiveness of an 8-session outdoor swimming intervention added to usual care versus usual care alone for adults with major depression.

The Gist

Imagine your mind is a garden. Sometimes, that garden gets overrun with weeds (depression). For a long time, doctors have offered two main tools to help: chemical fertilizers (medication) and talking to a gardener (therapy). These tools work, but sometimes the weeds grow back, or the garden just doesn't feel quite right.

Now, imagine a new tool: taking the garden outside to the ocean.

This paper is the "blueprint" (a study protocol) for a massive experiment called OUTSIDE 2. The researchers want to find out if simply swimming in cold, open water (like the sea, lakes, or rivers) can help clear the weeds from the garden of depression better than just sticking to the usual treatments.

Here is the plan, broken down into simple parts:

1. The Big Question

Can an 8-week course of outdoor swimming, done alongside your normal doctor visits, help people with depression feel better? And is it worth the money?

2. The Experiment (The "Race")

Think of this like a race with two teams of 480 runners (people with depression).

• Team A (The Swimmers): They get their usual care (meds, therapy, etc.) PLUS an 8-week outdoor swimming club. They meet once a week for an hour with a coach who knows how to keep them safe and help them feel brave in the water.
• Team B (The Control): They get only their usual care. They don't swim during the study, but the researchers promise that once the study is over, they will get the chance to join the swimming club too. This ensures no one feels left out.

3. The Rules of the Game

You can't just jump into the ocean if you have certain health issues, so there are strict safety rules:

• The "No-Go" List: If you are at high risk of hurting yourself, have a heart condition that cold water might trigger, or have severe asthma that gets worse in the cold, you can't join. Safety is the number one priority.
• The "Swim" Test: You need to be able to swim a little bit (like 25 or 50 meters) in a warm pool to prove you won't sink in the cold water.
• The Coach: Every swim group has a trained coach and a lifeguard. They aren't teaching you to be an Olympic swimmer; they are teaching you how to be comfortable and safe in nature.

4. How They Measure Success

The researchers aren't just guessing if people feel better. They are using a "ruler" to measure the changes:

• The Depression Score: They use a standard 9-question quiz (PHQ-9) to see how heavy the "garden weeds" feel. They check this before the study, right after the 8 weeks, and again 6 months later.
• The Anxiety Score: They also check how worried people feel.
• The "Mindfulness" Score: They check if people feel more "present" and calm.
• The Wallet Check: They track how much money is spent on healthcare and how much time people miss from work. They want to know if this swimming thing saves the NHS (the UK health system) money in the long run.

5. The "Secret Sauce" (Why this study is special)

In the past, people have talked about how cold water makes them feel amazing. But this is the first time scientists are doing a huge, scientific test to prove it actually works for depression.

• Real People, Real Places: They are testing this in 21 different spots across England, from the sea to lakes to outdoor pools.
• Listening to Voices: They aren't just looking at numbers. They are asking people to keep diaries and talk about their feelings. They want to know why the water helps. Is it the cold shock? The group feeling? The connection to nature?

6. The Goal

If the study proves that swimming is safe and helps people feel better, the researchers hope to change the rules of the game. They want to make outdoor swimming a standard "prescription" that doctors can give to people with depression, just like they prescribe pills or therapy.

In short: This paper is the map for a journey to see if getting wet in the cold ocean can dry out the tears of depression. If it works, it could be a game-changer for mental health care everywhere.

Technical Summary

Based on the provided document, here is a detailed technical summary of the OUTSIDE 2 study protocol.

1. Problem Statement

• Clinical Burden: Major depression affects at least 10% of adults globally. Current evidence-based treatments (pharmacotherapy, psychological therapies) often show modest clinical effectiveness and limited adherence.
• Emerging Interest: There is growing anecdotal and preliminary evidence suggesting that "blue exercise" (outdoor swimming) offers significant mental health benefits, including reduced depression and anxiety, and enhanced connection to nature.
• Evidence Gap: Despite the popularity of outdoor swimming, no full-scale Randomised Controlled Trial (RCT) has been conducted to rigorously evaluate its clinical effectiveness, safety, and cost-effectiveness as an adjunctive treatment for major depression.
• Previous Findings: A prior feasibility study (OUTSIDE 1) demonstrated high recruitment/retention rates and encouraging effect sizes (Cohen's d = 0.71) but was not powered to detect statistically significant differences.

2. Methodology

The study is designed as a pragmatic, parallel-group, superiority Randomised Controlled Trial (RCT) with an embedded economic evaluation and qualitative component.

• Study Design: 1:1 allocation comparing an 8-session outdoor swimming course (plus Treatment as Usual [TAU]) against TAU alone.
• Participants:
  • Target Sample: 480 adults (240 per arm) experiencing a current episode of major depression.
  • Inclusion Criteria: PHQ-9 score ≥ 8, meeting DSM-5 criteria for Major Depressive Episode (via MINI 7.0.2), aged 18+, and possessing basic swimming competence (varies by site: 50m for sea, 25m for lakes, none for heated lidos).
  • Exclusion Criteria: High suicide risk, active psychosis, severe cardiac/respiratory conditions contraindicating cold water, BMI < 17, and participation in other interventional trials.
• Intervention:
  • Structure: 8 weekly sessions (approx. 1 hour each) led by qualified open-water coaches trained in mental health first aid.
  • Content: Focuses on safe, enjoyable engagement with water rather than technique or fitness. Includes pre-swim safety briefings and group activities.
  • Setting: 21 sites across England (sea, rivers, lakes, lidos) during spring/summer (April–September) when water temperatures are 14°C–24°C.
• Control: Usual Care (TAU) alone. Participants in the control arm are offered the swimming course after the trial concludes.
• Outcomes & Measures:
  • Primary Outcome: Change in depression symptom severity (PHQ-9) from baseline (T0) to 12 weeks post-randomisation (T1).
  • Secondary Clinical Outcomes: Anxiety (GAD-7), depression at 38 weeks (T2), and mindfulness (FFMQ-15).
  • Economic Outcomes: Cost-utility analysis (QALYs via EQ-5D-5L and ReQoL-10) and cost-effectiveness (PHQ-9 reduction). Data collected via Client Service Receipt Inventory (CSRI) and productivity questionnaires.
  • Qualitative: Participant diaries, "swim-along" interviews (GoPro), and post-course interviews to explore mechanisms and experiences.
• Statistical Analysis:
  • Primary Analysis: Intention-to-Treat (ITT) using linear mixed-effects models with random intercepts for site/swim group.
  • Sensitivity Analyses: Complier Average Causal Effect (CACE) analysis for adherence; subgroup analyses by gender, ethnicity, and location.
  • Power: Calculated to detect a Minimal Clinically Important Difference (MCID) of 1.7 on the PHQ-9, accounting for clustering (ICC 0.01) and 25% attrition.

3. Key Contributions

• Rigorous Evidence Generation: This is the first definitive, large-scale RCT to test outdoor swimming as a specific intervention for major depression, moving beyond feasibility to definitive efficacy testing.
• Economic Evaluation: Includes a comprehensive health economic analysis to determine if the intervention is cost-effective for the NHS and society, addressing a critical gap in nature-based interventions.
• Safety Framework: Establishes strict eligibility criteria and safety protocols (qualified coaches, lifeguards, medical screening) to mitigate the inherent risks of cold-water immersion.
• Patient and Public Involvement (PPI): Extensive use of a Lived Experience Advisory Panel (LEAP) to co-design recruitment strategies, materials, and dissemination plans, ensuring the study is accessible and relevant to underserved communities.
• Mixed-Methods Approach: Combines quantitative clinical data with qualitative insights to understand how and why the intervention works (e.g., mechanisms like mindfulness, belonging, and facing challenges).

4. Results

• Status: As this document is a study protocol, no empirical results (data analysis or outcomes) are yet available.
• Timeline: Recruitment began in March 2024 and is planned to continue until June 2025, with follow-up data collection extending to February 2026.
• Expected Output: The study aims to produce peer-reviewed publications on clinical effectiveness, cost-effectiveness, and qualitative findings once the trial is complete.

5. Significance

• Clinical Impact: If proven effective and safe, the study could lead to the national implementation of outdoor swimming as a standard, evidence-based adjunctive treatment for depression within the NHS, offering a low-cost, accessible alternative to traditional therapies.
• Policy & Economics: Positive cost-effectiveness findings could justify public funding for "social prescribing" of outdoor swimming, potentially reducing long-term healthcare costs and improving workforce productivity.
• Broader Application: The methodology and findings could pave the way for evaluating other nature-based interventions (green/blue space activities) for mental health.
• Equity: By specifically targeting diverse recruitment (including ethnic minorities, men, and underserved communities) and offering bespoke courses, the study aims to address health inequalities in accessing nature-based mental health support.