Towards a Programme Theory for Nature-Based Health Interventions for People with Mild to Moderate Anxiety, Depression, and/or Stress: Identifying Target Group, Professionals, Mechanisms and Outcomes Through a Delphi-Based Study

This study aims to identify target group, professionals, mechanisms, and outcomes of nature-based health interventions (NBHIs) for people with mild to moderate anxiety, depression, and/or stress. A Delphi-based study was conducted to explore core components of NBHIs. Thirteen researchers with expertise related to the target group responded in Round 1, and eleven in Round 2. Respondents rated statements on a 7-point Likert scale and prioritised core components regarding target group, professionals, mechanisms, and outcomes. A thematic analysis was applied to synthesise qualitative responses.Consensus was achieved on 12 of 21 items across four domains: target group, professionals, mechanisms, and outcomes. Highest agreement concerned core mechanisms (nature interaction, social community, and physical activity), outcome priorities (mental wellbeing and quality of life), and professional competencies. Greater variation was observed regarding group composition and team delivery. Analysis of qualitative expert responses highlighted four key themes: (1) Balancing Group Composition, (2) Adapting Competencies to Context, (3) Core Mechanisms for Change, and (4) Weighing Perspectives in Outcome Selection.By setting out guiding principles for a programme theory, the study lays the foundation for the design and implementation of context-adapted NBHIs. The study underscores the complexity of developing and evaluating NBHIs, highlighting the need to approach them as complex interventions and contributing to a paradigm shift towards a new era of a bio-psycho-social health perspective.