Background: Plants for Joints (PFJ) is a multidisciplinary intervention centered around a whole-food plant-based diet, physical activity, and sleep and stress management. The PFJ intervention successfully improved disease activity and symptoms in people with rheumatoid arthritis (RA) or osteoarthritis (OA), respectively, and metabolic health. To investigate how these effects were achieved a mixed methods process evaluation was conducted to understand the context, implementation, and mechanism of impact of the PFJ intervention. Also, the relationship between degree of implementation and lifestyle changes was explored.

Methods: Quantitative and qualitative data were collected across the evaluation domains context (i.e. reach), implementation (i.e. recruitment and delivery), and mechanism of impact (i.e. responsiveness) of both the participants and coaches (incl. dietitians, sport coaches) according to the UK MRC guidelines for process evaluations. Data was collected from the participants via focus groups and questionnaires after the intervention, and interviews with coaches. Qualitative data were analyzed thematically, and quantitative data were assessed with descriptive statistics and linear regression analyses. Degree of implementation was quantified using a theory-driven implementation index score composed of different process evaluation constructs.

Results: Of the 155 participants who participated in the PFJ intervention, 106 (68%) took part in the questionnaire and 34 (22%) attended a focus group. Participants felt the intervention was complete, coherent, and would recommend the intervention to others (mean score 9.2 (SD 1.4) out of 10). Participants felt heard and empowered to take control of their lifestyle and health outcomes. Components perceived as most useful were self-monitoring, social support, practical and theoretical information, and (individual) guidance by the multidisciplinary team. Participants perceived the intervention as feasible, and many indicated it effectively improved their health outcomes. In an explorative analysis there was no significant difference in healthy lifestyle changes across implementation index score groups.

Conclusion: This process evaluation offers important insights into why the PFJ intervention works and how the intervention can be optimized for future implementation. Results indicating the intervention's high satisfaction, feasibility, and perceived effectiveness, further support the use of plant-based lifestyle interventions as an additional treatment option for patients with RA, OA, or other chronic diseases.

Trial registration: International Clinical Trial Registry Platform numbers: NL7800, NL7801, and NL7802, all registered 17-06-2019.