Patients with chronic somatic diseases such as obesity often develop comorbid depressive symptoms. E-mental health interventions are an innovative and effective treatment option within a stepped care approach. Studies have shown that acceptance and adherence are higher when they are tailored to the specific needs of the target group. This study protocol describes a randomised controlled trial (RCT) of an internet-based self-help intervention, Fit4Mood, to improve mental health in the high-risk group of adults with obesity. The objective of the @ktivPLUS research project is to evaluate the effectiveness, acceptability and cost-effectiveness of the intervention in comparison to an online bibliotherapy.

Eligible individuals will be randomly allocated to an intervention group (access to an internet-based intervention) or to an active control group (access to an online bibliotherapy). Assessments will be conducted before the start of the intervention (baseline (BL)) and 4 months after BL (follow-up (FU)). The primary outcome is the reduction in depressive symptoms (Beck Depression Inventory-II) in n=190 participants. Secondary outcomes are anxiety, quality of life, activity, self-efficacy, resilience, mental and digital health literacy, stress, sleep quality, weight loss, weight management activities and readiness to lose weight, weight self-stigma, uptake, adherence and satisfaction with the intervention, workability and cost-effectiveness at follow-up. Additionally, sociodemographics, health, comorbidities and disabilities, as well as internet-specific information, will be assessed at BL. Intention-to-treat analysis using generalised linear mixed models will be applied.

Approval for this study has been granted by the ethics committee of the University of Leipzig (ID: 140/25-ek). All participants will provide informed consent prior to participation in the study. Results will be disseminated in peer-reviewed journals and presented at national and international conferences. In the case of a successful evaluation, the internet-based self-help intervention Fit4Mood will be provided as freeware, which will be easily accessible and free of charge.

The current RCT study has been registered at the German Clinical Trials Register (Identifier: DRKS00036178, Registered 24 June 2025; https://www.drks.de/search/de/trial/DRKS00036178).