Abstract
INTRODUCTION: Allergic rhinoconjunctivitis (ARC) is a highly prevalent immune-mediated condition associated with substantial symptom burden, impaired quality of life and increased healthcare use. Emerging evidence highlights the role of the gut microbiome in immune regulation and allergic disease. Fermented foods may contain live microbes (when unpasteurised or uncooked) and bioactive postbiotic metabolites that can modulate immune responses. Despite growing interest in dietary strategies targeting the microbiome, no randomised controlled trial has compared fermented versus unfermented red cabbage for ARC. METHODS AND ANALYSES: This single-centre, randomised, controlled trial with a sensory-matched, unfermented cabbage comparator investigates the effects of daily consumption of fermented red cabbage for 8 weeks compared with an unfermented red cabbage control in young adults (18-35 years) with ARC. A total of 158 participants will be randomly assigned (1:1). The primary outcome is change in Total Nose and Eye Symptom Score from baseline to week 8. Secondary outcomes include daily symptoms and medication use captured via mobile ecological momentary assessments, quality of life, psychological well-being, gastrointestinal symptoms, systemic inflammatory markers, total IgE, immune cell profile and metagenomic characterisation of stool samples. A nested qualitative component explores participants' experiences and acceptability of the intervention. Analyses will include mixed-effects models, time-series analyses incorporating daily pollen counts and comprehensive microbiome statistics. Safety outcomes and adverse events will also be assessed. ETHICS AND DISSEMINATION: This study was approved by the Ethics Committee of Charité-Universitätsmedizin Berlin (EA4/043/25) and is conducted in accordance with the Declaration of Helsinki and Good Clinical Practice. Results will be disseminated through peer-reviewed publications, conference presentations and a lay summary provided to participants. Anonymised datasets and analysis scripts will be made available in public repositories, and metagenomic sequencing data will be deposited in an international sequence archive to ensure transparency and reproducibility. TRIAL REGISTRATION NUMBER: DRKS00036475.