Thymus serpyllum Extract Improves Gastrointestinal Symptoms and Modulates Fecal Gut Microbiome Composition in a Multicenter, Randomized, Placebo-Controlled Clinical Trial

Background/Objectives: Functional gastrointestinal discomfort is highly prevalent and affects up to 40% of the global population and remains challenging to manage due to its multifactorial nature. Usual interventions include dietary and lifestyle modifications, such as targeted phytochemical-based interventions. This multicenter, double-blind, randomized, placebo-controlled trial investigated the efficacy and exploratory microbiome-related effects of a standardized wild thyme extract (WThE) administered for 8 weeks on gastrointestinal symptoms, health-related quality of life, and fecal microbiome dynamics in adults without diagnosed organic gastrointestinal disease who reported gastrointestinal discomfort. Methods: A total of 178 adults without diagnosed organic gastrointestinal disease and with persistent gastrointestinal discomfort were randomized to WThE (n = 90) or placebo (n = 88); 131 participants had observed week-8 outcome data. Results: The analysis showed a greater reduction in overall gastrointestinal symptom burden with WThE than placebo, measured by the GSRS total score, with the largest exploratory domain-level differences observed in indigestion, constipation, and diarrhea-related symptoms. Improvements were also observed in health-related quality of life. At the microbiome level, no significant between-group differences were detected in α- or β-diversity; however, taxon-level analyses identified specific changes in fecal microbial composition, and predicted functional profiling suggested associated pathway-level differences. Moreover, the extract was well tolerated, with no product-related adverse events reported during the 8-week intervention. Conclusions: Overall, WThE reduced self-reported gastrointestinal symptom burden and improved selected quality-of-life outcomes in adults with persistent gastrointestinal discomfort, supporting Thymus serpyllum as a safe and well-tolerated botanical intervention linked to favorable modulation of the fecal microbiome and enhanced gastrointestinal function. ClinicalTrials.gov no: NCT06639126.