Probiotic Supplementation in Ultramarathon Runners: Effects on Gastrointestinal Symptoms and Intestinal Barrier Integrity — A Narrative Review

Background: Gastrointestinal (GI) symptoms are common during endurance exercise, particularly ultramarathons, where prolonged exertion may impair intestinal barrier function. Aim: To review evidence on the effects of probiotics on GI symptoms and intestinal barrier integrity in ultramarathon and long-distance runners. Methods: This narrative review synthesised scientific literature published between 2016 and 2026, identified through PubMed and Scopus, focusing on probiotic supplementation, gastrointestinal symptoms and intestinal barrier integrity in endurance athletes, particularly ultramarathon runners. Results: Some probiotic interventions reduced selected GI symptoms, but findings varied across strains, doses, supplementation periods and exercise protocols. Evidence for improvements in intestinal barrier function and inflammatory markers remains inconsistent. Symptom relief did not necessarily coincide with changes in objective biomarkers. Direct intervention studies in ultramarathon runners are scarce. Conclusions: Current evidence does not establish that probiotics reliably prevent GI symptoms or protect intestinal barrier integrity during ultramarathons. No optimal strain, dose or supplementation duration can be recommended. Further placebo-controlled trials in real-world ultramarathon settings are needed.