An eight-week placebo-controlled RCT on the efficacy of a probiotic nutritional intervention for subclinical gastrointestinal symptoms in students

Despite evidence for substantial placebo effects in the treatment of gastrointestinal (GI) complaints, there is still no methodologically sound study on placebo effects in their probiotic treatment. This randomized controlled trial (RCT) aimed to determine the contribution of a placebo effect in probiotic fruit bit intake in a sample of individuals with mild to moderate gastrointestinal complaints. A sample of 83 individuals were randomly assigned to a probiotic, placebo or to a no-intervention group. All participants received the same information, and the intervention groups took the respective fruit bits daily for eight weeks. Before and after the intervention gastrointestinal symptoms and burden were assessed. Participants in the probiotic (d = -1.08) and placebo group (d = -1.17) benefitted significantly more compared to the no-intervention group with respect of gastrointestinal symptoms, but no significant difference was seen between the probiotic and the placebo group (d = 0.04). At the end of the intervention phase, no significant group differences regarding stress, somatization, and well-being were noted. In both intervention groups, self-reported treatment expectation was not correlated with symptom improvement. Interestingly, in the probiotic group only, subjectively perceived improvement correlated significantly with the change in GI symptoms. While individuals benefitted from a novel probiotic food supplement, data indicated a substantial placebo response. Explicit treatment expectation seems to play a minor role in accounting for the observed intervention effects, yet might contribute to continued use of probiotics.