A randomized, double-blind, placebo-controlled trial found that a 4-week probiotic intervention significantly reduced self-reported negative mood in healthy adults, with effects emerging after 2 weeks of daily use.
Published in npj Mental Health Research, the study used daily self-reports to detect changes not captured by conventional psychological questionnaires.
Eighty-eight healthy participants were randomized to receive either a multispecies probiotic mixture (Ecologic® Barrier) or a placebo for 28 days. Participants submitted daily electronic reports rating their mood and bowel symptoms. While standard mood and emotion regulation questionnaires showed no significant effects, a statistically significant reduction in daily self-reported negative mood was observed in the probiotic group compared with placebo.
"We find a clear signal that probiotics improve negative mood in healthy volunteers based on daily reporting," said Katerina V.-A. Johnson from Clinical Psychology Unit, Leiden University, Institute of Psychology, Leiden, The Netherlands and Department of Psychiatry, University of Oxford, Oxford, UK.
The intervention did not affect positive mood ratings or bowel consistency. No group differences were observed in attentional bias as measured by the emotional dot-probe task. In the facial expression recognition task, a marginally significant group-by-session interaction suggested that the probiotic group slightly improved in emotion recognition accuracy.
Post-intervention questionnaire scores—including the State–Trait Anxiety Inventory (STAI), Penn State Worry Questionnaire (PSWQ), and Positive and Negative Affect Schedule (PANAS)—did not differ significantly between groups. An exception was the not-distracting subscale of the Multidimensional Assessment of Interoceptive Awareness (MAIA), on which the probiotic group reported a greater tendency to ignore or distract from discomfort. The placebo group showed a significant reduction in worry scores on the PSWQ.
Regression analysis indicated that baseline risk aversion, measured by the LEIDS-R subscale, significantly predicted improvement in negative mood in the probiotic group. No similar predictive relationships were found in the placebo group.
Participants met eligibility criteria that included no prior psychiatric diagnoses, no recent antibiotic or probiotic use, a body mass index between 18 and 30, and no significant dietary changes. The probiotic contained nine bacterial strains—including species of Bifidobacterium and Lactobacillus (some formerly classified under different taxonomic names)—delivered at 2.5 × 10⁹ CFU/g.
Although the study was not designed to assess clinical outcomes in individuals with diagnosed mental health conditions, the findings suggest that probiotics may influence mood regulation in nonclinical populations, particularly when mood is tracked with frequent self-reports. The authors noted that daily mood assessments were more sensitive in detecting emotional changes than traditional pre- and post-intervention questionnaires, with a clear divergence between groups becoming most apparent after the 2-week mark.
The study was funded by a Veni grant from the Netherlands Organization for Scientific Research. Probiotics and placebo products were provided by Winclove Probiotics B.V.
