This study examined the efficacy of a probiotic in reducing depressive symptom severity in people with subthreshold depression. In a double-blind, randomised, placebo-controlled trial, a probiotic (1 × 10^9 live cells per strain: Limosilactobacillus fermentum LF16 (DSM26956), Lacticaseibacillus rhamnosus LR06 (DSM21981), Lactiplantibacillus plantarum LP01 (LMG P-21021) and Bifidobacterium longum 04 (DSM23233)) or placebo was taken daily for 12 weeks. Data were collected at baseline, 6 and 12 weeks including psychological symptom severity (Beck Depression Inventory, BDI; Patient Health Questionnaire, PHQ; Hospital Anxiety Depression Scale, HADS; and Depression Anxiety and Stress Scale, DASS). Biomarkers of glycaemia, inflammation (high-sensitivity C-reactive protein, hs-CRP), antioxidant status (total glutathione (GSH)) and stress (cortisol awakening response, CAR) were also measured. Thirty-nine participants (nineteen probiotic; twenty placebo) were enrolled. There were no significant between-group differences in the examined psychological symptom severity scores, despite certain significant within-group changes observed in both groups from baseline to 6 and/or 12 weeks of follow-up. Regarding biomarkers, the probiotic group showed reduced hs-CRP (–1520; 95 % CI –273·7, −2766·2 ng/dl) and CAR (–0·28; 95 % CI −0·05, −0·51 μg/dl) at 12 weeks, but increased total GSH (3·9; 95 % CI 0·1, 7·5 ng/dl) at 6 weeks, compared with the placebo. The current study reported favourable decreases in depressive symptoms in both groups. Although the within-group changes observed in the probiotic group were supported by favourable inflammatory, antioxidant status and stress biomarker changes compared with the placebo, further research is required to shed more light on the role of gut microbiota modulation on emotional regulation.
