Background: Subclinical psychological symptoms—such as low mood, perceived stress, and poor sleep—affect a large portion of the population and can impair quality of life despite remaining below clinical thresholds. The gut–brain axis has emerged as a promising target for interventions that support emotional and psychological resilience. Probiotics and postbiotics are gaining attention for their potential to modulate mood and stress via microbiome-related mechanisms, but human evidence remains limited, particularly in non-clinical populations
. Objectives: We aimed to assess the effects of a two-strain combination of live microorganisms alongside a two-strain combination of heat-treated inactivated microorganisms on outcomes associated with anxiety, mood, perceived stress, and quality of life in healthy adults experiencing mild stress.
Methods: This study was conducted in two parts. In Part I, a randomized, double-blind, placebo-controlled study, 100 participants were randomized to receive either a blend of live microorganisms (
Bifidobacterium longum CECT 7347 and
Lactobacillus rhamnosus CECT 8361) or an identical placebo once daily for 12 weeks. In Part II, a pilot feasibility study, a subset of eight placebo non-responders from Part I received the heat-inactivated preparation of the same bacterial strains in a 6-week trial extension phase. For Parts I and II, the primary outcome was the change in the Hamilton Anxiety Rating Scale (HAM-A). Secondary outcomes included measures of mood (Beck Depression Inventory (BDI); Patient Health Questionnaire-9 (PHQ-9)), stress (state and trait anxiety inventory (STAI); Perceived Stress Scale (PSS)), sleep (Pittsburgh Sleep Quality Index (PSQI)), quality of life (36-item Short Form Survey (SF-36)), gastrointestinal symptoms (Gastrointestinal Symptom Rating Scale (GSRS)), salivary cortisol and microbiome modulation.
Results: In
Part I, there were no significant effects of the live blend on the HAM-A, indicating that the primary endpoint was not met. In addition, no significant effects were seen on the STAI or PSS scores when compared to the placebo. However, participants consuming the live blend trended toward a reduction in total PHQ-9 scores compared to placebo (
p = 0.089), whilst preliminary exploratory analyses suggested an improvement in anhedonia (
p = 0.045). Furthermore, there was a significant improvement in the vitality domain of the SF-36 compared to placebo (
p = 0.017). On microbiome analysis, it was noted that consumption of the live blend was linked to the preservation of butyrate-producing bacteria, particularly members of the
Pseudoflavonifractor genus and the
Clostridium SGB6179 species. Furthermore, the abundance of
B. longum species was found to be inversely associated with the total PSS Scores. In
Part II, supplementation with the inactivated preparation resulted in significant within-group improvements for the vitality (
p = 0.006) and social functioning (
p = 0.010) domains of the SF-36 and improvements in PSS scores compared to baseline (
p = 0.050).
Conclusions: Supplementation with either the dual-strain live or inactivated formulations was associated with significant improvements in the vitality domain of the SF-36, whilst participants receiving the inactivated formulation demonstrated lower perceived stress and improved social functioning compared to baseline. Overall, the findings from this pilot study suggest that these two biotic consortia are well-tolerated and may be associated with improvements in measures of vitality in individuals with subclinical psychological symptoms. The subtle observations detected for stress and anhedonia suggest that further well-powered trials are needed to better characterize these findings, potentially in populations with greater baseline symptomatology.
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