A Randomised Controlled Trial Investigating the Probiotic-mediated Suppression of Inflammatory Cytokines as a Mechanism for Improving Wellbeing

The gut microbiota can influence host immune function through interactions with the intestinal barrier, microbial metabolites and systemic immune signalling. In turn, systemic inflammatory mediators have been associated with mood, wellbeing and cognitive function. This study will investigate a proposed gut-immune-brain pathway in which modulation of the gut environment by a probiotic intervention may alter systemic inflammatory signalling and thereby influence psychological wellbeing and cognitive function.

This is a 12-week, randomised, double-blind, placebo-controlled, parallel-arm pilot study in 40 generally healthy adults. Following baseline assessments, participants will be randomly allocated to receive either a daily multi-strain probiotic supplement or a matched placebo for 12 weeks. Assessments will take place at baseline and at the end of the 12-week intervention, with a remote adherence and tolerability check approximately halfway through the intervention.

The primary objective is to determine whether probiotic supplementation reduces systemic pro-inflammatory activity compared with placebo. The primary endpoint is the change from baseline to week 12 in a pre-specified pro-inflammatory composite comprising IL-6, TNF-α, IL-1β, IL-8, IFN-γ and high-sensitivity C-reactive protein (hsCRP). Each marker will be standardised relative to its baseline variability and combined into a direction-consistent composite in which higher values indicate greater inflammatory activity.

Secondary assessments will include the anti-inflammatory cytokine IL-10, serum cortisol and validated questionnaires assessing quality of life, depressive symptoms, anxiety, wellbeing and prospective and retrospective memory.

Exploratory mechanistic outcomes will include faecal short-chain fatty acids and related organic acids measured by gas chromatography-mass spectrometry, quantification of selected major bacterial groups using fluorescence in situ hybridisation coupled with flow cytometry, and descriptive 16S rRNA gene sequencing to characterise gut microbial community features. Urine samples collected at baseline and week 12 will undergo nuclear magnetic resonance (NMR) spectroscopy-based metabolomic profiling to assess changes in urinary metabolites associated with the intervention. These analyses will provide an additional measure of host-microbial metabolic activity and will be explored in relation to changes in the gut microbiota, inflammatory markers and wellbeing outcomes.