Probiotics gave healthy workers a small mood lift, and cortisol fell too

Stress, anxiety and low mood among working adults carry costs for individuals, organisations and society, yet most research on mental health interventions focuses on people who are already unwell. At the same time, evidence has been building for a link between the gut microbiome and mental well-being, raising the possibility that probiotic supplements could support people who are otherwise healthy. This review asked a specific question: do probiotics measurably improve depression, anxiety, stress, sleep and the biological markers that track them in healthy employed adults rather than patients?

Following PRISMA 2020 guidelines, the authors searched EMBASE, the Cochrane Library and PubMed for randomised controlled trials of probiotic supplementation in healthy employed adults aged 18 or over, excluding anyone with psychiatric, neurodegenerative, genetic, infectious or endocrine disorders, and excluding pregnancy. Primary outcomes were perceived symptoms of depression, anxiety and stress plus sleep quality. Secondary outcomes were physiological markers of mental health such as cortisol and C-reactive protein. Twelve studies with 3,350 participants met the criteria and were pooled.

Probiotic consumption had a modest but statistically significant benefit on subclinical symptoms of depression, anxiety and stress, with a standardised mean difference of -0.21. Cortisol, a key biomarker of physiological stress, also fell significantly with supplementation, at a standardised difference of -0.26. C-reactive protein, a marker of systemic inflammation, showed no significant change. For sleep quality and markers of oxidative stress there was too little evidence to draw any conclusion. The main results held despite moderate disagreement between studies, which the authors attribute to varying strains, dosages and durations.

Most probiotic and mental health research has looked at clinical populations, so this review extends the gut-brain story into healthy working adults with everyday, subclinical stress. The cortisol finding gives the psychological results a plausible biological echo, while the null result on inflammation shows the effect is not a blanket one. The authors themselves describe the evidence as preliminary, and the biggest open questions are practical ones: which strains, at what doses, and for how long.

If you are a healthy working adult, this is a reason to think your gut microbiome may have some relationship with how stressed and low you feel day to day, and that probiotics are worth watching as the evidence matures. The effect found here was modest, and because the trials used different strains, doses and durations, the research cannot yet point to any particular product or regimen. It is a promising signal, not a recommendation.