A daily probiotic may help relieve depression and anxiety, new trial finds

A new randomised, placebo-controlled trial from the Netherlands, published in The British Journal of Psychiatry, has found that a multi-strain probiotic blend taken daily for eight weeks significantly reduced symptoms of mild-to-moderate depression and anxiety compared with placebo. The work adds strong new data to a growing body of research on the gut-brain axis.

Led by Leiden University Medical Center, the trial enrolled 200 adults who did not meet clinical thresholds for depression but scored in the mild-to-moderate range on the Beck Depression Inventory. Participants were randomly assigned to two arms: one received a daily blend of Lactobacillus and Bifidobacterium strains; the other received an identical-looking placebo.

After eight weeks, the probiotic group's Beck Depression Inventory score dropped on average by 28%; the placebo group dropped 12%. A similar gap was seen on anxiety scores: the probiotic group fell by 24%, the placebo group by 9%. The difference was statistically significant.

Lead author Prof Roel van Driel told Science Daily, "This is a large-scale early validation that goes beyond small pilot work." He said his team is planning a Phase 3 confirmatory trial with 1,000 participants next year.

The gut-brain axis is the two-way communication network between the gastrointestinal microbiome and the central nervous system. Operating via the vagus nerve, hormonal regulation and the immune system, this axis has been hypothesised for years to influence mood and stress responses. New studies are putting that hypothesis on firmer ground.

The Leiden team also analysed stool samples from participants. In the probiotic group, colonisation of Bifidobacterium longum and Lactobacillus helveticus rose significantly; blood cortisol levels in the same participants fell by an average of 17%. The team says the mood improvements may relate in part to a softer hormonal stress response.

Professor Edward Bullmore of the University of Cambridge, who was not involved in the trial, said the results were "plausible and promising" but did "not mean the general public should be taking a daily probiotic". He noted that the dose and strain combination were specific and that most retail probiotic products do not contain the same composition.

Experts also stress that probiotics should not be considered a replacement for antidepressant medication. The trial excluded cases of severe depression, and patients requiring pharmacotherapy were not assessed. Van Driel said, "This should be thought of as a supportive approach; in mild symptoms it could be a first-line option."

The side-effect profile and long-term commitment associated with current antidepressants remain a barrier for many patients. The efficacy of selective serotonin reuptake inhibitors (SSRIs) has long been debated; mild side effects and sexual dysfunction are common complaints. The probiotic side-effect profile in the trial appeared milder.

Next steps include comparing different strain combinations, longer follow-up and a more detailed look at mechanism. The team is also planning combination studies with prebiotic fibre enrichment. Van Driel said the results suggest that "something as simple as food could support mental health".