Probiotic brain effects lingered weeks after stopping

The idea that gut bacteria influence the brain via the gut-brain axis has driven a large market in probiotics aimed at mood and cognition. Almost all of that work assumes the effect requires continuous daily intake, so studies stop measuring when supplementation stops and intake recommendations are built around ongoing use. Whether anything persists after people stop is essentially unknown, and it is not a trivial question: it affects how trials should be designed, how long washout periods need to be, and whether crossover designs are even valid. This follow-up set out to test persistence directly using brain imaging alongside psychological questionnaires.

The parent study was a six-week randomised, placebo-controlled trial in community-dwelling, self-reported healthy older adults, blinded to investigators, nurses and participants, comparing two formulations of Lacticaseibacillus rhamnosus HN001 of identical appearance and taste with placebo. It ran in Örebro, Sweden, in autumn 2023 with 32 participants aged 68.0 ± 5.4 years, 21 women and 11 men. A discontinuation follow-up was performed 4-6 weeks after intake ceased, using brain imaging targeting both function and structure — resting-state functional connectivity being the primary outcome of this report though exploratory in the main study — plus questionnaires covering mood, perceived stress and sleep quality. Participants analysed per group were 8 of 29 for encapsulated probiotic, 12 of 31 for non-encapsulated probiotic and 12 of 30 for placebo.

Previously reported immediate effects had included altered anxiety symptom scores and distinct resting-state functional connectivity patterns. At the discontinuation follow-up, some of these changes persisted while others were no longer detectable. Comparing the two probiotic formulations, connectivity between the left superior parietal lobule and right occipital, cuneal, intracalcarine and supracalcarine cortex differed significantly (T = +7.38, cluster-size p = 0.0001, FDR-corrected and additionally corrected for the number of tests). Hospital Anxiety and Depression Scale total score and its anxiety subscore also showed significant time-by-group interaction effects between weeks 10-12 and week 6, but not relative to baseline. No related adverse events were reported at follow-up.

This challenges the working assumption that gut-brain effects of probiotics switch off as soon as supplementation stops, suggesting instead that functional brain measures may carry a residue for weeks. It also underlines that formulation matters, since the clearest persistent differences were between the encapsulated and non-encapsulated versions rather than against placebo. The design limits are severe: very few participants were analysed per arm, the brain outcomes were exploratory in the parent trial, and the psychological findings were significant against week 6 rather than baseline, which complicates interpretation. The authors themselves frame the implication mainly for future study design and washout periods rather than for users.

For anyone taking probiotics with brain or mood benefits in mind, this is a reason to think the timing story is more complicated than 'stop and the effect stops' — but not evidence that a course of probiotics delivers lasting mental health benefit. With single-digit numbers in one arm and exploratory outcomes, this is a hypothesis-generating study, and the clearest signals were comparisons between formulations rather than against placebo. The wider point is about interpreting probiotic research: trials that assume a short washout may be misreading their own control conditions. Treat any lasting-benefit claim built on this as premature until replicated in far larger samples.