One of the earliest and most cited human trials came from Messaoudi and colleagues (2011). In a double-blind, placebo-controlled study, healthy volunteers took Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 for 30 days and showed reduced psychological distress versus placebo, with anxiolytic-like effects in a companion rat experiment.[1][2]
Evidence at the level of the brain
In 2013, Tillisch and colleagues at UCLA showed a fermented product could change human brain function: women who consumed a probiotic fermented milk twice daily for four weeks showed altered activity across a brain network responding to an emotional-attention task, measured by fMRI.[3] A 2016 trial by Allen and colleagues tested the single strain Bifidobacterium longum 1714 in healthy men, and found it attenuated the rise in the stress hormone cortisol and subjective anxiety to an acute stressor.[4]
Specific strains can produce measurable psychological and neurophysiological effects in humans — but sample sizes were small, most participants were healthy rather than depressed, and results have not always replicated.
For a plain-language summary aimed at the common question "what are the best probiotics for mood and anxiety?" — and why strain and dose change everything — see our companion guide. If a recent antibiotic course is part of your picture, see rebuilding your microbiome after antibiotics.