A perspective published in July 2026 in Nature Mental Health argues that the microbiome-gut-brain axis is becoming a serious lever in the treatment of anxiety, depression and stress-related conditions.
The paper, led by researchers at APC Microbiome Ireland in Cork with co-authors from Brazil, South Africa, Kenya and the UK, urges scientists and funders to widen the research beyond wealthy countries so future gut-based therapies work for diverse populations.
The direction is backed by growing clinical data. A December 2024 meta-analysis in Nutrition Reviews pooled 23 randomized trials with 1,401 patients. Probiotics produced a moderate reduction in anxiety scores and a large reduction in depression scores in people with a diagnosed mental health condition.
Two strains that keep coming up
Most of the interest centers on two families of bacteria, Lactobacillus and Bifidobacterium. In mice, Lactobacillus rhamnosus JB-1 reduced anxiety-like behavior and altered brain chemistry through the vagus nerve, the long nerve running from gut to brainstem.
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The human picture is more sober. When Cork researchers gave JB-1 to 29 healthy men in a 2017 crossover trial, the strain did not shift stress hormones, mood or anxiety. Mouse results do not automatically translate to people.
Bifidobacterium longum 1714 has done better in humans. A randomized, placebo-controlled trial of 89 adults published in Scientific Reports in February 2024 found that eight weeks of the strain improved sleep quality, energy and social functioning, though it did not lower anxiety scores directly.
How the gut talks to the brain
Two main routes connect the gut to the brain. One is the vagus nerve. The other is the immune system, which reacts to chemicals produced by gut bacteria.
When the gut lining is inflamed, the brain often feels it too. That is the biology behind the field known as psychobiotics, an area already linked to depression in a growing body of research.
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What actually shifts the microbiome
The most persuasive dietary evidence comes from fermented foods. In a 10-week Stanford trial published in Cell in 2021, 36 healthy adults added yogurt, kefir, kimchi, kombucha and cottage cheese to their meals.
Their gut microbiomes grew more diverse and blood levels of 19 inflammatory proteins fell, compared with a group assigned a high-fiber diet.
"This is a stunning finding," said Justin Sonnenburg, a Stanford microbiologist and co-senior author, in the university's announcement. "It provides one of the first examples of how a simple change in diet can reproducibly remodel the microbiota across a cohort of healthy adults."
Prebiotic fiber from vegetables, legumes, oats and fruit feeds bacteria that produce short-chain fatty acids, which help regulate inflammation. The everyday diet you keep is one of the most consistent influences on gut composition.
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Where the evidence stops
Probiotics are not a substitute for treatment. Effects are strain specific and vary between people. The strongest signals come when they are used alongside standard care, not instead of it.
Anyone dealing with persistent anxiety or low mood should talk to their doctor before changing medication or reaching for supplements.
"The greatest discoveries may come not from studying more of the same populations, but from embracing the remarkable diversity of people, environments and cultures that shape the human microbiome," said John F. Cryan, one of the paper's senior authors, in the University College Cork announcement.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.