The relationship between the gut, its microbial inhabitants and the brain is an active research area with real findings. It's also the basis of a substantial product category making claims the research doesn't support.
Separating them requires knowing roughly where the evidence currently stands.
What's established
Communication between gut and brain is real and operates through several routes.
The vagus nerve provides direct neural connection, carrying signals in both directions with the majority travelling from gut to brain.
The gut produces and responds to numerous signalling molecules, including a large proportion of the body's serotonin — though this is peripheral serotonin, which does not cross the blood-brain barrier, a point frequently misrepresented.
Immune signalling connects the two, with gut inflammation influencing systemic inflammatory state.
Microbial metabolites, particularly short-chain fatty acids produced by fermentation of dietary fibre, enter circulation and have effects on host physiology.
All of this is well documented. The question is what follows from it.
The animal evidence
Much of the excitement derives from animal studies, and they're genuinely striking.
Germ-free mice, raised without any microbiota, show altered stress responses, altered behaviour and altered brain development.
Transferring microbiota between animals with different behavioural characteristics has, in some studies, transferred aspects of the behavioural phenotype.
These findings are real and they establish that microbiota can influence brain and behaviour in these models.
The translation problem is substantial. Germ-free animals are a highly artificial condition with no human equivalent. Mouse and human microbiota differ considerably. Behavioural measures in rodents map imperfectly onto human psychological states.
The human evidence
Considerably thinner.
Observational studies have found differences in microbiota composition between people with and without various conditions including depression. These are correlations, in small samples, with substantial methodological heterogeneity and inconsistent findings between studies about which organisms differ.
Causal direction is a serious problem here. Depression affects diet, activity, sleep and medication use, all of which affect microbiota. Finding a difference doesn't establish which came first.
Intervention trials — giving probiotics and measuring psychological outcomes — exist. Meta-analyses have found small effects on depressive and anxiety symptoms in some analyses, with considerable heterogeneity, small samples, and concerns about bias.
The honest summary: promising, not established, and considerably less certain than product marketing implies.
The probiotic problem
Several issues specific to commercial probiotics.
Strain specificity. Effects, where demonstrated, are specific to particular strains. A product containing a different strain of the same species has not been shown to do the same thing. Most consumer products do not specify strains studied in trials.
Survival and colonisation. Many organisms don't survive gastric transit in useful numbers, and those that do generally don't establish permanently. Studies tracking supplemented organisms find they largely disappear after supplementation stops.
Individual variation. Research has found that colonisation resistance varies substantially between individuals — the same product produces very different results in different people, apparently depending on their existing microbiota.
Dose and viability. Testing has found products containing fewer viable organisms than labelled.
What has better support
Dietary approaches have more consistent evidence than supplements.
Dietary fibre. Fermentable fibre feeds existing microbiota and increases short-chain fatty acid production. This is well established, and most people consume substantially less than recommended.
Dietary diversity. Studies have found associations between the variety of plant foods consumed and microbial diversity, which is generally regarded as a marker of a healthy microbiome.
Fermented foods. A trial comparing high-fibre and high-fermented-food diets found the fermented food arm increased microbial diversity and decreased inflammatory markers, which is one of the more interesting recent findings.
Mediterranean-style dietary patterns. Have evidence for depression outcomes in their own right, through mechanisms that may or may not involve the microbiome.
Note that all of these are dietary patterns rather than products, which is a recurring theme in nutrition.
What I'd conclude
The gut-brain axis is real and the research is worth following. Claims that specific products will improve mood through the microbiome are ahead of the evidence.
The interventions with the best support — more fibre, more plant diversity, some fermented foods — are the same things recommended for other reasons, which is convenient and also slightly deflating for anyone hoping for something novel.
And if mood is the concern, the interventions with strong evidence remain the established ones. Diet quality is worth attending to and is not a substitute for treatment where treatment is indicated.
General information only. Anyone with a digestive or mental health condition should seek advice from a qualified healthcare professional.
Antibiotics and recovery
One area where the microbiome question becomes practically relevant. Antibiotic courses produce substantial disruption to gut microbial communities, with recovery over weeks to months and, in some studies, incomplete restoration of certain organisms.
Evidence for probiotics during or after antibiotics is mixed, with some support for reducing antibiotic-associated diarrhoea and considerably less clarity about restoring community composition.
What is uncontroversial is not taking antibiotics unnecessarily, which matters for resistance as much as for the microbiome, and eating a varied high-fibre diet during recovery.