The gut–brain axis: how your microbiome may influence mood and stress

One of the questions I'm asked most often is whether what we eat can genuinely change how we feel — not just physically, but emotionally. The short, honest answer is that the gut and brain are in constant two-way conversation, and your microbiome appears to be part of that dialogue. But there is a lot of hype here, and the human evidence is promising in some places and still early in others.

What is the gut–brain axis?

The "gut–brain axis" describes the continuous, bidirectional communication between your digestive tract — including the trillions of microbes living there — and your central nervous system. The most comprehensive review of the area, in Physiological Reviews, maps how gut microbes can signal to the brain through several routes at once.^[1] These include the vagus nerve (a direct line between gut and brain), the immune system (much of which lives in the gut), microbial metabolites such as the short-chain fatty acids produced when bacteria ferment fibre, and the metabolism of tryptophan, relevant to the mood neurotransmitter serotonin.^[1] A caution, though: much of the most striking mechanistic work comes from animal studies, which don't automatically translate to people.

What the human evidence actually supports

The strongest practical signal comes from overall dietary pattern. In the SMILES trial, a randomised controlled trial in BMC Medicine, adults with moderate-to-severe depression were supported to adopt a modified Mediterranean-style diet, and showed significantly greater improvement in depressive symptoms over twelve weeks than a social-support group.^[2] It was a small trial that tested a whole dietary pattern (with dietitian support), not a microbiome intervention as such — but an important early demonstration that improving diet quality can support mood, as an adjunct to standard care.

On probiotics, a systematic review and meta-analysis found small but statistically significant benefits for depression and anxiety symptoms, with larger effects in clinically diagnosed groups; prebiotic supplements showed no significant effect.^[3] "Small but significant" is the honest framing — probiotics are not a stand-in for treatment. And a randomised study in Cell found that a diet high in fermented foods increased gut-microbiome diversity and reduced markers of inflammation — supporting the plausibility of the pathway rather than proving a mood effect.^[4]

Practical, evidence-aligned steps

• Prioritise fibre diversity — different plant fibres feed different microbes and fuel short-chain fatty acid production.^[1]
• Include fermented foods regularly — small daily servings of live-culture yoghurt, kefir, sauerkraut or kimchi.^[4]
• Focus on whole-diet quality — the most robust mood signal comes from an overall Mediterranean-style pattern, not any single "superfood".^[2]
• Be realistic about probiotic supplements — a possible modest adjunct, not a foundation.^[3]

An important safety note

Everything here is about supporting wellbeing, not treating mental illness. Diet and microbiome strategies sit alongside — never instead of — professional mental health care. If you're struggling with your mood, please speak with your GP or a mental health professional.

References

1. Cryan JF, O'Riordan KJ, Cowan CSM, et al. The Microbiota-Gut-Brain Axis. Physiol Rev. 2019;99(4):1877–2013. PubMed
2. Jacka FN, O'Neil A, Opie R, et al. A randomised controlled trial of dietary improvement for adults with major depression (the 'SMILES' trial). BMC Med. 2017;15:23. PubMed
3. Liu RT, Walsh RFL, Sheehan AE. Prebiotics and probiotics for depression and anxiety: a systematic review and meta-analysis of controlled clinical trials. Neurosci Biobehav Rev. 2019;102:13–23. PubMed
4. Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137–4153. PubMed