Volume 30, Issue 173, July 2026

The Gut–Brain Axis in Depression: A Review of Current Evidence

Milena Kędzierska, Michał Wójcicki, Agnieszka Kowalska^♦, Michał Biernacki

Collegium Medicum at Jan Kochanowski University in Kielce, Poland

^♦Corresponding author
Agnieszka Kowalska, Collegium Medicum at Jan Kochanowski University in Kielce, Poland

ABSTRACT

Major Depressive Disorder (MDD) is no longer seen just as a problem with brain chemicals. Instead, it is a whole-body issue. A key part of this is the gut-brain axis, in which the brain and the digestive system communicate via nerves, hormones, and the immune system. This review looks at how an unhealthy balance of gut bacteria (dysbiosis) contributes to depression. This constant, low-level inflammation alters how the body processes amino acids; instead of producing serotonin (the "mood" chemical), the body produces toxins that can stress brain cells and reduce levels of growth factors like BDNF. Furthermore, signals from gut microbes can alter how the body responds to stress and how it produces calming neurotransmitters, such as GABA. Finally, this paper examines psychobiotics— specific probiotics that may improve mental health. Clinical trials show that certain strains of Lactobacillus and Bifidobacterium can reduce depressive symptoms and strengthen the gut barrier. While these results are encouraging, the studies are currently small and short-term. More standardized research is needed, but targeting gut health is clearly a promising new strategy for treating depression.

Keywords: gut-brain axis, depression, microbiota, inflammation, cytokines, shortchain fatty acids (SCFA), probiotics

Medical Science, 2026, 30, e124ms3888

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Published: 15 July 2026