European journal of pharmacologyMar 5, 2026

Gut bacteria-produced indole compounds in depression: how they may work and treatment possibilities

Gut-Brain Axis Weekly Brief ↗PubMed ↗DOI ↗

Updated Jun 27, 2026

Abstract

Essence

Gut microbiota-derived indole metabolites are framed as a potential depression pathway and treatment target.

Evidence

This review synthesizes recent evidence on gut microbiota-derived indole metabolites such as IPA and IAld, their proposed antidepressant mechanisms, and microbiome-targeted interventions.

Caveat

As a review, it summarizes proposed mechanisms and therapeutic potential rather than testing whether restoring indole metabolites improves depression outcomes in a trial.

Simplified

Depression, a prevalent neuropsychiatric disorder with complex pathophysiology and often insufficient treatment efficacy, is increasingly associated with disruptions in the gut-brain axis. This review focuses on the underappreciated role of the microbial indole pathway, a key route in tryptophan metabolism orchestrated by the gut microbiota. We synthesize recent evidence demonstrating that gut microbiota-derived indole metabolites, such as indole-3-propionic acid (IPA) and indole-3-aldehyde (IAld), are significantly reduced in depression. These metabolites exert multifaceted antidepressant effects by enhancing intestinal and blood-brain barrier integrity, suppressing neuroinflammation, and promoting neuroplasticity. Furthermore, we explore the therapeutic potential of targeting this axis through interventions like specific probiotics, prebiotics, dietary modifications, and fecal microbiota transplantation to restore microbial ecology and indole metabolite levels. By highlighting the microbiota-indole-brain pathway as a critical mechanistic and therapeutic frontier, this review provides a novel perspective on the pathogenesis and treatment of depression, moving beyond conventional monoaminergic theories.