Why Do Some Patients Remain Depressed Despite Multiple Treatments?
One of the greatest challenges in psychiatry is treatment-resistant depression (TRD).
Although many individuals with major depressive disorder respond to antidepressants, approximately one-third continue to experience significant symptoms despite multiple treatment attempts. These patients often endure years of persistent depression, functional impairment, and increased risk of hospitalization and suicide.
For decades, researchers have focused primarily on the brain when trying to understand depression. However, emerging evidence suggests that another organ system may play an important role:
The gut.
A newly accepted review article published in Progress in Neuro-Psychopharmacology & Biological Psychiatry explores how alterations in the microbiota–gut–brain axis may contribute to the development and persistence of treatment-resistant depression. The review was developed through a collaboration between investigators from UTHealth Houston and several Brazilian research institutions.
What Is the Microbiota–Gut–Brain Axis?
The microbiota–gut–brain axis refers to the complex, bidirectional communication network connecting:
Trillions of microorganisms residing in the gastrointestinal tract continuously communicate with the brain through multiple biological pathways. These microbes influence:
Increasing evidence suggests that disruptions in this communication network may contribute to psychiatric disorders, including depression.
The Gut in Treatment-Resistant Depression
Patients with treatment-resistant depression frequently demonstrate:
These abnormalities may create a biological environment that makes depression more difficult to treat.
The review highlights several interconnected mechanisms.
One of the strongest findings in TRD research is the presence of chronic low-grade inflammation.
Patients with treatment-resistant depression often show elevated levels of:
These inflammatory molecules can interfere with neurotransmitter systems and reduce the effectiveness of conventional antidepressants.
The gut microbiome may play a central role in generating and maintaining this inflammatory state.
The review discusses how disruptions in intestinal barrier integrity can allow bacterial products, such as lipopolysaccharide (LPS), to enter the bloodstream.
This process can trigger systemic inflammation and potentially affect the blood-brain barrier.
When these protective barriers become compromised:
Immune signals gain greater access to the brain.
Neuroinflammation increases.
Mood-regulating circuits may become disrupted.
This model provides a biologically plausible explanation for why some patients remain symptomatic despite adequate antidepressant treatment.
Tryptophan is best known as the precursor of serotonin.
However, under inflammatory conditions, tryptophan is increasingly diverted into the kynurenine pathway rather than serotonin production.
This shift may result in:
The review highlights growing evidence that gut microbiota can influence this pathway and potentially contribute to treatment resistance.
Beneficial gut bacteria produce short-chain fatty acids (SCFAs), including:
These compounds have multiple potentially beneficial effects:
Several studies suggest that patients with depression may have reduced levels of important SCFA-producing bacteria.
The review also examines the relationship between the gut microbiome and the hypothalamic-pituitary-adrenal (HPA) axis.
Chronic stress can:
Conversely, microbiome disturbances may worsen stress responses and contribute to persistent HPA axis activation.
This creates a vicious cycle that may perpetuate depressive symptoms.
Could the Microbiome Become a Treatment Target?
Perhaps the most exciting aspect of this research is the possibility of developing new interventions aimed at the gut microbiome.
Potential approaches include:
Although many questions remain, these strategies could eventually complement traditional antidepressants and neuromodulation therapies.
UTHealth Houston Perspective
At the Center for Interventional Psychiatry at UTHealth Houston, we increasingly recognize that depression is not solely a disorder of neurotransmitters or brain circuits.
Rather, it appears to involve a complex interaction among:
This review reinforces the importance of adopting a broader biological perspective when evaluating and treating difficult-to-treat forms of depression.
As precision psychiatry evolves, understanding the microbiota–gut–brain axis may help us identify new biomarkers, predict treatment response, and develop more personalized interventions for patients with treatment-resistant depression.
Final Thoughts
The future of depression treatment may not lie exclusively within the brain.
Increasing evidence suggests that the gut microbiome influences inflammation, neurotransmission, neuroplasticity, and stress regulation—all key mechanisms implicated in treatment-resistant depression.
While microbiome-based therapies are still evolving, this growing field offers a promising new direction for understanding why some patients fail to respond to conventional treatments and how we may better help them in the future.
The brain and gut are connected in ways we are only beginning to understand.
And that connection may hold important clues to overcoming treatment resistance.
Reference
Bertollo AG, Puntel CF, Scaini G, Quevedo J, Barichello T, Ignácio ZM, Réus GZ. The interplay between microbiota and the gut-brain axis in treatment-resistant depression. Progress in Neuro-Psychopharmacology & Biological Psychiatry. Accepted June 2026.
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Disclaimer
This article discusses findings from a recently accepted scientific publication and is intended for educational and informational purposes only. The author, Joao L. de Quevedo, MD, PhD, is a co-author of the publication discussed above. The manuscript was developed through a collaboration between investigators at UTHealth Houston and leading Brazilian academic institutions. The content presented here reflects the scientific findings and interpretations reported in the publication and does not constitute medical advice. Patients should consult qualified healthcare professionals regarding diagnosis and treatment decisions.
This article was created with the assistance of artificial intelligence (AI) to help organize and refine the presentation of scientific information. All medical and scientific content has been reviewed and approved by Joao L. de Quevedo, MD, PhD, Executive Director of the Center for Interventional Psychiatry at the John S. Dunn Behavioral Sciences Center at UTHealth Houston.