Association between small intestine bacterial overgrowth and psychiatric disorders

Alzheimer's DiseaseDepressionGut MicrobiotaGut-brain axiskynurenine pathway

Association between small intestine bacterial overgrowth and psychiatric disorders

Why this matters for your gut health

SIBO is a common but often overlooked gut condition, and this review connects it to major psychiatric and neurodegenerative diseases (autism, schizophrenia, Alzheimer’s, Parkinson’s, depression) through disrupted tryptophan metabolism. It also links thyroid dysfunction as a possible mediating factor. Understanding this gut-brain-thyroid connection could open new diagnostic markers and treatment approaches — such as treating gut dysbiosis alongside or instead of purely psychiatric pharmacotherapy — for conditions that are otherwise hard to manage.

Summary

This mini-review synthesizes literature (screened from over 2.6 million initial records down to a focused set of studies, 1992–2023) exploring how SIBO affects tryptophan metabolism via the serotonin and kynurenine pathways, and how this dysregulation is associated with several psychiatric and neurodegenerative disorders. It also examines how thyroid dysfunction (particularly hypothyroidism and autoimmune thyroid disease) interacts with both SIBO and gut dysbiosis to potentially worsen psychiatric symptoms. The authors conclude that causal relationships remain unclear and call for more longitudinal research, while proposing that gut-microbiome–targeted therapies could offer a novel treatment avenue for these conditions.

Key findings

Patients with SIBO show elevated kynurenine and quinolinic acid but decreased tryptophan and kynurenic acid, indicating a shift toward neurotoxic metabolite production.
Rifaximin (an antibiotic used for SIBO) treatment normalized the KYN/TRP ratio and reduced urinary 5-HIAA, suggesting gut treatment can affect serotonin pathway markers.
Parkinson’s disease patients show a significantly higher SIBO prevalence (up to 25-55%, odds ratio 5.22) and SIBO is linked to worse motor function and impaired levodopa absorption.
In Alzheimer’s disease, 49% of dementia patients studied were SIBO-positive versus 22% of controls; kynurenine pathway changes affect amyloid-beta clearance and tau hyperphosphorylation.
Depression and anxiety are more common in SIBO patients, with imbalances in kynurenic acid/quinolinic acid ratios correlating with brain structure changes (hippocampus, amygdala).
Autism Spectrum Disorder shows a 31% SIBO prevalence in one Chinese cohort (vs. 9.3% in controls), and SIBO correlates with more severe ASD symptoms.
54% of hyperthyroidism patients had SIBO compared to only 5% of controls, and thyroid hormone imbalance (TSH, FT3, FT4) differs between constipation-type and diarrhea-type SIBO.
Hashimoto’s disease and other autoimmune thyroid conditions carry a sixfold increased risk of depressive symptoms, tying together gut, thyroid, and mental health.

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Citation

Bogielski B, Michalczyk K, Głodek P, Tempka B, Gębski W, Stygar D (2024). Association between small intestine bacterial overgrowth and psychiatric disorders. Frontiers in Endocrinology 15:1438066. doi: 10.3389/fendo.2024.1438066