The connection between the gut and the brain has been recognized for decades, but it has only been more recently that researchers have begun to clarify how deeply interconnected these systems are. This communication network, often referred to as the gut–brain axis, describes the back-and-forth signaling between the gut and the brain, carried through systems including the nervous system, the immune system, and metabolic processes.

Early research focused largely on how stress and mood can influence digestion. Increasingly, however, studies are also showing the reverse: that disruptions in the gut itself can influence brain chemistry and mental health. In this post, we highlight two research studies that examine the relationship between imbalances in the gut’s microbial environment, including those observed in SIBO, and changes in mood, anxiety, and depression.

Association Between Small Intestinal Bacterial Overgrowth and Psychiatric Disorders

This 2024 publication by Bogielski et al. (published here) is a meta-analysis of thousands of publications from the last 20 years of research on SIBO and psychiatric conditions. It examines whether SIBO is associated with psychiatric and neurological conditions, and whether shared biological mechanisms might help explain these links. 

Key findings: 

• SIBO is consistently associated with mental health conditions across studies. Across the research reviewed, people with SIBO were more likely to experience conditions such as depression and anxiety compared with control groups.

• SIBO is associated with lower availability of tryptophan, a key building block for serotonin. Tryptophan is an essential amino acid used by the body to produce serotonin, a neurotransmitter central to mood regulation. Evidence summarized in this analysis shows that people with SIBO often have reduced tryptophan levels, which may limit serotonin production.

• SIBO may also alter how tryptophan is processed in the body, further affecting mood-related pathways.
  In addition to reduced availability, tryptophan in people with SIBO appears more likely to be diverted into alternative metabolic pathways rather than being used to support serotonin production. Some of the resulting metabolic compounds are thought to be linked to depression and other psychiatric conditions.

• SIBO is associated with broader neurological, immune, and endocrine changes.
  The analysis reports high rates of SIBO in certain neurological populations, such as Parkinson’s disease, as well as more frequent markers of thyroid autoimmunity, suggesting that gut dysbiosis may interact with multiple systems involved in brain and hormonal regulation.

What this study does—and does not—show:
Because this meta-analysis is based on observational studies, it does not establish that SIBO causes psychiatric disorders. Instead, it demonstrates that SIBO is consistently associated with biological changes in systems known to influence brain function and mood. There is no proven causation, only correlation.