The gut-brain connection describes the bidirectional communication between the gastrointestinal tract and the central nervous system. Growing evidence shows that shifts in gut microbiota can influence neurological and psychiatric conditions through neural, immune, endocrine, and metabolic pathways. This article summarizes key diseases tied to the gut-brain axis and how microbiome assessment can inform research and clinical approaches.

Common diseases linked to the gut-brain axis

Neurodegenerative disorders such as Parkinson’s and Alzheimer’s disease display consistent associations with gut microbial imbalances. In Parkinson’s, prodromal gastrointestinal symptoms (for example, chronic constipation) often precede motor signs, and some studies have reported microbial patterns that correlate with alpha-synuclein pathology. Alzheimer’s research has identified reduced abundance of butyrate-producing bacteria and increased pro-inflammatory taxa in some cohorts, which may contribute to neuroinflammation and amyloid-related processes.

Autoimmune and inflammatory conditions affecting the nervous system, notably multiple sclerosis (MS), also show altered gut communities. Animal transfer experiments have demonstrated that microbiota from MS patients can modulate immune activity and susceptibility in germ-free models, implicating microbial modulation of peripheral immunity as a mediator of central nervous system inflammation.

Neurodevelopmental and behavioral conditions, including autism spectrum disorder (ASD), frequently present with gastrointestinal symptoms and decreased microbial diversity. Certain bacterial groups associated with toxin production and immune activation have been observed at higher levels in some ASD cohorts, and alterations in gut-derived metabolites can intersect with neural development and behavior.

Mood, cognition and functional GI disorders

Mental health conditions such as depression and anxiety have reproducible links to gut microbiota composition. Reduced abundance of genera like Bifidobacterium and Lactobacillus and lower overall diversity are common findings in major depressive disorder studies. Mechanistically, dysbiosis can change neurotransmitter availability, heighten systemic inflammation, and dysregulate the hypothalamic-pituitary-adrenal axis, all of which affect mood and stress responses.

Functional gastrointestinal disorders such as irritable bowel syndrome (IBS) and gastroparesis exemplify the mutual influence of gut and brain: altered motility, visceral hypersensitivity, and mood disturbances often coexist and correlate with specific microbial signatures. In IBS, for example, reduced levels of Faecalibacterium prausnitzii and changes in fermentative bacteria have been implicated in symptom generation.

Role of microbiome assessment and emerging directions

Microbiome testing offers non-invasive profiling of bacterial composition, diversity, and potential inflammatory markers, providing a window into gut-derived influences on the nervous system. For an overview of the conditions associated with this axis, see this review of gut-brain connection diseases. Specialized resources explore microbial contributors to metabolism and aging, such as research on Roseburia intestinalis and weight regulation and developments in longevity-focused gut health in the longevity and healthy aging module. A related announcement summarizes platform updates in an external briefing: platform longevity module announcement.

While causality is still being established for many conditions, integrating microbiome data with clinical, immunological, and neuroimaging measures is a promising path. For researchers and clinicians, validated testing protocols and reproducible biomarkers will be key to translating microbiome findings into targeted interventions.

Overall, the gut-brain axis is implicated across a spectrum of diseases—from neurodegeneration and autoimmune neuroinflammation to mood disorders, developmental conditions, and functional GI disorders—highlighting the importance of multidisciplinary investigation into microbial contributions to brain health.