From IBS to Alzheimer’s: How Gut Microbes Impact Brain Function

The connection between the gut and the brain is deeper than once believed, with gut microbes playing a key role in shaping brain health. These microscopic organisms influence everything from emotional balance to cognitive function, offering new insight into both mental and neurological disorders.

Table of Contents:

1. The Microbiota–Gut–Brain Axis as a Bidirectional Communication Network

2. Irritable Bowel Syndrome (IBS): A Brain–Gut Disorder

3. The Gut’s Role in Alzheimer’s

4. Microbial Messengers: How the Gut Talks to the Brain

5. From Mice to Humans: Translating Microbiota Science

6. Microbiome-Based Therapies

About me

I am Adriano dos Santos, MSc, rNutr, IFMCP, MBOG, RSM, a Functional Registered Nutritionist, Sleep Medicine & Microbiome Researcher and Educator.

Introduction

The gut and brain, once thought to operate independently, are now known to be deeply interconnected through a complex communication network involving nerves, hormones, immune signals, and microbial messengers. This gut–brain axis plays a critical role in both digestive and cognitive health. Conditions like irritable bowel syndrome (IBS) and Alzheimer’s disease (AD), though seemingly unrelated, may share common roots in gut microbiota imbalance. Research shows that changes in the gut microbiome can influence mood, memory, and neurodegeneration, offering new insight into disease prevention and treatment. With emerging therapies like probiotics, diet interventions, and even fecal transplants, scientists are now exploring how modifying the gut could benefit the brain. Understanding this powerful link may be the key to treating some of the most challenging disorders of our time.

The Microbiota–Gut–Brain Axis as a Bidirectional Communication Network

The microbiota–gut–brain axis refers to the complex, bidirectional communication between the central nervous system (CNS), the enteric nervous system (ENS), and the trillions of microbes inhabiting the gastrointestinal (GI) tract (Margolis K. et al., 2021). These microorganisms interact with the brain through neural, hormonal, immune, and metabolic pathways. Gut bacteria can produce neurotransmitters such as serotonin and gamma-aminobutyric acid (GABA), influence inflammatory processes, and modify vagal nerve activity, all of which contribute to brain function and behavior (Margolis K. et al., 2021).

serotonin gaba

Disruption of this axis has been implicated in disorders including irritable bowel syndrome (IBS), anxiety, depression, Parkinson’s disease, and Alzheimer’s disease (Vasilev G. et al., 2024; Margolis K. et al., 2021).

Irritable Bowel Syndrome (IBS): A Brain–Gut Disorder

Irritable Bowel Syndrome (IBS) is the most common functional gastrointestinal disorder, affecting nearly 5% of the global population (Margolis K. et al., 2021). Although it primarily presents with digestive symptoms, IBS is now recognized as a brain–gut disorder. Up to 50% of individuals with IBS also meet the diagnostic criteria for anxiety disorders, highlighting the strong involvement of the central nervous system (Vasilev G. et al., 2024).

Changes in the gut microbiome, such as reduced microbial diversity or differences in species that produce important metabolites, have been observed in people with IBS, although findings are not always consistent across studies (Margolis K. et al., 2021). These alterations may impact serotonin signaling, which plays a key role in regulating both digestion and mood (Vasilev G. et al., 2024; Margolis K. et al., 2021).

The Gut’s Role in Alzheimer’s

Emerging research suggests that the gut microbiome may play a role in the development of Alzheimer’s disease. One notable study showed that fecal microbiota transplantation (FMT) from healthy mice to Alzheimer’s model mice improved cognitive function, reduced amyloid-β deposition, and restored gut microbial diversity (Jiang X. et al., 2025).

Additionally, heritability studies have revealed that although Alzheimer’s disease is highly heritable (60 to 80 percent), plasma biomarkers such as neurofilament light (NfL) and glial fibrillary acidic protein (GFAP), both associated with neurodegeneration, also have strong genetic components. This suggests that host biology, including gut–brain signaling, may influence disease onset and progression (Rousset R. et al., 2024).

Interestingly, while NfL and GFAP showed high heritability, the plasma Aβ42/40 ratio, a core biomarker for Alzheimer’s, exhibited only modest genetic influence and appeared to be more strongly affected by environmental factors. This may reflect the potential impact of the gut microbiota and lifestyle on disease expression (Rousset R. et al., 2024).

Microbial Messengers: How the Gut Talks to the Brain

Gut microbes produce a variety of metabolites that can reach the brain or affect it indirectly. Short-chain fatty acids (SCFAs), produced during fiber fermentation, influence immune regulation, gut motility, and even neurogenesis (Margolis K. et al., 2021). Serotonin, 90% of which is produced in the gut, is also under microbial control. Specific bacteria, such as spore-forming Clostridiales, can boost serotonin synthesis via short-chain fatty acid production (Margolis K. et al., 2021).

This microbial signaling can affect CNS function through activation of the vagus nerve, modulation of immune responses, and alteration of neuroinflammation, all of which are mechanisms linked to both IBS and Alzheimer’s disease (Vasilev G. et al., 2024; Margolis K. et al., 2021; Jiang X. et al., 2025).

From Mice to Humans: Translating Microbiota Science

Preclinical models have been instrumental in uncovering the role of the microbiome in CNS disorders. For instance, germ-free mice show exaggerated stress responses, impaired cognition, and altered neurotransmitter levels. Colonizing these mice with “normal” microbiota early in life can reverse many of these deficits, but late colonization does not, suggesting a critical developmental window for gut–brain communication (Margolis K. et al., 2021).

These findings highlight how important early exposure to gut microbes is, as it may affect conditions like autism, anxiety, IBS, and brain diseases later in life (Vasilev G. et al., 2024; Margolis K. et al., 2021).

Microbiome-Based Therapies

Given the influence of gut microbes on brain health, modifying the microbiota has become a promising therapeutic approach. Fecal microbiota transplantation has already shown success in improving cognition in Alzheimer’s mouse models (Jiang X. et al., 2025). Dietary interventions like the low-FODMAP diet are effective in IBS symptom relief, though long-term effects on the microbiome remain uncertain (Margolis K. et al., 2021).

Probiotics have shown mixed results in mood disorders and IBS, indicating the need for strain-specific approaches and better-designed trials (Margolis K. et al., 2021). Meanwhile, personalized dietary and psychobiotic interventions are under development, aiming to reshape the microbiota-gut-brain axis for lasting benefits.

Conclusion

From abdominal pain to memory decline, the gut microbiome appears to play a key role in a range of brain-related conditions. The growing understanding of gut-brain communication is revealing how microbes, the immune system, and genetics work together to influence health. This insight is paving the way for innovative treatments that focus on both gut and brain. Targeting the microbiota could transform how we approach disorders like IBS and Alzheimer’s disease.

References:

1. Vasilev G., Georgieva Miteva D., Gulinac M., Chervenkov L., Kitanova M., Velikova T. (2024). Exploring Gut–Brain Interaction Disorders: Mechanisms and Translational Therapies Crossing Neurology to Gastroenterology. MDPI Gastroenterology Insights.  https://doi.org/10.3390/gastroent15030041

2. Jiang X., Zheng Y., Sun H., Dang Y., Yin M., Xiao M., Wu T. (2025). Fecal Microbiota Transplantation Improves Cognitive Function of a Mouse Model of Alzheimer's Disease. CNS Neuroscience & Therapeutics. doi: 10.1111/cns.70259

3. Rousset R., Braber A., Verberk I., Boonkamp L., Wilson D., Ligthart L., Teunissen C., de Geus E. (2024). Heritability of Alzheimer's disease plasma biomarkers: A nuclear twin family design. Alzheimer's Association. doi: 10.1002/alz.14269

4. Margolis K., Cryan J., Mayer E. (2021). The Microbiota-Gut-Brain Axis: From Motility to Mood. Gastroenterology aga. doi: 10.1053/j.gastro.2020.10.066