Gut Microbiota and Cognitive Function

Authors: Katherine Leung and Sandrine Thuret
Affiliation: Institute of Psychiatry, King’s College London, UK
Contact Information: Sandra Thuret, email: sandrine.1.thuret@kcl.ac.uk, Tel: +44-020-7848-5405
DOI: 10.3390/healthcare3040898
Received: 27 May 2015
Accepted: 24 September 2015
Published: 29 September 2015

Field of Study: Microbiology, specifically gut microbiota's role in physiology and health.
Key Concept: Introduction of the microbiota-gut-brain axis, showing how gut microbiota influences brain function, behavior, plasticity, and cognitive aging.
Main Conclusion: Gut microbiota play a significant role in cognitive function during aging and could present new therapeutic opportunities for cognitive decline.
Keywords: gut microbiota, inflammation, gut-brain axis, cognitive function, aging, probiotics, adult hippocampal neurogenesis, fatty acids.

Interdisciplinary Connection: There's been limited interaction between neuroscience and microbiology until recent developments in gut microbiome studies related to health.
Evidence Base: Increasingly, studies link gut microbiota with cognitive processes including learning and memory.
Focus of Review: Exploring microbiota's impact on brain plasticity in older adults and therapeutic strategies, including probiotics and diet.

Diversity of Microbiota: Hosting over 100 trillion bacteria in the human gastrointestinal tract, microbiota is far greater than eukaryotic cells in the body (10-100 times more).
Colonization Timeline: Initiates at birth and stabilizes by the age of three due to co-evolution leading to mutualistic symbiosis.
Functions of Gut Microbiota:
- Metabolism: Breakdown of complex polysaccharides.
- Regulation: Affects gut motility, GI barrier maintenance, fat distribution, immune function (development of gut-associated lymphoid tissues).
- Energy Influence: Impacts host energy metabolism and mitochondrial function.
Dysbiosis Impact: Disruption of gut microbe balance leading to diseases.

Concept Definition: Established connection between CNS and GI tract, extending beyond gut functions to mental health via various signaling pathways (neurological, hormonal, metabolic, immune).
Comorbidities: Gut inflammation correlates with altered gut-brain interactions, notably higher anxiety levels in those with inflammatory bowel disease.
- Animal Studies: Germ-free environments used to study microbiota's cognitive effects.

Study Findings:
- HPA Axis Development: Germ-free mice showed altered HPA responses correlating with behavior; corrected with Bifidobacterium infantis enhancing BDNF expression, critical for brain plasticity.
- Cognitive Deficits: Germ-free mice exhibited reduced memory and altered neural mechanisms, linked to NMDA receptor subunit expression.
Potential Links to ASD: Probiotics showed potential in ameliorating ASD-related traits in mouse models.

Infection Study Insights: Chronic gut inflammation increases anxiety and alters BDNF levels.
- Inflammatory Markers: Changes were reversed in treated mice with probiotics.

Age-related Changes: The microbiota profile stabilizes until ~75 years, with significant shifts observed in centenarians.
- Reported Changes: Increase in facultative anaerobes and decrease in probiotics like Bifidobacteria linked to reduced gut functions in elderly.
Medical Influences: Prescription medications (opioids, antibiotics) adversely affect gut flora, impacting gut-brain communication.

Inflammation in Ageing: Chronic low-grade inflammation leads to “inflammageing.” Pro-inflammatory cytokines associated with changes in microbiota composition.

Vagus Nerve Pathways: The role of the vagus nerve in gut-brain communication; vagotomy disrupts microbiota-mediated changes in behavior.
Immune Activation Pathways: Gut microbiota's modulation of the immune system influences CNS, especially altered in aged hosts.
- Microglial Regulation: Microbiota metabolites can influence microglial function impacting CNS health.

Definition: Live organisms with beneficial health effects when consumed in adequate amounts.
Therapeutic Effects: Evidence for probiotics in treating anxiety, cognitive decline, and inflammation.

Diet-Lifestyle Influence: Links between dietary patterns and cognitive functions; changes in the gut microbiome could influence AHN and cognitive health.
Specific Diets: Caloric restriction and intermittent fasting positively affect cognitive decline in aged populations.

Final Remarks: The connection between microbiota and cognitive function invites further exploration, especially regarding human therapeutic strategies based on animal research results.