Gut-Brain Axis in Ageing: A Comprehensive Review

Abstract

Ageing is accompanied by complex physiological changes that affect multiple organ systems, including the brain and the gastrointestinal tract. Emerging research has identified the gut-brain axis as a crucial bidirectional communication network linking the central nervous system (CNS) and the gut microbiota, with profound implications in brain ageing and associated cognitive decline. This review synthesizes current evidence on the interplay between gut microbiota composition, its metabolic activity, and brain function during ageing. Age-related dysbiosis, characterized by reduced microbial diversity and altered metabolite profiles, promotes systemic inflammation and compromises the integrity of the blood-brain barrier, thereby facilitating neuroinflammation and neuronal dysfunction. Microbial metabolites such as short-chain fatty acids (SCFAs), tryptophan derivatives, and neuroactive compounds (e.g., gamma-aminobutyric acid) modulate neural plasticity and neurotransmission, impacting cognitive processes. Dietary interventions, particularly polyphenol-rich diets, demonstrate potential in modulating gut microbiota and attenuating inflammaging, thereby supporting brain health. Moreover, probiotic and prebiotic therapies hold promise for restoring microbial balance and improving cognitive outcomes. However, mechanistic insights from human studies remain limited, and translational challenges persist due to interindividual variability. This review highlights key molecular pathways underlying the gut-brain communication in ageing, discusses therapeutic prospects, and identifies critical research gaps that must be addressed to harness the gut-brain axis for neuroprotection in the elderly population.

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Introduction

The ageing process involves multifactorial physiological alterations, including neuronal and glial dysfunction, vascular deterioration, and increased oxidative stress, which collectively contribute to cognitive decline. The gut microbiota, a complex community of microorganisms residing in the gastrointestinal tract, has been shown to play a pivotal role in maintaining systemic and cerebral homeostasis through the microbiota-gut-brain axis (MGBA). This axis encompasses neural, immune, endocrine, and metabolic pathways facilitating bidirectional communication between the gut and brain. Alterations in microbiota composition, or dysbiosis, frequently observed in ageing, have been implicated in the pathogenesis of neurodegenerative disorders and cognitive impairment. This review aims to integrate findings from recent literature on the gut-brain axis in ageing, with a focus on microbiota-mediated modulation of brain health and potential therapeutic strategies.

Gut Microbiota Changes with Ageing

Ageing is associated with significant shifts in gut microbiota diversity and composition, often characterized by a decrease in beneficial commensals such as Bacteroidetes, and an increase in potentially pro-inflammatory Proteobacteria. These alterations negatively influence the production of microbiota-derived metabolites, including SCFAs, which are essential for the maintenance of intestinal barrier integrity and modulation of systemic immune responses. Dysbiosis may precipitate increased intestinal permeability, facilitating the translocation of endotoxins such as lipopolysaccharides (LPS) into systemic circulation, thereby triggering chronic low-grade inflammation or “inflammaging.” This chronic inflammatory state is recognized as a key contributor to neuroinflammation and subsequent neuronal damage observed in ageing brains.

Evidence from animal models reveals that dietary fiber deficiency accelerates cognitive decline through dysbiosis-mediated hippocampal synaptic loss and microglial activation, implicating SCFA-dependent pathways. Similarly, shifts in microbiota composition have been linked to impaired blood-brain barrier integrity, amplifying neurodegenerative processes. These findings underscore the critical role of maintaining microbial homeostasis for healthy brain ageing [1].

Mechanisms Linking Gut Microbiota and Brain Ageing

The gut-brain axis utilizes several molecular and cellular pathways to regulate brain function during ageing. SCFAs such as butyrate influence microglial maturation and exert anti-inflammatory effects within the CNS. Moreover, microbial metabolites derived from tryptophan metabolism, including kynurenine pathway intermediates, modulate neuroimmune interactions and neurotransmission, affecting mood and cognitive functions. Dysregulation of these pathways has been implicated in the pathogenesis of Alzheimer’s and Parkinson’s diseases. For instance, amyloid-beta aggregation, a hallmark of Alzheimer’s disease, can be exacerbated by gut microbiota-induced neuroinflammatory responses driven by bacterial amyloids and LPS [2].

Inhibitory neurotransmitters such as gamma-aminobutyric acid (GABA), produced by specific gut bacteria, have emerged as potential mediators of the gut-brain axis, influencing anxiety, depression, and stress resilience. Changes in circulating and brain GABA levels correlate with alterations in gut microbiota, suggesting possible therapeutic targets in ageing-related neuropsychiatric conditions [3].

Neuropeptides like neuropeptide Y (NPY) further modulate stress responses and gastrointestinal functions, providing an intersection of neuroendocrine regulation critical for maintaining homeostasis in ageing individuals [4].

Nutritional Modulation of the Gut-Brain Axis in Ageing

Diet is a significant modulator of gut microbiota and the MGBA during ageing. Polyphenols—naturally occurring plant compounds—exhibit potent antioxidant and anti-inflammatory properties and have been shown to beneficially influence microbiota composition and metabolite production. Polyphenol consumption correlates with improved cognitive performance and decreased neuroinflammation, partly via restoration of SCFA levels and enhanced gut barrier function. However, mechanistic human data remain scarce, highlighting the need for rigorously designed clinical trials to validate these findings [5].

In addition to polyphenols, theaflavins from black tea have demonstrated efficacy in mitigating ageing-induced cognitive decline by modulating gut microbiota diversity, enhancing antioxidant defenses, and upregulating neurotrophic factors, suggesting a dietary avenue for neuroprotection [6].

Probiotic supplementation offers another promising strategy for reinstating gut microbial balance and curbing age-associated inflammatory cascades that impair cognition. Studies employing multi-strain probiotics have shown improvements in memory and reductions in neuroinflammatory markers in ageing models, mediated via toll-like receptor (TLR) pathways and suppression of nuclear factor-kappa B (NF-κB) signaling [7].

Gut Microbiota and Neurodegenerative Diseases in Ageing

Age-related neurodegenerative diseases, including Alzheimer’s and Parkinson’s diseases, are increasingly recognized as disorders influenced by gut microbiota alterations. Dysbiosis-induced systemic inflammation and compromised intestinal and blood-brain barriers foster pathological processes such as amyloid-beta deposition and alpha-synuclein aggregation. Microbiota-mediated modulations of immune responses and metabolite signaling contribute to disease progression and cognitive decline [8],[9].

Emerging interventional approaches targeting the microbiota through antibiotics, probiotics, prebiotics, synbiotics, and fecal microbiota transplantation (FMT) hold therapeutic potential, although clinical translation requires deeper mechanistic insights and longitudinal human studies to ascertain efficacy and safety [10],[11].

Limitations and Research Gaps

Despite compelling evidence from animal and preclinical studies supporting the gut-brain axis role in ageing and cognition, there are significant gaps when translating findings to heterogeneous human populations. Variability in microbiota composition due to genetics, environment, diet, and medication complicates the identification of universal biomarkers and therapeutic targets. Many human studies remain correlational, lacking causative proof and mechanistic detail. The complexity of interactions between microbial metabolites, immune modulation, neuroendocrine factors, and central nervous system function demands integrative systems biology approaches.

Moreover, current interventions such as probiotics and dietary supplementation require standardization regarding strains, dosages, and treatment durations. Future research should emphasize well-controlled longitudinal clinical trials, multi-omics data integration, and personalized medicine frameworks to harness the gut-brain axis for brain health during ageing.

Conclusion

The gut-brain axis represents a critical frontier in understanding the biological underpinnings of brain ageing. Age-associated dysbiosis and resultant changes in microbial metabolites significantly influence neuroinflammation, blood-brain barrier integrity, and cognitive functions. Nutritional and probiotic interventions offer promising strategies to modulate this axis and mitigate age-related cognitive decline. However, comprehensive mechanistic insights and robust clinical evidence are necessary to translate this knowledge into effective therapies. Continued multidisciplinary research promises to unlock novel avenues to promote healthy brain ageing via gut microbiota modulation.

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This review thus integrates significant findings on the microbiota-gut-brain axis and presents a comprehensive understanding of its pivotal role in brain ageing and neurodegeneration, while outlining promising therapeutic directions and current challenges.

References

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