Symbiotic in Alzheimer's disease: modulating the gut-brain axis for neuroimmune homeostasis and cognitive protection

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder defined by progressive cognitive impairment, neuroinflammation, oxidative stress, amyloid-β (Aβ) accumulation, synaptic dysfunction, mitochondrial impairment, and tau hyperphosphorylation. The gut-brain axis (GBA) is a crucial regulatory signaling cascade that links intestinal microbiome composition with both neural health and disease through the vagus nerve. Gut dysbiosis has increasingly been implicated in AD pathogenesis by exacerbating systemic and neuroinflammatory signaling, disrupting intestinal and blood-brain barrier (BBB) structural stability, and promoting microglial activation, thereby facilitating Aβ aggregation and neurodegeneration. Preclinical studies indicate that symbiotic interventions restore microbial balance and improve gut-brain communication, contributing to neuroprotective effects. Additionally, it has been demonstrated that symbiotics can restore synaptic plasticity and cognitive resilience by suppressing pro-inflammatory cytokines, as exemplified by interleukin-1β (IL-1β) and tumour necrosis factor-α (TNF-α), and by upregulating neurotrophic factors, particularly brain-derived neurotrophic factor (BDNF). These effects are associated with normalised glial reactivity, attenuation of oxidative stress, and improved mitochondrial bioenergetics, together contributing to enhanced synaptic function, reduced neuroinflammation, and preservation of cognitive performance. This review highlights a critical assessment of the treatment potential of symbiotic interventions in modulating the GBA in AD, emphasising mechanistic insights into neurodegenerative pathways and evaluating their capacity to mitigate symptoms and delay disease progression, as supported by current preclinical evidence.