The Gut Microbiome and Alzheimer's Disease
The balance of microbial populations making up the gut microbiome changes with age, both a loss of microbes generating beneficial metabolites and an increase in the number of inflammatory microbes. Separately from this harmful process, a number of studies have shown that that aged gut microbiome is distinctly different in patients with Alzheimer's disease, suggesting that there may be a meaningful contribution to disease onset and progression arising from the gut. The precise mechanisms involved have yet to be identified. While inflammation has an important role in Alzheimer's disease, the contribution of an Alzheimer's-like gut microbiome may not be as simple as increased levels of chronic inflammation in comparison to other older individuals.
Unlike the typical aging process, Alzheimer's disease (AD) is a progressive neurodegenerative condition characterized by a range of cognitive impairments affecting various aspects of daily life. These impairments impact memory, thinking, decision making, communication, problem solving, personality, and mobility. In AD, the formation of amyloid-beta (Aβ) plaques and hyperphosphorylated tau neurofibrillary tangles (NFTs) leads to inflammation and a gradual decline in cognitive function. Despite various hypotheses about the development of AD, its onset and progression remain unclear.
Recent evidence suggests that the gut microbiota-brain axis could offer insights into the early diagnosis and treatment of neurodegenerative disorders, including depression and AD. Gut health is significantly influenced by microbiota, which is largely composed of diverse microorganisms and resides primarily in the gastrointestinal tract. The gut microbiota's role in AD pathogenesis has been extensively explored, revealing that individuals with AD and mild cognitive impairment (MCI) exhibit a lower gut microbiota diversity index than healthy controls.
Additionally, studies indicate similarities in the gut microbiota of individuals with MCI and AD, offering potential insights into pre-dementia pathogenesis and the identification of at-risk individuals. Moreover, numerous studies are pursuing the goal of understanding and mitigating changeable risk factors for AD pathology, such as lifestyle, different types of dietary patterns, and obesity. These external factors play a critical role in AD development. Conversely, research has shown that a healthy diet may offer a non-pharmacotherapeutic approach to modulating AD neuropathological markers. Therefore, researchers are studying several lifestyle and dietary patterns in order to determine which patterns are most effective in preventing AD, focusing primarily on the Mediterranean diet, DASH diet, MIND diet, and ketogenic diet. Gut microbiota can be affected by several factors, including genetics, age, antibiotics, and diet. Hence, this review aims to enhance our understanding of gut microbiota function, the role of diet, and the connections of these factors to AD.