Is There Any Link between Cognitive Impairment and Gut Microbiota? A Systematic Review

Background: Symptoms of cognitive impairments vary from mild without clinical manifestation to severe with advanced signs of dementia or Alzheimer's disease (AD). Growing evidence in recent years has indicated the association between the brain and gut microbiota, which has been described as the "gut-brain axis. " This systematic review seeks to summarize the primary results from recent human and animal studies regarding the alteration of gut microbiota composition in cognitive disorders. Methods: A systematic literature search was conducted on PubMed, Scopus, and Web of Science databases up to August 2020. The full texts of the papers were analyzed to retrieve the relevant information. Results: Totally, 24 observational studies (14 animal and 13 human studies) were included. Most of the animal studies were performed on mouse models of AD. Human studies were conducted on patients with Parkinson's disease (3 studies), AD (4 studies), poststroke cognitive impairment patients (1 study), and patients with mild to severe cognitive impairment without mention to the cause of disease (5 studies). More recent evidence suggests that throughout aging Firmicutes and Bifidobacteria decrease but Proteobacteria increases. Conclusions: The gut microbiota may alter brain function or trigger various psychiatric conditions through the gut-brain axis. Prospective studies are needed in order to explore the role of the gut microbiota in the etiology of dementia and to achieve clinical recommendations.