Gut microbiome characteristics in subjective cognitive decline, mild cognitive impairment and Alzheimer's disease: a systematic review and meta‐analysis

Abstract Background and purpose The gut microbiome has been reported to be closely related to Alzheimer's disease (AD) progression. Here, a comprehensive meta‐analysis of gut microbial characteristics in AD, mild cognitive impairment (MCI) and subjective cognitive decline (SCD) was performed to compare gut microbial alterations at each stage. Methods A total of 10 databases (CNKI, WanFang, VIP, SinoMed, WOS, PubMed, Embase, Cochrane Library, PsycINFO and Void) were searched and 34 case–control studies were included. α and β diversity and the relative abundance of gut microbiota were analysed as outcome indices. Data analysis was performed using Review Manager (5.4.1) and R. Results Chao1 and Shannon index levels in AD were significantly lower compared with healthy controls (HCs), and the Chao1 index was significantly lower in MCI compared with HCs. There was a significant difference in β diversity of gut microbiomes in patients (SCD, MCI, AD) compared with HCs. The relative abundance of Firmicutes at the phylum level was significantly lower in patients with AD and MCI than HCs. However, the relative abundance of Bacteroidetes at the phylum level was significantly higher in patients with MCI than HCs. There was an increasing trend for Enterobacteriaceae and a decreasing trend for Ruminococcaceae, Lachnospiraceae and Lactobacillus during AD; Lactobacillus showed a decreasing trend early in SCD. Conclusion Our results indicated that there were gut microbiological abnormalities in AD, even as early as the SCD stage. The dynamic, consistent changes in gut microbes with the disease process showed that they might serve as potential biomarkers for early identification and diagnosis of AD.