Altered gut microbiota, SCFAs, and barrier integrity markers in Alzheimer's and Parkinson's disease patients.

Parkinson's disease (PD) and Alzheimer's disease(AD) have significant gut-brain axis interaction via microbial dysbiosis. In this study, Iranian patients with PD (n = 25), ad (n = 25), and neurological disorders (ND, n = 20) were compared with healthy controls (HC, n = 20) in terms of gut microbiota abundance, short-chain fatty acid levels, and gut inflammation markers (calprotectin and zonulin). Stool and blood samples were collected from all participants and analyzed using real-time PCR, high-performance liquid chromatography, and enzyme-linked immunosorbent assay. Significant reductions in Bacteroidetes, Faecalibacterium prausnitzii, Bacteroides spp., Bifidobacterium spp., and Lactobacillus spp. were observed in PD and ad patients versus HC. Enterobacteriaceae levels were elevated in PD, ad, and ND groups, while Proteobacteria were significantly higher only in ND patients. ad patients showed reduced Actinobacteria and increased Akkermansia muciniphila compared to HC. Inflammatory markers calprotectin and zonulin were markedly elevated in all patient groups, indicating intestinal inflammation. These findings suggest that microbial dysbiosis, particularly in PD and ad, may contribute to gut barrier dysfunction and systemic inflammation. Due to sample size and methodological limitations, future studies should incorporate advanced sequencing and longitudinal designs to validate these associations and explore therapeutic implications.