The gut–eye axis: microbiota and their role in diabetic retinopathy: a systematic review and meta-analysis

To systematically evaluate the relationship between gut microbiota dysbiosis and diabetic retinopathy (DR), exploring microbial diversity, composition, metabolic function, and causal associations via the gut–eye axis. A systematic review and meta-analysis were conducted following PRISMA guidelines. Searches across PubMed, Scopus, Embase, and Web of Science identified studies examining gut microbiota in diabetic patients with and without DR. Eighteen eligible studies—including observational, cohort, and Mendelian randomization (MR) designs—were critically appraised. Meta-analyses pooled standardized mean differences (SMDs) for alpha diversity indices (Chao1, ACE, OTUs, Shannon, Simpson) between DR, diabetes without retinopathy (DM), and healthy controls (HC), using random-effects models with heterogeneity assessments. Gut microbiota in DR patients showed inconsistent alpha diversity changes but consistent beta diversity shifts, indicating distinct microbial community structures. Meta-analysis across eight studies (268 DR, 269 DM, 99 HC) revealed no significant differences in alpha diversity between DR and DM (e.g., Shannon SMD 0.01, 95% CI -0.44 to 0.45; I²=74%) or DR and HC (e.g., Shannon SMD 0.02, 95% CI -1.30 to 1.33; I²=71%), with moderate to high heterogeneity. DR cohorts exhibited altered Firmicutes/Bacteroidetes ratios, reduced short-chain fatty acid (SCFA)-producing genera (e.g., Faecalibacterium, Roseburia), and increased pro-inflammatory taxa (e.g., Escherichia-Shigella, Pseudomonas). Functional analyses revealed dysregulated amino acid and lipid metabolism, with specific taxa-metabolite correlations. This review substantiates gut microbiota dysbiosis as a potential contributor to DR pathogenesis via the gut–eye axis. While no robust changes in alpha diversity were found, compositional and functional alterations highlight specific microbial taxa and pathways as potential therapeutic targets. Microbiota modulation through diet, probiotics, or fecal transplantation may offer novel strategies to complement conventional DR management. However, high heterogeneity, demographic limitations, and methodological variations warrant further longitudinal and ethnically diverse studies to validate these findings and guide clinical translation.