Gut Microbiota Composition in Maintenance Hemodialysis Patients: Associations with Sex, Age, and Body Composition

Background/Objectives: Patients receiving maintenance hemodialysis (HD) commonly exhibit chronic low-grade inflammation, nutritional disturbances, altered body composition, and metabolic imbalance. Gut dysbiosis may contribute to these abnormalities through the gut–kidney axis; however, the relationship between the gut microbiota composition and host phenotype in HD patients remains incompletely characterized. This study aimed to characterize the gut microbiota composition in maintenance HD patients and assess its cross-sectional associations with demographic, inflammatory, nutritional, dialysis-related, and bioimpedance-derived body composition parameters. Methods: This single-center cross-sectional study included 96 patients with end-stage kidney disease undergoing maintenance HD. The primary objective was to characterize the gut microbiota composition in maintenance HD patients. Secondary objectives were to assess cross-sectional associations with demographic factors (sex, age) and bioimpedance-derived body composition (specifically VAT). Clinical and laboratory data, inflammatory markers, nutritional indicators, malnutrition–inflammation score (MIS), dialysis-related variables, and bioimpedance-derived body composition parameters were collected. Stool samples were analyzed using full-length 16S rRNA sequencing. The gut microbiota composition was assessed using taxonomic profiling, alpha-diversity and beta-diversity analyses, subgroup comparisons, and exploratory distance-based analyses. Associations were interpreted within a descriptive and hypothesis-generating framework. Results: The gut microbiota composition showed marked inter-individual heterogeneity at the genus level, with dominant taxa including Blautia, Faecalibacterium, Streptococcus, Gemmiger, Ruminococcus, Escherichia-Shigella, and Enterococcus. Chao1 richness was higher in men than in women. Shannon entropy and Chao1 richness were positively associated with age and visceral adipose tissue (VAT), while Faith’s phylogenetic diversity increased with age. In contrast, the Gini index was negatively associated with age and VAT, indicating a more even microbial community structure in older individuals and in those with higher visceral adiposity. Beta-diversity analyses suggested modest differences in microbial community structure according to sex and selected body composition-related categories, particularly in sex-stratified analyses. Exploratory distance-based analysis showed a modest association between overall microbiota dissimilarity and host phenotype dissimilarity, although this finding was limited by reduced sample overlap. Conclusions: The gut microbiota composition in maintenance HD patients was highly heterogeneous and showed cross-sectional associations, mainly with sex, age, visceral adiposity, and broader host phenotype. These findings suggest that microbiota variation in HD reflects multidimensional demographic, inflammatory, nutritional, metabolic, and body composition-related factors rather than a single clinical determinant. Larger longitudinal studies integrating standardized dietary, medication, metabolic, and clinical outcome data are needed to determine the prognostic relevance of these microbiota patterns.