Diet Quality Modulates Gut Microbiota Structure in Blastocystis‐Colonised Individuals from Two Distinct Cohorts with Contrasting Sociodemographic Profiles

Diet and gut microbiota are key determinants of host health, but their interaction in Blastocystis-colonised individuals remains poorly understood. We evaluated the rela-tionship between diet quality, faecal microbial diversity, and Blastocystis colonisation in two cohorts: university students (FACSA) and institutionalised children with their caregivers (PAVILA), all Blastocystis-positive subjects. We assessed diet quality using the Healthy Eating Index-2020 (HEI-2020), and we characterised faecal microbiota by 16S rRNA sequencing. Alpha and beta diversity were analysed, and genus-level trans-formed data were further evaluated using PERMANOVA, PCoA, and db-RDA. The FACSA cohort presented higher microbial richness and diversity (Shannon and Simpson indexes) than PAVILA (p < 0.01), and microbial composition differed signifi-cantly between cohorts (PERMANOVA R² = 0.39, p = 0.002). Total diet quality was as-sociated with microbial structure (R² = 0.26, p = 0.016), with protein (R² = 0.23, p = 0.017) and vegetable (R² = 0.17, p = 0.044) components being the main contributors. Multivariate analysis showed higher protein and vegetable intakes were associated with genera such as Sellimonas, Murimonas, Alistipes and Desulfovibrio (FACSA group). In contrast, Hydrogenoanaerobacterium, V9D2013_group and Haemophilus were related to lower-quality diets (PAVILA group). Despite universal colonisation by Blastocystis, differences in microbial diversity and structure reflect a modulating effect of diet con-ditioned by the sociodemographic environment. These findings suggest the relevance of dietary quality as a modulator of the gut microbiota in populations colonised by Blastocystis and may guide future nutritional interventions in vulnerable groups.