Vitamin D

Background: Vitamin D is an endocrine secosteroid with recognized extra-skeletal functions, including modulation of immune responses, epithelial barrier integrity, antimicrobial peptide expression, and inflammatory signaling. In parallel, the intestinal microbiota is increasingly recognized as a metabolic and immunological interface relevant to obesity, diabetes, inflammatory bowel disease, and systemic health. This review evaluates the evidence linking vitamin D status or supplementation to the composition and diversity of the human gut microbiota. Methods: A systematic narrative review was conducted using PubMed, ScienceDirect, and Google Scholar for studies published from 2010 to 2025. Human interventional and observational studies, systematic reviews, and mechanistic reports addressing vitamin D status or supplementation and gut microbiota outcomes were considered. The search and selection process was summarized using a PRISMA-style flow diagram. Findings were synthesized qualitatively because of substantial heterogeneity in populations, vitamin D regimens, microbiome sequencing methods, and reported endpoints. Results: Seventy-four records were identified; after duplicate removal and screening, 25 full-text reports were assessed and 11 evidence sources were retained for detailed synthesis. Higher serum 25-hydroxyvitamin D was generally associated with a more favorable microbiota profile, including greater microbial diversity and enrichment of taxa such as Bifidobacterium, Lactobacillus, Akkermansia, Bacteroides, and butyrate-associated organisms. Randomized supplementation studies showed biologically plausible but inconsistent effects on alpha diversity, beta diversity, and phylum-level changes, particularly the Bacteroidetes/Firmicutes balance. Conclusions: Current evidence supports a plausible vitamin D-gut microbiota axis, but causality remains unproven. Vitamin D supplementation should not yet be considered a targeted microbiota- modifying therapy outside correction of deficiency. Larger standardized randomized trials with integrated metabolomic, endocrine, and clinical endpoints are required.