The Gut-Skin Axis in Atopic Dermatitis and Inflammatory Bowel Disease: Mechanisms, Microbiota, and Therapeutic Implications.

The gut-skin axis is a bidirectional communication network linking the gastrointestinal tract, skin, microbiota, and immune system. As major barrier organs, the gut and skin harbor complex microbial communities that contribute to tissue homeostasis, immune regulation, and protection from environmental insults. Increasing evidence indicates that dysbiosis of the gut and skin microbiota is associated with inflammatory diseases through interconnected microbial, metabolic, and immune pathways. Clinical observations further reveal strong associations between gastrointestinal disorders, including inflammatory bowel disease (IBD) and celiac disease, and cutaneous manifestations. Atopic dermatitis (AD) is characterized by epithelial barrier dysfunction, immune dysregulation, microbial imbalance, and environmental influences. Patients with AD frequently exhibit reduced gut microbial diversity, depletion of beneficial short-chain fatty acid (SCFA)-producing bacteria, and enrichment of potentially pathogenic microorganisms. Cutaneous dysbiosis, particularly expansion of Staphylococcus aureus, can further impair the skin barrier and sustain inflammation. Microbial metabolites, including SCFAs, tryptophan-derived aryl hydrocarbon receptor ligands, and bile acid metabolites, may mediate gut-skin communication by regulating epithelial integrity, immune tolerance, and inflammatory signaling. IBD is also increasingly recognized as a systemic disorder involving alterations in skin microbiota and cutaneous immunity. Intestinal inflammation may disrupt immune tolerance to skin commensals and promote cutaneous inflammation through cytokine signaling and immune-cell trafficking, while emerging evidence suggests reciprocal skin-to-gut effects. Epidemiologic, genetic, and clinical studies indicate an association between AD and IBD, potentially reflecting shared genetic susceptibility, barrier dysfunction, dysbiosis, and immune pathways. This review summarizes current evidence linking the gut-skin axis to AD and IBD.