Abstract: The invention relates to a method of diagnosing, assessing, and monitoring impaired gastrointestinal barrier function in a subject, comprising comparing the level of one or more markers in a tissue or body fluid of the subject relative to a reference value for the one or more markers, wherein the one or more markers are selected from the group consisting of: desmoglein 1 or a fragment thereof; desmoplakin or a fragment thereof; SPP 24 or a fragment thereof; Paraoxonase or a fragment thereof; CD14 protein precursor or a fragment thereof; Nck-associated protein 1 or a fragment thereof; Claudin-1 or a fragment thereof; Claudin-3 or a fragment thereof; Fatty acid-binding protein 5 or a fragment thereof; Occludin or a fragment thereof; Alpha-1-microglobulin or a fragment thereof; Guanylate cyclase activator 2a or a fragment thereof; Serglycin or a fragment thereof; Propanoic acid; Hydroxybutyric acid; Citric acid; Proline; Valine; 4-hydroxy-benzoic acid; and Anthranilic acid, and to compositions, devices and kits