Bile Acid Insufficiency as an Underrecognized Upstream Driver of Increased Intestinal Permeability

Increased intestinal permeability (“leaky gut”) is widely discussed in relation to inflammation, dysbiosis, immune activation, and dietary triggers. However, many individuals experience limited or transient improvement when interventions focus exclusively on microbiota modulation or inflammatory suppression. This preprint proposes bile acid insufficiency as an underrecognized upstream driver of intestinal barrier dysfunction. Beyond fat digestion, bile acids regulate intestinal motility, exert antimicrobial effects, support mucosal repair, and activate key signaling pathways such as FXR and TGR5. When bile flow is reduced, these protective mechanisms may fail, creating conditions that predispose to bacterial overgrowth, endotoxin accumulation, mucosal inflammation, and increased permeability. Rather than positioning leaky gut as a primary intestinal disorder, this work reframes it as a downstream manifestation of disrupted digestive flow. By integrating established physiological mechanisms with longitudinal observational insights, this framework aims to clarify why microbiome- or inflammation-centered approaches alone may be insufficient in certain cases. This preprint is intended to encourage upstream-focused investigation into bile-mediated regulation of intestinal integrity and to support more structurally informed models of gut barrier dysfunction.