Dietary iron imbalance disrupts enterohepatic homeostasis via the gut microbiota-bile acid-FXR signaling axis.

Dietary iron deficiency and excess may both disturb enterohepatic homeostasis, yet their differential effects on organ-specific iron distribution and the microbiota-bile acid-FXR signaling axis remain unclear. Here, we integrated multi-cohort cross-sectional analyses with mouse experiments, cell co-culture assays, organoid models, metabolomics, and fecal microbiota transplantation experiments. In population analyses, higher dietary iron, rather than serum iron, was associated with diarrhea, fatty liver, and liver fibrosis. In mice, iron deficiency induced widespread reductions in tissue iron content, whereas iron excess caused marked iron accumulation in the small intestine, colon, liver, spleen, kidney, and skeleton. Both iron-deficient and iron-excess diets impaired colonic barrier homeostasis, as indicated by reduced goblet cell abundance, lower Muc2 expression, disrupted tight junctions, and increased intestinal permeability. However, iron excess caused more profound gut microbiota dysbiosis, perturbed bile acid-related bacterial taxa, depleted primary bile acids, suppressed intestinal and hepatic farnesoid X receptor (FXR) signaling, and exacerbated hepatic steatosis, fibrosis, inflammation, and dysfunction. Importantly, chenodeoxycholic acid, cholic acid, and fecal microbiota transplantation partially restored bile acid-FXR signaling, improved barrier integrity, and alleviated liver injury under iron-excess conditions. Collectively, dietary iron imbalance disrupts enterohepatic homeostasis via the gut microbiota-bile acid-FXR signaling axis, with iron excess emerging as the predominant driver of bile acid dysregulation, impaired FXR signaling, and gut-liver injury. These findings identify dietary iron as an important determinant of enterohepatic homeostasis and support bile acid- and microbiota-targeted strategies for iron-related metabolic disorders.