FGF23 and Crohn disease: FGF23 inhibits parathyroid hormone and induces cytochrome P450 family 24 subfamily A member 1 (Cyp24A1), which results in decreased vitamin D production.37 Disruptions in the microbiota and homeostasis of the gastrointestinal tract are also associated with IBD.38 Patients with active Crohn's disease have lower vitamin D levels.39 With more severe inflammation that occurs following Citrobacter rodentium infection at 2 weeks, vitamin D and 25D levels were lower.