Vitamin D, immune regulation, the microbiota, and inflammatory bowel disease.

The inflammatory bowel diseases are complex diseases caused by environmental, immunological, and genetic factors. Vitamin D status is low in patients with inflammatory bowel diseases, and experimental inflammatory bowel diseases are more severe in vitamin D-deficient or vitamin D receptor knockout animals. Vitamin D is beneficial in inflammatory bowel diseases because it regulates multiple checkpoints and processes essential for homeostasis in the gut. Vitamin D inhibits IFN-γ and IL-17 production while inducing regulatory T cells. In addition, vitamin D regulates epithelial cell integrity, innate immune responses, and the composition of the gut microbiota. Overall, vitamin D regulates multiple pathways that maintain gastrointestinal homeostasis. The data support improving vitamin D status in patients with inflammatory bowel diseases.