VDR and irritable bowel syndrome: The binding of the 25-OHD-vitamin D receptor complex results in the expression of 1-α-hydroxylase, which converts 25-OHD to 1,25-dihydroxyvitamin D. This 25-OHD metabolite has been shown to regulate epithelial barrier function and bowel inflammation, suggesting that a vitamin D-deficiency may directly affect bowel function and IBS symptomology [27,28].