A research paper published in the United European Gastroenterology Journal showed that if you are experiencing a period of remission with Cohn’s disease that Vitamin D confers additional benefit in restoring/maintaining appropriate gut permeability.
In this small study – some 27 people were involved, all of whom were determined to be in remission at the time of the oral supplementation with either 2000 iu of vitamin D or a placebo for 90 days. They found, that patients treated with the supplementation were more likely to maintain their intestinal permeability, whereas this deteriorated in the placebo group. Increased intestinal permeability is considered a measure of gut leakiness, which is shown to predict and precede clinical relapse in CD. In addition, patients with the highest blood levels of vitamin D had signs of reduced inflammation (measured by C-reactive protein and antimicrobial peptides), and these patients also reported better quality of life.
This finding supported animal studies that had shown a number of related barrier benefits with the exogenous supply of vitamin D. Plus others that have shown a reduced Vit D status also predisposes to increased responses to infectious agents and the associated inflammation.
Vitamin D could also influence colonic commensal bacterial profiles through regulation of anti-microbial peptides. Epithelial VDR signalling may also regulate autophagy, another molecular event that has been implicated in IBD. Finally, as a well-known immune regulatory factor, vitamin D-VDR signalling can certainly control mucosal inflammation by regulating the immune system.
It may be extrapolated that the the Vitamin D Receptor (VDR) found in the gut has important roles to play in the management of numerous aspects of immune, microbiota and barrier function in the small and large intestines, and also that during periods of inflammation these recptors appear to be blocked by specific molecules, making the VDR blunted to suitable agonists.
As such determining Vit D status a normal part of human health analysis these days should be looked at with greater intensity if someone presents with active or quiescent IBD and that their levels of it D may need to be monitored more closely and adjusted accordingly.
However, at the risk of making Vitamin D seem to be the only prohormone/nutrient necessary for the gut it is important to remember that retinoic acid (vitamin A) is also essential for gut health and that too high a level of either in relation to the other will also interfere with receptor adhesion and development of mucosal tolerance.
 T. Raftery, A. R. Martineau, C. L. Greiller, S. Ghosh, D. McNamara, K. Bennett, J. Meddings, M. O’Sullivan. Effects of vitamin D supplementation on intestinal permeability, cathelicidin and disease markers in Crohn’s disease: Results from a randomised double-blind placebo-controlled study. United European Gastroenterology Journal, 2015; 3 (3): 294 View Full Paper
 Chen SW, Ma YY, Zhu J, Zuo S, Zhang JL, Chen ZY, Chen GW, Wang X, Pan YS, Liu YC, Wang PY. Protective effect of 1,25-dihydroxyvitamin D3 on ethanol-induced intestinal barrier injury both in vitro and in vivo. Toxicol Lett. 2015 Jun 9;237(2) View Abstract
 Assa A, Vong L, Pinnell LJ, Avitzur N, Johnson-Henry KC, Sherman PM. Vitamin D deficiency promotes epithelial barrier dysfunction and intestinal inflammation. J Infect Dis. 2014 Oct 15;210(8):1296-305 View Abstract