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 Studies suggest that Vitamin D and the it's downstream Antimicrobial Peptides (AMPS) products may have beneficial effects in Inflammatory Bowel Disease (IBD):

• Antimicrobial peptides in gastrointestinal inflammation

Don't miss this excellent summary table in the above article:

Table 1-Antimicrobials in the gastrointestinal tract.

• Clinical trial: vitamin D3 treatment in Crohn's disease - a randomized double-blind placebo-controlled study
• Direct and indirect induction by 1,25-dihydroxyvitamin D3 of the NOD2/CARD15-defensin beta2 innate immune pathway defective in Crohn disease
• 25 (OH) vitamin D level in Crohn's disease: association with sun exposure & disease activity
• Novel role of the vitamin D receptor in maintaining the integrity of the intestinal mucosal barrier
• Vitamin D receptor gene polymorphism:  association with Crohn's disease susceptibility

 And while epidemiological studies do not necessarily point to a root cause, the studies below are interesting in that they seem to find that Inflammatory Bowel Disease occurs at higher rates in northern latitudes and also increases with affluence (whereby you might assume with greater affluence, people may spend more time indoors, etc):

• The epidemiology and natural history of Crohn's disease in population-based patient cohorts from North America:  a systemic review
• Inflammatory bowel disease:  epidemiology, pathogenesis, and therapeutic opportunities
• Incidence of juvenile-onset Crohn's disease in Scotland: association with northern latitude and affluence

 Lastly, below is an article on a recent study that discusses just how complicated the whole disease process for Inflammatory Bowel Disease may really be, namely that it may take genetic susceptibility (and potentially low Vitamin D levels) and then potentially even something like a virus (in this case Norovirus) that can alter the gut microbe balance before disease occurs:

• Virus Works With Gene to Cause Crohn's-Like Illness

Here's a link to the actual study referenced above:

• Virus-plus-susceptibility gene interaction determines Crohn's disease gene Atg16L1 phenotypes in intestine 

Additional articles on Vitamin D in Inflammatory Bowel Disease can be found on the Vitamin D Council page on Vitamin D and Inflammatory Bowel Disease:

• Inflammatory Bowel Disease