Thankfully, nature has provided an alternative path for obtaining Vitamin D: human skin converts cholesterol into Vitamin D on exposure to ultraviolet-B light (290 to 320 nanometer wavelength).

The form of vitamin D generated by sunlight on skin, vitamin D3, seems to be particularly important for immune regulation, for regulation of the absorption of calcium, and for regulation of the production of various hormones, including sex hormones such as testosterone, estrogen, etc.

http://ods.od.nih.gov/factsheets/vitamind.asp
http://sunlightandvitamind.com/
http://www.osteo.org/newfile.asp?doc=n502
http://www.shirleys-wellness-cafe.com/cholesterol.htm

Why is this?

It has been reported that CD is seasonal, with the highest possibility of relapse in Autumn(Fall) and Winter. Read the following URL for details

http://crohn.ie/archive/research/misc/season.htm

One possible conclusion is that seasonal factors, e.g. lack of sunlight, different nutrition, are involved in causing Crohns disease.

Another explanation is bacterial. The bacterium Mycobacterium paratuberculosis, which has been postulated as the cause of Crohns disease, has been detected to be present in retail samples of milk on a seasonal basis, i.e. in the Autumn and Winter. The bacterium is not found in milk during the Spring and Summer. See the following URL for details

http://crohn.ie/archive/htmilk.htm
Does Mycobacterium paratuberculosis cause Crohn’s disease?
http://crohn.ie/archive/welcome.htm