Zizzle wrote:Assuming they have the same sun exposure or access to supplements as a healthy person,
I agree with Sara - I don't believe that we can make that assumption. Many people don't get very much sun exposure, and they don't take supplemental vitamin D. Also, many people who do get sun exposure, tend to wash off the oils and chemicals that are part of the intermediate chemical reaction that converts sunlight to vitamin D. That reaction takes 24 to 48 hours to complete, and few people are willing to leave a sweaty, oily film on their skin that long, these days - they promptly wash it off.
I also believe that it
may be true that
all autoimmune diseases "deplete" 25(OH)D levels at an accelerated rate. (We know for a fact that IBDs do that - I can site a reference if you want). And, as Sara suggests, it's even possible that the mechanism by which vitamin D absorption is interrupted, when an IBD is present, may have something to do with chemical and/or neurological signals between the mucosa of the gut, and the skin - they are both epidermal tissues, with similar characteristics. When the epidermal surface of the gut is inflamed, it
might communicate something to the exterior epidermal surface, (skin), that interferes with vitamin D synthesis. That would imply that a food-sensitivity might not only interfere with vitamin D absorption in the gut, but the effect might extend to vitamin D synthesis on the skin.

Obviously, that's just speculation, on my part, though - I'm just thinking out loud, here.
One other consideration is cholesterol and/or statins.
Cholesterol is necessary for the synthesis of vitamin D from sunlight. Is your cholesterol level high enough? My post from a year ago, at the following link describes what happened to me when my cholesterol level plummeted drastically - so did my vitamin D blood level.

There are probably a lot of people running around these days with compromised vitamin D synthesis capability, because they are taking a statin, or for some other reason, have a lower than ideal cholesterol level, (and my idea of an "ideal cholesterol level" most definitely does not coincide with the "ideal cholesterol level" concept promoted by most doctors).
http://www.perskyfarms.com/phpBB2/viewtopic.php?t=12008
Normally, about 90% of bile acids are absorbed, (for recycling), in the terminal ileum, and the terminal ileum, (along with the ascending colon), is one of the most likely locations for the most intense inflammation markers, associated with MC. Therefore, it is very likely that when MC is active, bile acid recycling is attenuated, (due to the malabsorption problems caused by the inflammation), resulting in a much greater demand by the body to produce bile for the digestive system, which then depletes the normal reserves from which cholesterol is normally synthesized. The end result is a lowered availability of cholesterol that can be used to synthesize vitamin D.
My own example, (described in the thread at the link above), is a good illustration of this effect, because part of my terminal ileum was surgically removed, prior to my cholesterol "crash", resulting in a huge reduction in my vitamin D level. It went from 96.8 ng/mL, to 46 ng/mL.
Tex