So I thought perhaps I'd been too quick to diss vitamin D as potentially having benefit for patients with eczema. A PubMed search turned up this review paper, which I am inclined to trust as credible for several reasons:
- one of the two authors is Donald Leung, head of pediatric allergy and immunology at National Jewish Health Center in Denver; editor-in-chief of the Journal of Allergy and Clinical Immunology; and principal investigator for the Atopic Dermatitis Research Network
- the authors cite 73 papers instead of just one or two selected to support their cause; they present a balanced view with opposing data
- they don't use scary headings like "Is This Nutritional Deficiency Ruining Your Heart?" or provide a convenient link to an online store with products such as krill oil, vitamin spray, and bidets.
That's not enough to make me rush out and buy barrels of vitamin D though, especially since there are multiple side effects of an overdose, including kidney damage. As Caroline commented after my last post, the issue is really whether you're deficient or not, not that it's a good idea to take an excessive dose. (I sent her a copy of the paper so we can look forward to learning her perspective.)
A reading of the paper turns up a bewildering number of studies in cells and mice that produced conflicting results. E.g. mice deficient in vitamin D had helper T cell responses greatly slanted toward type 1 instead of type 2; but it's known that, in humans, eczema patients have overactive type 2 helper T cells. If you were to take that one paper as gospel, you'd try to deprive yourself of vitamin D to cure your eczema.
Another paper shows that adults who received vitamin D supplements as infants were more likely to develop atopy and allergy. But then opposing papers balance this out. Dosage and timing could be crucial.
So, as with most science, the conclusion is the familiar "more research needed."