An Australian/British study connecting eczema and exposure to sunshine (in press) in the Journal of Allergy and Clinical Immunology has been getting a lot of media attention of late. [media summary] [paywall link to paper, which I mysteriously got for free a few days ago]
The media coverage has been misleading, at least in the titles. Makes it sound like you should send your kids out in the sun. "Playing in the sun ‘reduces risk of eczema and food allergies in children.'" "Sunlight may help ward off allergies." Irresponsible, really, when Australia faces an epidemic of melanoma caused by exposure to UV through the ozone hole.
That doesn't mean the results aren't interesting. The authors of the paper analyzed data from a national database, for two cohorts of 3312 and 4331 children (aged 4-5 and 8-9) so the number of subjects isn't a concern. They stratified the kids by geographical location from north to south and found that the incidence of eczema and allergy to peanut and egg increased from north to south. (Asthma showed no correlation.)
Strangely, the authors don't quantify how much sunlight children might be exposed to in these regions, even in crude average numbers. You'd think that if they are going to make the claim, as they do, that their numbers provide support for the hypothesis that kids are developing eczema and allergies because they aren't getting enough sunlight--or to go further and say that the lack of sunlight is reducing vitamin D levels in these kids and THIS is what is causing the eczema and allergies--that they might make some effort to refer to how much sun kids are exposed to. But no.
This leaves me wondering what other factors might account for the incidence of eczema and allergies increasing as you go southward through Australia. Also, I wonder whether it might be true that sunlight helps keep eczema down (I have no personal experience with this) by some other mechanism than vitamin D production. Perhaps exposure to long-wavelength--safer--UV radiation affects skin cells in some beneficial way that I'm not aware of.
I had my own vitamin D level measured last week. According to Kaiser Permanente, the normal range for 25-hydroxy vitamin D in the blood is 25-79 ng/ml. My level is 32 ng/ml. Relatively low, but in the normal range. So it's unlikely that for me personally a lack of vitamin D is what's causing my eczema.
Once I'm done with my habit-reversal trial, it might be worth taking some supplemental vitamin D to see whether it has any effect.