Dietary sources of vitamin D are not ideal; it is much better to get vitamin D from natural sunlight exposure. Dietary supplementation of vitamin D may actually be related to the increase in allergies and asthma.
Vitamin D3 supplementation has been linked to asthma, which regulates many "allergy" genes and also regulate an antimicrobial response within the immune system. Oral vitamin D supplementation early in life may lead to fewer infections -- but this in turn results in a less-prepared immune system and more allergies.
But while too much vitamin D in the diet can increase the incidence of allergic diseases, too little vitamin D can make allergic diseases worse. However, it is impossible to overdose on vitamin D from sunlight exposure, because the process destroys any excess vitamin D. However, it's easy to get hyper-doses of vitamin D through oral supplementation.
* AchooAllergy.com March 21, 2008