The objectives of this study are : 1 ) to compare the effect of ` high dose ' ( 2000 IU/day ) vs. ` standard dose ' ( 400 IU/day ) vitamin D supplementation in achieving reductions in laboratory confirmed URTI and asthma exacerbations during the winter in preschool-aged Canadian children ; and 2 ) to assess the effect of ` high dose ' vitamin D supplementation on vitamin D serum levels and specific viruses that cause URTI .