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