Vitamin D can be made in the skin by exposure to sunlight and can be found in certain foods. Vitamin D levels are alarmingly low in many North American children. Several health issues have been linked with low vitamin D. These include colds caused by viruses and asthma attacks. However, no study has determined whether vitamin D supplementation can reduce the risk of these conditions in young children where they are most common and most severe.

The goals of this study are to determine whether wintertime high dose vitamin D supplementation of preschoolers can prevent colds and asthma attacks. The investigators also aim to work out how much money would be saved by the health care system and society if preschoolers were routinely supplemented with Vitamin D during the winter. The investigators believe that preschoolers receiving 'high dose' vitamin D supplementation during the wintertime will be less likely to have colds, asthma attacks, and low vitamin D levels and will be less likely to use the medical system and keep their parents away from work.

Inclusion Criteria:

1. Healthy children by parental report
2. Have reached their 1st birthday but not past their 6th birthday
3. Present to a TARGet Kids! practice for routine primary healthcare prior to viral season (September through November)
4. Parents provide informed consent to participate.

Exclusion Criteria:

1. Children with gestational age < 32 weeks
2. Children with chronic illness (except for asthma) on parental report which is known to interfere with vitamin D metabolism and increase the risk of respiratory infection
3. Children with a sibling participating in the study to reduce clustering effects.