Inclusion Criteria:  Healthy children aged 4-11 years attending school in Northern Ireland.  Exclusion Criteria:  Any children on long-term prescription medication; and/or diagnosed with a long-term or exacerbated health condition or disease (does not exclude children with minor or mild health conditions). Those previously taking a vitamin D supplement and those not willing to abstain from starting vitamin D supplements. Those using home sun beds. Those who are planning a family holiday to a sunny country (latitudes below 37°N) during the duration of the intervention.