The primary purpose of this study it to investigate total circulating 25-hydroxyvitamin D (25(OH)D) in school children (aged 4-11years) in Northern Ireland; thereby establishing the prevalence of vitamin D status (deficiency, insufficiency, and sufficiency (SACN 2016)) in this cohort. Additionally, the study aims to determine if 10µg/day vitamin D3 supplementation over 12 weeks maintains vitamin D status. This area of research is lacking in the quality of prospective studies. While previous literature has focused on the prevalence of vitamin D status in adults, there is a paucity of data available for children. This study will add to the existing knowledge and provide more specific analysis for children. Hypothesis 1: Increase in vitamin D concentrations will be associated with an improvement in health outcomes including anthropometric measurements, musculoskeletal outcomes, markers of inflammation, nutritional status and bone turnover, lipid profiles, cognitive function, and handgrip strength.