An 8 week, double blind, randomised controlled trial will be carried out in Chester, UK. The 8 week intervention period was selected since more than 1 month is appropriate to generally enlarge the erythropoietic marrow and observe changes in red blood cell indices, especially in non-anaemic groups. Most vitamin D studies have used a supplement concentration of 1000 IU and reported no adverse effects in the participants. Some studies have used higher doses of 1600 or 4000 IU with no reported adverse outcomes in the participants.