- Prior to start of study, participants are required to have received a daily supplement of at least 400 IU (10 microgram) of vitamin D (ergocalciferol or cholecalciferol) for at least 3 months or 25hydroxyvitamin D levels above 50 nmol/l. Subjects may be treated with ergocalciferol or cholecalciferol during a run-in period of three months before entering the study.