It is increasingly being recognized that infants and children living at northern latitudes are at risk of vitamin D deficiency, especially if their skin is darkly pigmented. The study by the PI (Pediatrics 2006;118:603) was the first to demonstrate that infants with light skin pigmentation living at 41 degrees north are at risk of vitamin D deficiency. During winter (December - April) a full 78% of infants were vitamin D deficient if they did not receive vitamin D from an external source. Very few breastfed babies are currently receiving supplemental vitamin D. The recommended dose is 200 IU/day. However, there are questions about the adequacy of this dose of vitamin D for the prevention of vitamin D deficiency. The present trial is designed to determine whether a dose of 200 IU/day is effective or whether doses of 400 IU/day, 600 IU/day or 800 IU/day are required to prevent vitamin D deficiency reliably.