Until recently scientists haven’t appreciated the fact that breastfeeding mothers need daily Vitamin D3. Nursing mums could have 50-60 ng/ml of measurable 25(OH)D (a metabolite of Vitamin D3) in their blood but no measurable Vitamin D3 in their breast milk. Why? After Vitamin D3 has been turned to 25(OH)D it cannot enter the breast milk. Vitamin D3 has a half life in the autocrine system of roughly 24 hours, so in order for it to have a meaningful impact on cellular functions, you need a new supply of it every day. The only way to get Vitamin D3 in breast milk is by dosing daily - either by sun, diet or supplementation. The latest research of Grassrootshealth and Dr. Bruce Hollis, Professor of Pediatrics, College of Medicine, Medical University of South Carolina indicates that 6,400 IU/day is necessary for breast feeding mothers to attain sufficient vitamin D (to the value of 400 IU/day for the infant) in their breast milk. More on appropriate daily sun exposure in Healthy Parents, Healthy Baby and Healthy Parents, Healthy Toddler.