It may not be the breastmilk that has a vitamin D deficiency, but the dietary vitamin D recommendation for the lactating mother!
The common notion that low concentrations of vitamin D in human milk is a defect in design has prompted the recommendation for vitamin D supplementation for breastfeeding infants starting within the first few days after birth. This misconception was revealed in a study that supplemented the lactating mother with 6400 IU of vitamin D per day.The problem is not in the composition of human milk but in the dietary vitamin D recommendation for the lactating mother. The current recommendation of 400 IU per day for the lactating woman is insufficient to maintain blood concentrations of the parent vitamin D compound, the form that crosses from the maternal circulation into human milk. This results in minimal vitamin D transfer into breastmilk. The resulting vitamin D deficiency in the breastfeeding infant, especially darker-pigmented infants, can be significant.This study shows that with appropriate vitamin D intake, the lactating mother can fully transfer from her blood to her milk the vitamin D required to sustain optimal vitamin D nutrition in the nursing infant with no additional supplementation required for the infant.
Hollis BW, Wagner CL, Howard CR, et al. Maternal versus infant vitamin D supplementation during lactation: a randomized controlled trial. Pediatrics 2015; 136:625-634.