Check out the article in the Journal of Pediatrics, 1989, Vol 114,
pgs.204-212 entitled "Bone Mineral Content, Serum Vit D Metabolite
Concentration, and UVB Light Exposure in Human Milk-fed Infants with and
without Vitamin D Supplementation by Greer and Marshall.
We were able to prove that in Caucasion infants, in the climate of
Southcentral Wisconsin (not always the most sunny -place in America)
Breastfeeding infants, who's mother's diet met their minimum daily
requirements of Calcium, Phosphorous, and Vit D- the infants did not require
supplementation.  We monitored the sunlight exposure using docimeters
developed by NASA, and found, even in the coldest of winter here, that
CAUCASION babies were fine.  We did not include infants with darker
pigmented skin.  This definitely required further study.  Rickets in
breastfeeding infants is a devastating thing.

Sharon Marshall, BSN, RN, IBCLC