The Food Standard Agency recommends taking 10 micrograms of Vitamin D a day if breastfeeding. They say that you should get enough of other vitamins with a balanced diet.
www.eatwell.gov.uk/agesandstages/baby/breastfeed/
I think Vitamin D is needed for bones and teeth. My fingernails have all broken during the past couple of weeks and wonder if it could be connected. Does anyone take Vitamin D?