While I used to think otherwise, I firmly believe that nursing mothers should supplement both themselves and their babies with Vitamin D. The exceptions to this are mothers who live south of Atlanta, who together with their babies are out of doors without sunscreen between 11:00 am and 1:00pm and who don’t use soap when they wash (because it washes away the D2 oil involved in the process of making D3.) We used to have government programs to encourage parents to take children out into the sun. See this informative and entertaining video from UCSD.
In Finland, babies were routinely given cod liver oil providing 4500 iu of D (and modern cod liver oil is NOT recommended for it due to lower D/A ratios). Then, since research showed that rickets could be prevented at 400 iu, the recommended daily amount was lowered. As a result, Type 1 Diabetes skyrocketed. We have very good data showing that 90% of Type 1 Childhood Diabetes can be eliminated by Vitamin D supplementation.