I remember that the children in the study were all dark skinned and
exclusively BF.  I think the key factor was no exposure to sunlight.  If I
remember correctly the moms practiced muslim dress (chador) and didn't
venture out of the house.  It only takes about 10 minutes a day of sunlight
on 1 square inch of skin to make enough vitamin D. The message for the
article should have been - get those babies outdoors in the sunlight and
fresh air, not supplements.
Pat Young in SNJ, USA