From James Akre's 1991 "Infant feeding:  the physiological basis" (WHO)

"... it is now understood that the optimal route for vitamin D ingestion in
humans is not the gastrointestinal tract, which may permit toxic amounts to
be absorbed.  Rather, the skin is the human organ designed, in the presence
of sunlight, both to manufacture vitamin D in potentially vast quantities
and to prevent the absorption of more than the body can safely use and
store.  It takes only brief exposure to sunlight to produce sufficient
vitamin D;  to satisfy a week's requirements for white infants in a
midwestern US city the exposure time is 10 minutes unclothed or 30 minutes
with only the head and hands exposed."

Since that amounts to 6 minutes on hands and bare head 5 days a week, I've
always figured for most of the year that means just running errands with
children in tow - something some daycare babies don't get.

Diane Wiessinger, MS, IBCLC, LLLL Ithaca, NY