Just a second, here, what signs of dehydration?  This is important,
because if there are truly signs of dehydration, this is one thing.
But if the baby is merely spitting up the extra he gets, and continues
keeping down lots, why on earth does the mother need to give soy
formula?  Who is so good as to be able to diagnose dehydration in a
four day old.

Maybe we are missing something here.  Galactosemia (rare: 1/60,000
live births) could present with vomiting that would not occur with a
soy formula (since it does not contain lactose).  I suppose
sensitivity to ingested milk by the mother might cause it.  Can't
think of anything else.

Jack Newman, MD, FRCPC