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When my daughter was very young, I was lucky to find a book by Dr Lendon
Smith [pediatrician] called FEED YOUR KIDS RIGHT.  In it, he said to
give a child only a tbs or so of a new food on and empty stomach, wait
five min and then check the anus.  If it is red [often beet red], then
your child is allergic to whatever was fed.  It is as simple as that.
He also advocated rotating the diet, and not giving the same food group
more than once every four days.

the book is not longer in print, but you can probably find it and others
by him in your library.  A wonderful book....I gave it to every new
mother and a few older ones, too.

Ann