Ah Kathy, that is a difficult one.  Breastmilk actually does contain
some vitamin D, but not enough to depend on it solely.  Even in Toronto,
though, children are taken outside, especially during the warm season,
which, I hear, is due to begin any time now.  They will get sun exposure
which should help (actually outside exposure, as ultra violet rays pass
through cloud).

We all used to get cod liver oil as a source of vitamin D.  It tasted
awful.  Luckily, those children who are growing up today get it in their
cow's milk.

The most common causes of rickets we see in 4-10 year olds, at least in
my experience, are renal disease, inflammatory bowel disease or
malabsorption problems, and metabolic problems.  Except where a child
has been starved and kept in a locked cupboard, I cannot imagine a child
getting vitamin D deficiency rickets in our society.  It must be
virtually unheard of here.

All that to say "I don't know how long prenatal stores last".  Probably
less than a year, but I don't know if anyone has looked into it.

Jack Newman, MD, FRCPC