This article is from the Solid Food for Infants FAQ, by David M. Poduska email@example.com with numerous contributions by others.
An additional worry concerning early introduction of supplemental foods
is recent research linking Type I (juvenile, or IDDM) diabetes to
including dairy products in the diet before a child is a year old.
This research is quite recent, so not all pediatricians are aware of
the connection to warn their patients' parents about it. The research
is also quite controversial. While it's clear that a correlation exists,
the degree of correlation is not clear, and causation is not established.
Basically, the antibodies that may arise when whole cow's milk proteins
are absorbed in the infant's gut can sometimes lead to the destruction of
the cells in the pancreas that make insulin. It has been suggested, as a
potential mechanism, that some protein on the surface of the cells 'looks'
like a cow's milk protein, as far as the antibodies are concerned. While
most infants who receive cow's milk before they are old enough for it do
not go on to develop diabetes, some researchers have claimed that most
cases of Type I diabetes could be prevented by avoiding cow's milk in the
diet of under-one-year-olds; the risk is not large for any individual child,
but diabetes is an extremely serious disease, so it's worth some effort to
try to reduce the risk. It appears that milk leads to diabetes only in
susceptible individuals, but it's difficult to tell who is susceptible
without tests (for HLA markers) which are not commercially available.