It is important to remember that the grain and insulinoma hypothesis
which your vet put forward is still a hypothesis, albeit one based on
some interesting evidence.  Of course, the best hypotheses have some
decent evidence behind them but still even a number of those hypotheses
don't bear out in the long run.
 
An example of one that has not borne fruit the last I heard is the idea
that anaphylactic reactions in our critters and later cardiomyopathy
may be linked; although the possible physiological mechanisms were
interesting to date i have not read of it being proven or disproven --
though the experts I've talked to in human medicine the last time I asked
again (about a year ago) said that they did *not* expect a correlation
to exist.  (Has anyone heard anything more recent in terms of research
results?)
 
An example of an interesting hypothesis by a noted ferret expert that
didn't pan out was the idea that blood tests may diagnose lymphoma.
Unfortunately, since she is a noted expert too many people treated the
hypothesis as if it were fact from the start of inquiry rather than
waiting to see if the idea turned out to be valid after research into it.
 
Some pan out and some don't.
 
It's fine to take precautions based upon hypotheses and I think that
pretty much everyone (including us) does so, but it is essential to
always remember what is still unproven.  Doing so really cuts down on
later confusion if things don't pan out.
[Posted in FML issue 3934]