The reason B-6 is considered too dangerous in large amounts even though
it's a water-soluble one is because in large amounts it sometimes causes
neurologial damage.  The reasons C is considered dangerous in large amounts
are: 1.  It is preferentially taken up by the body instead of the Bs so if
an individual has any problem with uptake of Bs (NOT uncommon in some human
genetic groups; am sure no one knows if the problem can exist in ferrets) or
with not enough in the diet (also not uncommon, esp. in human females)
there can be a short fall in at least one of the Bs which can have
unpleasant (sometimes very unpleasant) results.  Too much IN the body
appears to sometimes interfere with copper metabolism which can result in
anemia down the chain of interactions (also more likely to be a problem for
females if also menstruating or reproducing).  When a large amount is taken
for an extended period and then reduced or stopped some people have had
their bodies adjust the to larger amounts.  If they do not re-adjust rapidly
to the smaller amount then rebound-scurvy can happen; this is scurvy despite
taking in enough C simply because the body is removing too much of it into
the waste.  have no idea if this can happen in ferrets but there's NO reason
to suspect it can't.
 
Yes, there are also a number of minerals which present real hazards (some
fatal, some neurological damage, etc.) in large amounts.  Moderation is
healthy.
 
Hey, there are reasons a varied diet is so healthy: wide range of nutrients,
probably some nutrients we don't even know are needed, yet, fiber, a natural
moderation of how much of any given thing is taken in at any time or day,
etc.  Do use sense and don't give things which aren't likely to digest
easily since there could be a blockage resulting from that action, and don't
over-do anything.
 
Now, what I put up here is like most of what you'll find on nutrition: it's
for humans but there are common sense things which carry to the ferrets,
too.  Do NOT expect anyone to have hard and fast answers.  This field is
just recently getting a bit of the attention it should have long ago
received and even in the most studied species there are still way, way, way
more questions than answers.
[Posted in FML issue 2321]