I am not sure what kind of "medical proof" you need.
 
I think the basic requirements for a ferrets nutritional standards are
pretty well known... although higher levels are debatable I think everyone
agrees that a ferret needs:
Minimum 34% Protein
Minimum 18% Fat
 
Everyone also agrees ferrets are carnivores and the protein source must be
meat correct?
 
The 8in1's food that is sold in the small boxes at WalMart has:
Minimum 28% Protein
Minimum 8% Fat
 
The Kaytee and Nutriphase foods: The levels LOOK okay but look at the
sources of the ingredients: Poultry Meal, Fish Meal, Animal Fat, Soy
Flour, Kibbled Corn, Dried Beet Pulp, Dried Blood Meal.
 
Ferrets are OBLIQUE CARNIVORES..  not Herbivores or even Omnivores... they
must have a meat source for protein.  Hunks of dried fruits and vegetables
should not be in their staple diet.
 
I have witnessed dozens of ferrets who were "fine" on these foods get
switched to better quality foods and the owners amazed at how much better
they looked, how much more active they were etc etc etc  I have watched 7
ferrets in the past 8 months who were all fed these foods for years dies
of complications that the vets marks down as "due to inadequate food"...
there has been numerous studies of ferrets and nutritional needs which all
say the same thing.
 
Exactly what more "proof" was needed?
 
Am I saying no ferret fed these foods ever lived a good long healthy life?
No of course not... we are talking percentages and odds here.  Does every
single person who smokes cigarettes die of lung cancer?  No... but are the
percentages higher?  Yes.
 
Saying a vet told you so, in my book is not good enough.  I know vets here
who tell people generic cat food is okay and that ferrets get vaccinated
for Feline Leukemia... just because someone is a vet doesn't mean they
have followed the research on ferrets.
 
Let's look at vet knowledge: A vet here see's ferrets and "treats" ferrets.
We are currently in an argument with someone to get another vet to look
at their ferret.  An almost 5 year old Marshall Male who suddenly stopped
using his litterbox, is having trouble urinating, and is sexually attacking
the other ferrets.
 
This wonderful vet told the person this is NORMAL behavior and not to worry
about it... now she refuses to believe me (who is not a vet) that I think
the ferret had Adrenal Disease and to get a second opinion.
[Posted in FML issue 3161]