Ferrets are strict carnivores that require diets based on highly digestible meat protein with little to no carbohydrates. Most ferret owners choose a dry diet for convenience and safety. Dry ferret kibbles should have proteins no lower then 35% but some of the best have up to 60% protein. The fat content of 18-22% is ideal.
The less carbohydrates the better. Ferrets have a very short gastrointestinal tract. The intestine of the ferret is unusual in that there is no external anatomical division between ileum and colon. That basically means they have no functioning large intestines. With a system like this, they do not respond well to fiber or plant proteins. The proteins found in plants are deficient in one or more essential amino acids and not available to the ferrets body. They are designed to digest meat and meat proteins only. That is one of the reasons ferrets have a very messy soft stool if not on a high meat protein low fiber diet.
Messy stool and nutritional deficiency isn't the only consequence of an inappropriate diet. The ferrets pancreas is very sensitive to excessive insulin production. Over time if fed too much carbs and sugars the pancreas can become hyperplastic, in turn contributing to insulinoma. This is a very common problem in ferrets and is like the opposite of being a diabetic, its an overproduction of insulin.
So what is the best diet for ferrets? Experts are now saying that as close to a whole prey diet as possible is the ideal. Feeding whole prey has its downsides, parasites, mess, and the possibility of bacterial infections that are contagious to the owner. The recommended diet would be a commercial diet that has all the positive attributes of whole mice, without the parasites and bacteria issues.
I have two favorites, one is Wysong ferret food. It is a freeze dried/ cold pressed product. It has all the benefits of raw, whole prey diet but has been pasteurized. Its very high in protein, at 60% and fat content of 20%, and its first 4 ingredients are meat proteins. The fiber is 3.5%, thats pretty low for a commercial diet.
Another excellent diet is Evo Ferret. The protein level is 50.2%, fat content is 22.53%, and has a very nice fiber content of 1.39%
All diets should be switched very slowly, over the course of two weeks the foods should be slowly mixed until the new diet is all that remains.
Stool samples should also be checked annually for intestinal parasites, and don't forget to update rabies vaccines as well!