Ferrets have no way to break down plant based protein. Do not ever feed them fruits, vegetables, anything with a high sugar content, or any junk food you happen to be eating.
Ferrets imprint on foods when they are young. This means that they may not see that the expensive ferret food you just bought is food at all. If you adopt one, be sure and find out what brand food the ferret is used to. Then you can try mixing that food, a little bit at a time, with other ferret foods until the ferret thinks it’s food, too.
Some folks skimp on the ferret foods, and give it food meant for other animals. Not a good idea. If you get a ferret that is used to say, kitten food, try switching it to a premium ferret food. Ferret foods are getting better than they have been in the past, and kitten food doesn’t cut it any more. If you absolutely have to feed it kitten food, make sure it has at least 30 to 40 percent high quality animal-based protein, NEVER plant based! It also needs to have 20 to 30 percent fat, and very low carbohydrates and fiber, less than 3 percent. NEVER use store brand or generic foods.
The bottom line? Give you ferret premium ferret food, and try to use a mixture of several brands, in case one is no longer available.