Forgive me if I am wrong on anything, its been about 7 years since I have owned ferrets. From my experience, they have never had any problems while being on the kibble diet.
The three that I use to own were Marshall's Ferrets, and I fed them the Marshalls Kibble. My oldest was 7, my other two were 5 and 4 1/2, and none of them had any health problems what so ever. They were never under weight or over weight, and I took them to the vet on a yearly basis, all clean bills of health every time. And all were fixed at an early age.
When I was 13, my parents made me get rid of them, I still miss those lil boogers.
Keep in mind, that I am not trying to argue, or state that anyone's info is wrong, I am just stating my experiences on owning ferrets.