Mice and rats don't have a lot of other options besides instinctive eating, and put them in the wrong environment, and they'll get sick from their food. Best way to get them diabetic is a high sugar, high fat "cafeteria" or buffet sort of diet, in that situation, they'll optimize for sickness. On the other hand, take a diabetic animal, and offer them fat, protein and carbohydrate separately--and they'll manage their own blood glucose by preferring fat and protein over carbohydrate. I think the key point here is that instinct doesn't get you past ice cream, or oreos. If my body craves fat, even if rightly so--the sugar will still make it more enjoyable, it's liable to be the cookies or the ice cream that gets my attention. Butter wins if you take out the unfair competition.