Rats are quite tolerant when it comes to food, like we are, but eating the world's biggest muffin with glazing would probably not be healthy to a human so it's probably not exactly healthy to a rat. But, on the other hand, pet rats have very short lives in any case (~2 year average in Norway), much of which can be attributed to horrible breeding, so I doubt it would have much effect on an average sort of lifespan unless it was a regular thing. I believe this is a picture from Atticus' birthday celebration.
Rats can tolerate theobromine which is the stuff in choclate which is poisonous to some animals like dogs. Whether or not it's poisonous to squirrels, I've no idea, but it's possible. Adult animals typically can't properly digest milk either, but I guess that should just give them the runs.
