Fruit isn’t lethal to rabbits, but it should be served in moderation. Sugar is easy-access food for gut microbes and may promote the growth and proliferation of the “wrong” gut microorganisms. That means the delicate balance of microorganisms working together in the bunny gut to break down food and help make nytrients available to the system can get disrupted by too much sugar.
Rabbits that are getting a healthy, well-balanced diet of grass hay, appropriate vegs and a small amount of good quality pellets can have a small piece of fruit now and then as a treat.
Rabbits often get sick from an inappropriate diet, Their humans give them whole carrots, whole apples, seeds and nuts, breakfast cereal, hard bread and a lot more pellets than the bun needs. This is all done with love. Rabbits do go crazy for treats and have no clue whats good for them long term. What makes it worse is that pet stores and also the pet shelves in grocery stores carry a lot of rabbit-inappropriate treats and muesli-type food. There are pictures of cute rabbits on the packages so naturally people buy them for their rabbits, thinking it’s ok stuff. Very often it isn’t.
I give my bun a thin slice of apple now and then. He eats lots of hay, for which i am so very grateful. Buns that won’t eat hay have a lot more tummy trouble than hay eating buns do.