Welcome to TCS! Can you give us a little background information, e.g. age of your cat, vet diagnosis, reason your cat was prescribed Hills ID, what was your cat's diet before, etc.? There are many knowledgeable members and I'm sure someone can help with a little more info. about your cat. Thanks.
I don't know if it's any lower in fat, but Royal Canin makes a gastro food. I have a cat who is on i/d, but we blend it with regular grain-free canned food.