hi and welcome....truth is Vets know very little about nutrition. cats need protein, even now studies are finding high protein is even ok for cats with CRF. so don't you worry about getting too high, as long as all the ingredients are balanced. and they are with commercial foods, its homemade diets that you have to be careful with.
I will send you two excellent websites that will answer all your questions about nutrition...on this first one you can also click on all the links on the right hand side for best commercial diets on the market, Orijen is good but there are better ones.
good luck and please post anytime.
Yout vet IS right, high protein diets will harm you cat's kidneys...but the thing I don't get, you cat is diabetic or you're just scared that diets with carbs will cause diabetes?? Because it's not carbs that cause diabetes, it's a really high sugar diet that causes it in animals, commercial food have just the carbs and proteins and fats that your cat need (the good ones, of course there are some brands that can damage pets).
The truth is that cats need more protein that dogs, but they still need cabs, if he eats only proteins and low carbs and fats, he'll have ketosis...which can cause liver damage and if your cat is diabetic it will cause ketoacidosis that goes from vomiting to coma an death. The thing is that your cat is healthy now, but they have tons of kidney problems when they get old...and a diet that's stressing the kidneys or liver since they're small can increase the chances of kidney and livet failure a lot.
My advice as a vet student is to always trust the vet, I have to dissagree with opus, at least I have a lot of subjects that have to do with nutrition because it's the most important thing when it comes to food production (cows, pigs and chicken mostly) and to pets health too because a bad diet causes tons of problems in cats and dogs that go from vomiting to hair loss, anemia and even death.
here is an article from the JAVMA (journal american veterinary medical association) that discusses protein and carbohydrates in the feline diet...
also another that discusses the controversial subject of restriction (or not) of proteins in cats with crf.
I am not a Vet and its always best to follow the recommendations of a trusted Vet on any health matter, but do your homework too.
from the world association veterinary congress