This is part of something I posted in reply to Kirsten's thread on Luna. Very good food info here.

My own vet is overjoyed that both my cats will eat the no-grain dry food - apparently it keeps the cat's urine at the right pH so they don't develop crystals. But ask your vet about that.

The great thing too is that on the no-grain food, my boys are leaner and more energetic - and they eat less of the food!

Here are some links that with info you can mention to your vet:

http://uk.answers.yahoo.com/question...5040119AAU2sZm

Very good info here on what to look for on a cat food label:
http://ezinearticles.com/?Five-Cat-F...e-UTD&id=27939

Very good info here too:
http://www.greenlittlecat.com/?p=56

There are more sites if you Google "cat food and struvite crystals". All the sites I looked at mentioned high protein as preventing the struvite crystals. The crystals themselves are diet-related, say some.