The low-protein myth
In the past, it was believed that a low protein diet was essential in controlling CRF. The idea behind this was to supposedly cut down on the kidneys' load. However, studies done on dogs in renal failure, show that a low protein diet did not help said dogs. Since cats have an even higher protein requirement than dogs, it seems unlikely that they could thrive (or even properly exist) on low protein diets.
As an obligate carnivore, they need the nutrients only available from meat. My personal (and I like to think, informed) opinion is that cats with CRF should eat the same food that other healthy cats eat. Please check the references especially 9 and 10 for more arguments for feeding CRF cats a diet high in protein.
Cats are unique when it comes to diet. In all cats, their high protein requirement is due to high activity of hepatic enzymes which are responsible for ureagenesis i.e. the conversion of ammonia to urea in the liver. Cats do not have the ability to decrease the activity of these hepatic enzymes.
So it is dangerous to feed them a low protein diet as their bodies break down proteins via through the urea cycle on a continual basis regardless of the amount of protein fed. If cats do not get protein from diet, they will suffer from muscle wasting as their bodies will literally try to utilize protein internally.
Please do not deprive your cat of valuable protein (from real meat)! That said, only in very late stages of kidney failure, one can consider this purely for palliative relief from symptoms of uremia and azotemia.
There are several mouse studies all of which have found a positive relationship between protein and amount of "erythropoietic units" in spleen cells. Cats on a raw diet (at least 95% meat, including organs and bones) do not typically develop the muscle wasting or anemia commonly seen in cats with CRF because adequate protein amounts provide their body's basic building block not the least of which is RBC production.
The bottom line is that protein must come from an animal source, and must be of highest quality i.e. not primarily beaks, feathers, fur, claws, etc, just what comes with a whole bird/animal. The food should be digestible i.e. utilized by the cat's body so the kidneys do not have to process colors and preservatives such as dyes, BHA, BHT, etc.
If you cannot feed a raw diet, look for grainless canned foods that contain real meat. Examples of good brands – grain-free versions of Nature's Variety (Instinct), Merrick, Wellness, Natural Balance, EVO, etc. All of them contain real meat but not as much as a mouse/rat would have (mid-50%), all have low to mid Phosphorous levels, and Ca:Phos of 1.2:1 as well as moisture amounts equivalent to cat prey. Feed these better brands in rotation.
This is the same food one would give a non-CRF cat. Alas, the specialty foods fall short on many levels. For example, since meat is high in Phosphorous, far too many brands especially the ones that purportedly help CRF cats bring the amount down by adding grain and other filler in place of meat . This also has the effect of lopsided Calcium:Phosphorous ratios e.g. Hills K/D's is 1.7:1 (ideal for cats is 1.2 – 1.4:1) as well as of being rather unpalatable to cats (big shocker!).
What to feed
The best thing you can do for your CRF cat is to feed him/her a home-made diet from organic meat. For recipes, check the Keeping Cats Well-fed section. There is some evidence that one should keep a closer eye on Phosphorous in the diet, so one way to do this is to feed meats low in phosphorous, and/or swap out a small % of the meat with lightly cooked egg whites.
If your cat has elevated Phosphorous levels, look into eggshell powder which has been researched in Japan on human kidney failure patients, and shown to work as an Oral Phosphate Binder (OPB). More on this under treatments.
If a home-made diet is not possible, endeavor to reduce the load on your cat's kidneys by feeding commercial food with the least amount of fillers such as grains and other harmful ingredients like dyes and preservatives. Look for grain-less commercial wet foods such as Wellness, Nature's Variety, Natural Balance venison and pea, and Merrick (all except CA Roll and Southern Delight).