The current veterinary recommendations are to reduce phosphorus in cats with kidney disease. Foods high in protein tend to also be high in phosphorus, so that's why foods developed for cats with kidney disease also have lower protein levels.
That said, cats with kidney disease also tend to have nausea/upset stomach that makes eating difficult, and low protein foods tend to be less palatable than "normal" cat foods. It is more important that a cat with kidney disease EATS FOOD (preferably wet food) than doesn't eat food and starts to break down its own body tissues for fuel.
Two studies that support reduced phosphorus/protein levels.
Strong evidence supports the provision of renal diets, which are
protein and phosphorus restricted; compliance is improved by gradual
dietary transition. Additional phosphorus restriction is achieved by
the use of phosphate binding agents, although it is unknown if these
yield similar survival benefits to those provided by renal diets.
- Feline CKD Current therapies – what is achievable?
Rachel M Korman, Joanna D White.
Journal of Feline Medicine and Surgery September 2013 vol. 15 no. 1 suppl 29-44
The following table/chart are taken from a study where a group of cats was fed a Reduced Phosphate Diet (RPD) and a Normal Phosphate Diet (NPD). The cats in the RPD group lived longer and tended to die of non-kidney related reasons (though the sample size is small).
- Survival of cats with naturally occurring chronic renal failure: effect of dietary management
J. Elliott, J. M. Rawlings, P. J. Markwell and P. J. Barber
Journal of Small Animal Practice
Volume 41, Issue 6, pages 235–242, June 2000