The usual treatment for renal failure is a low-protein diet. This makes sense: proteins are the main source of nitrogen (vitamins contribute a negligible amount), and there is no urea or creatinine without nitrogen, so on a low-protein diet the levels of of these two compounds go down, meaning that (hopefully) the body is not poisoning itself as much. Even in a diet completely devoid of protein (not recommended), though, there will always be some protein breakdown as a part of "business as usual".
Unless you're a cat. If you're a cat, your metabolism, unlike those of your human slave-monkeys, is so attuned to the life of a carnivore that its preferred fuel source is not carbohydrates, but proteins. This means that for cats, protein is not just something that builds muscles and makes them strong--protein is also their main source of energy. To a cat, substituting protein with carbohydrates is like trying to run a diesel truck on jet fuel. It'll go--most likely with an explosive bang.
I realize that this doesn't really make sense--cats need to break down protein in order to build up carbohydrates so that their bodies can be properly fueled. Note the word "properly". Because cats are also extremely efficient at utilizing carbohydrates. Too efficient--the energy they cull from carbohydrates, if it's not burned off, is usually stored as fat. Even skinny cats may have a larger percentage of body fat than is strictly good for them.
For a cat with renal failure, then, protein is not the bane it is in the human counterpart of the disease. Protein is the fuel on which their bodies run most efficiently, meaning that there's less waste for the kidneys to filter, and giving them more energy to fight the disease. A diet high in carbohydrates, on the other hand, jacks everything up to hi-speed, too high--generating lots of waste and putting lots of stress on the kidneys.
So goes the theory, anyway. The reality is: it's complicated.
It's complicated because kidneys are complicated, and kidney failure even more so. It's not just the loss of filtering ability or concentrating urine. Kidneys generate erythropoietin, which stimulate the production of red blood cells. They regulate blood pressure--25% of the body's blood volume passes through the kidneys every minute. They regulate blood pH, calcium levels, sodium and potassium. Since renal failure isn't obvious until 70% of kidney function is lost, maintaining whatever kidney function is left becomes critical.
And it could very well be that decreasing azotemia at any cost may be the best course of action for this. I theorize that it has to do with the stage of the disease. If you caught it early--as I did (so early that the vet had to do a urinalysis to be sure)--then perhaps a diet of high-quality protein might buy you more time. As the disease progresses, perhaps low quantities of protein will be more important.
I'm not a vet. What I write is based entirely on my understanding of metabolism and biochemistry from my few years in medical school. Your vet will most likely think that my advocating a raw diet--which is not actually a high-protein diet, as meat is only about 20% protein--is heresy to begin with, and doubly so for a sick cat.
Yet the Tweeb is doing well. Questioning dogma is something I do regularly anyway, not necessarily with concrete evidence. To have a living, breathing, healthy counterpoint to accepted practice merely enforces what some might consider a bad habit.