We have always been told to support potassium levels. We gave them both Sundown potassium-a third or a half of a tab twice a day for Cooper, four chunks a day for Myrna; Cooper had Tumil K twice a day (and now she will finish the pills.) By giving the following oils, we are giving omegas and antioxidants and whatever else comes from the mix of these oils: a gel cap of Krill oil 300mg (this replaces the usually recommended cod liver oil which is no longer pure and cod is almost extinct from over fishing), COQ10 100mg, and 1/2 of 100IU vitamin E, mixed with 1ml of water. I give this orally via a syringe because neither was eating all of their food if the oil was mixed in. I put a smidge of Halo xtra C vitamin C powder (made for cats) in their food. They take 1/2 of a B12 (for energy), 1/4 magnesium (needed electrolyte and muscle/nerve help), 1/4 iron (helps to make blood cells for kidneys)-every other day (it had been every day but I was worried about hypervitaminosis but perhaps more doses could be given.) If the phosphorous level begins to creep up and is above a 4, give a phos binder. Cooper was on a phosphorous binder which binds with iron and causes constipation so I had just decreased his iron to a weekly amount.
In addition, we need to decrease protein intake a tad if kidney values are creeping up and decrease it a lot if kidney values are high. Protein in food should be 6-9% for cats needing to lower protein. Watch those treats-they have about 30% protein. With Myrna, we mix lower protein foods with her regular food and I am breaking up treats when she gets them. She gets a lot due to her litter box encouragement process. (She needs us to take her to the box, encourage her to go, and treat and praise her, and when she goes, treat and praise her again. Helps her go in the box and not have accidents.)
Some Science Diet canned cat food sold at the pet store has 6-9%. It was hard to find an over the counter, low phos cat food because the stores did not carry all of the types flavors in a brand that are produced. Fancy Feast makes pouches of broth with fish. If you buy the "Classic", it is broth. There is another version with milk which I would not use. The broth had very low protein due to it being mostly water. But when poured over his food, this broth enticed Cooper to eat. Tuna fish in a can is low in phos but may not be the best thing to give due to pollution and such. But sometimes, that was all he would eat. Or if we put some on top of his food, he would eat everything.
And finally, water. To support kidneys, the cat must take in water. If your cat gets sick and stops taking in enough water, the cat will become dehydrated and kidney function can be impaired. If the cat stops drinking enough, you can gently give liquids in a syringe or eye dropper by mouth, about 6-9mls every couple of hours.