Phosphorous is an important mineral usually found in nature combined with oxygen as phosphate. Around two thirds of cats with CRF have high blood phosphate levels as the kidneys are normally responsible for excreting phosphorous from the body. Whilst an important component of cells and bone in the mammalian body, too much phosphate (hyperphosphataemia) contributes towards hyperparathyroidism (over production of parathyroid hormone which regulates the amount of calcium and phosphorus in the body).
The use of low protein diets will achieve a reduced phosphorous intake, but this will not always be sufficient to prevent the hyperphosphataemia encountered in CRF. If hyperphosphataemia persists despite dietary restriction, or if the cat will not eat a prescription renal diet, then oral phosphate binders should be given with meals.
Oral phosphate binders stick to phosphate present in the diet and limit what is absorbed by the bowel. Aluminium hydroxide, aluminium carbonate or aluminium oxide have been most commonly used, however some of these compounds are no longer available. Calcium-based agents are another option (calcium carbonate and calcium acetate) but their use may lead to excess levels of calcium in the body (hypercalcaemia), so these agents should be used with care and the blood calcium levels monitored.