Dietary management also plays a role in treating problems associated with diseases of major internal organs, such as the heart, liver and kidneys.
Prescription diets, available from veterinarians, are usually recommended after hospitalization and treatment have stabilized the animal. With heart disease, the main dietary consideration is a low sodium level, which helps reduce blood pressure and the work load on the heart. Prescription diets for treating kidney disease have reduced levels of protein (but high quality protein), phosphorus and sodium, because the diseased kidney does a poor job of excreting these. They also contain extra amounts of B-complex vitamins, which tend to be lost through the diseased kidney. A more recently marketed prescription diet is aimed at increasing excretion of nitrogenous waste (from protein metabolism) through the intestinal tract to make up for the lost kidney function.
Dogs with liver disease are frequently fed these same diets designed for kidney disease. http://www.espomagazine.com/vet/jul98.htm