Kidneys role is to filter waste and toxins from the blood, and maintain the normal balance of fluid and minerals within the body. As pets are getting older, damage to kidney tissues may occur.  This process is irreversible, and will lead to further loss of kidney function.  Renal Cats may also experience loss of appetite and their caloric intake might decline, which may lead eventually to loss of muscle mass.
Hill's nutritionists and veterinarians developed Prescription Diet™ k/d™, clinical nutrition to support your cat's kidney function and help sustain muscle mass.  In fact, the nutrition of k/d is clinically tested nutrition to improve & lengthen quality of life.