Hi there @zarinameykhori,
Thank you for your question. I am sorry to hear your cat is having problems.
Cats have specific nutritional needs that need to be taken into consideration when choosing a diet for them. As cats get older their individual requirements can change depending on behaviour (generally less mobile- therefore less calories needed) and any disease processes they may be encountering (heart, kidney, thyroid dysfunction, arthritis).
It is important to have dietary modification in cats with kidney disease as this can help improve quality of life and slow the progression of the disease.
Often diets with reduced protein and reduced levels of Phosphorus are suggested for cats with renal problems. For heart disease, a reduction in salt (Sodium) in the diet is important.
There are a wide choice of diets available to your pet from supermarket brands through to prescription diets. Often the trickiest part is getting our feline friends to try new foods. This can be challenging at times. Remember to always introduce a new food gradually (over several days), by mixing a small amount of the new diet in with the old diet. The amount of new food can then be increased slowly.
With any medical condition; along with potential dietary changes, there are usually multiple drug therapies involved and it is important to discuss with your veterinarian on the type of treatment and diet that is best for your cat.