Not protein, but phosphorus. A conversation with Marlena Stodolak-Podziewska about cats with kidney disease

In September 2026, the first new iCatCare guidelines on this disease in a decade were published. I covered them separately, from the medical side — what changed in them and what stayed the same. This conversation is about something else: the bowl. About what the owner should do tomorrow morning.

I asked Marlena Stodolak-Podziewska, a dog and cat nutritionist who runs the brand Zoodietetyka. We spoke after the new guidelines were announced, so her answers take them into account.

A cat is an obligate carnivore — after a diagnosis of chronic kidney disease (CKD) it is easy to fall into the trap of thinking “protein in the diet must be reduced”. This stems from the fact that protein is a natural source of phosphorus, and limiting phosphorus is key in managing chronic kidney disease. It must be remembered, however, that restricting protein too much can mean a loss of muscle mass, especially in an older cat. The 2026 iCatCare consensus guidelines on the diagnosis and management of chronic kidney disease in cats point above all to controlling protein intake and monitoring body weight and muscle condition.

What does this mean in practice?

After a CKD diagnosis, the first thing to change in the bowl is above all the amount of phosphorus taken in — not reducing protein at all costs.

Restricting protein is justified only in specific situations, and even such extreme situations are not an indication for a meat-free diet in a cat.

— Marlena Stodolak-Podziewska

Besides a controlled phosphorus intake, it is necessary to monitor fluid intake. That is why wet food and/or a cooked diet are the only correct forms of feeding a cat with kidney disease.

In an ideal world, after hearing a diagnosis (not only CKD), the vet, having consulted a nutritionist, would hand the worried owner a package of food perfectly suited to their pet.

Unfortunately, a 2021 study of the German market showed that in many commercial products the phosphorus content far exceeds recommended or legally set values. Over 90% of the complete foods tested supplied twice the required amount of phosphorus, and the problem also affected dietetic foods for patients with kidney failure, where the standards were also often drastically exceeded.

The problem lies mainly in inorganic phosphorus — it does not occur naturally in food, but is added during technological processes to give the food the properties manufacturers want (such as colour or texture). These forms are fully digestible and lead to a rapid, high rise in blood phosphorus after a meal.

— Marlena Stodolak-Podziewska

Complete commercial foods — leaving aside the previously mentioned issues of incorrect amounts of micro- and macronutrients — have one important drawback: they are produced for the general population, not for a particular case. They assume a specific scenario: a cat weighing X, aged Y, without allergies and so on. Unfortunately, life writes different scenarios, and very often there simply is no food with those specific parameters that would be ideal for a given cat.

The premise of a home-prepared diet — whether raw or cooked — is different: the individual case and the pet's preferences are put on a pedestal, along with responding to a changing (often rapidly changing) situation.

Whether the animal can stay on the established diet depends solely on its health and well-being, and also on the owner's own capabilities.

Introducing wet food into the diet is the most sensible decision when kidney disease is diagnosed, and also as prevention. A small amount of water can gradually be added to wet food while watching whether the cat still eats willingly. The amount of water can be increased if the texture and taste remain acceptable to the cat. It is also worth swapping crunchy treats for paste treats — they contain much more water.

I do not recommend, however, giving milk and dairy products, juices, drinks for people or ready-made “flavoured” additives. They are not needed to hydrate a cat and may supply unwanted ingredients, including phosphates.

Importantly — there is no need to push the cat to drink a set amount of water “by force”. In CKD, what counts is total fluid intake — including the water in the food. That is why a cat eating wet food with added water may be taking in much more fluid than watching it drink from the bowl would suggest.

Any supplements should always be used after consulting a vet and a nutritionist. What I usually pay attention to is that the supplements bought contain as few substances as possible — the point is not to give “vitamins for the kidneys”, but substances that genuinely improve the animal's comfort and address its current problems.

For one cat this will be an extra source of fibre (such as psyllium gel) to prevent constipation, for another B vitamins, in particular B9 and B12 — there is no single, perfect recipe for ideal supplementation.

The forbidden kind of supplementation is the chaotic, ill-considered kind.

— Marlena Stodolak-Podziewska

Any fast lasting longer than a few hours in a cat is reason enough to see the vet. A prolonged lack of food can lead to health problems such as hepatic lipidosis, which is dangerous to the cat's life.

— Marlena Stodolak-Podziewska

In a cat with CKD, as kidney function worsens, metabolic waste products accumulate in the body and affect the centres responsible for nausea and appetite. The cat may then approach the bowl, sniff the food and walk away, smack its lips, lick its mouth and drool.

The 2026 guidelines point even more strongly to the view that cats with CKD may have an appetite that changes from time to time, and that it is worth having several nutritionally suitable options.

It is also worth not being afraid, during a crisis, to give favourite treats agreed in advance with the vet — sometimes even a small amount of a tasty paste on the food encourages the animal to eat.

These days, the most important thing — we don't analyse test results with artificial intelligence. Reading blood tests means understanding the correlations between individual parameters, and the fact that “something is flagged red” doesn't mean that parameter is already working against the cat's health. Sometimes it is a sign that, for example, the cat was very stressed during the blood draw, or that the time between drawing the blood and testing it in the lab was much longer than recommended.

The stages of chronic kidney disease were introduced to give a better idea of how advanced the disease is in a given patient. For example — stage 1 is when, say, abnormal kidney imaging is found on ultrasound with no clinical signs, or very subtle ones, while stage 3 already means moderate azotaemia (abnormally high urea and creatinine) and clear clinical signs. On this basis, the vet may additionally divide stage 3 into an early and a late phase.

The diet for each stage differs mainly in the use of supplements for specific problems (e.g. increased trouble passing stools or a critically low appetite), but the main principles remain the same — restricted phosphorus, easily digestible protein and hydration.

A CKD diagnosis often sets off an avalanche: new food, supplements, several medications, drinking bowls, a fountain, fluid therapy, tests, weighing, watching every pee — and preferably all on the same day. We can't change everything at once — what matters is setting an individual plan, adapted to the cat and to the owner's capabilities.

In addition, and importantly — we don't force the cat to eat the perfect food at any cost. I mean the situation where the owner, out of care, removes everything the cat liked so far and then tries to “wait it out” until the cat eats the new food. In CKD it is especially important to maintain adequate food intake. The feeding plan should be adapted to the particular cat, its appetite and any concurrent diseases.

And most importantly of all — we do everything we can for the cat's comfort, and we don't try to prolong its life at any cost at the expense of its quality.

— Marlena Stodolak-Podziewska

The goal of CKD treatment is at the same time to control symptoms, maintain quality of life and — where possible — slow the progression of the disease. In later stages, symptom control and comfort become especially important.

Don't panic, don't say goodbye to your pet already — CKD is not a death sentence, but a condition a cat can live with for a long time. The outlook is promising: CKD is usually progressive, but the rate of progression varies greatly between cats. According to the 2026 iCatCare guidelines, cats with stable disease can stay in good condition for many years.

Chronic kidney disease is special in that it has no single disease profile — every case is slightly different, which is why changes must not be made on your own without agreeing them with the vet. Here I mean consulting test results and vets' decisions with artificial intelligence or groups on social media.

It's worth keeping a folder with all of the cat's test results and noting down any changes — different meal times, a bigger or smaller appetite, or even a worse-looking coat. Seemingly small details like these are very important at the clinic.

— Marlena Stodolak-Podziewska

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References

1. Dobenecker, B. (2021). Phosphataufnahme durch auf dem deutschen Markt befindliche Alleinfutter und Nierendiäten für Hunde und Katzen, Tierärztliche Praxis Ausgabe K: Kleintiere/Heimtiere, 49(4), 247–254doi:10.1055/a-1487-8625
2. Taylor, S., Finch, N., Caney, S., Elliott, J., Geddes, R., Mortier, F., Parker, V., Quimby, J., Segev, G. & White, J. (2026). 2026 iCatCare consensus guidelines on the diagnosis and management of chronic kidney disease in cats, Journal of Feline Medicine and Surgery, 28(9)doi:10.1177/1098612X261466553
3. International Renal Interest Society (2026). IRIS Treatment Recommendations for CKD in Cats, IRIS Kidney↗

Frequently asked

Does a cat with chronic kidney disease have to go on a reduced-protein diet?

Not as the first step. The nutrient that matters most is phosphorus, and protein is its natural source — which is where the whole misunderstanding comes from. Restricting protein itself is justified only in specific situations, and cutting it too hard costs muscle mass, which worsens the outlook in an older cat. Even in extreme cases, a meat-free diet is not appropriate for a cat.

Do I have to give up BARF or home-cooked food after the diagnosis?

Not necessarily. A home-prepared diet has the advantage of putting the individual cat first and allowing a response to a changing situation, which ready-made food by definition does not do, because it is made for the general population. Whether the animal can stay on it depends on its health and well-being, and on what the owner can realistically manage. Balancing such a diet in kidney disease requires working with a vet and a nutritionist.

What can I add to food to keep a cat with kidney disease hydrated?

Water — gradually, to wet food, while watching whether the cat still eats willingly. It is also worth swapping crunchy treats for pastes, because they contain much more water. Milk and dairy products, juices, drinks for people and ready-made flavour additives are not recommended: they are not needed for hydration and may supply unwanted ingredients, including phosphates.

My cat with kidney disease has stopped eating. How long can I wait?

You should not wait. In a cat, going without food for more than a few hours is reason enough to see the vet, because a prolonged lack of food risks hepatic lipidosis, which is life-threatening. In kidney disease, a loss of appetite is usually caused by accumulating metabolic waste products that act on the centres responsible for nausea — this is a medical situation, not a whim.