If your cat's blood phosphorus level is over 6 mg/dl (USA) or over 1.9 mmol/L (international), it is too high and you need to get it under control.

Keeping phosphorus levels under control can help slow the progression of the CKD, reduce the risk of serious problems and make your cat feel better.

Feeding a food low in phosphorus is the first and best step. Ideally you want a food with a level below 0.5% phosphorus on a dry matter analysis basis. Therapeutic kidney diet foods are the only complete foods which meet this criterion.

If your cat will not eat the therapeutic kidney diet, feeding a food as low in phosphorus as possible and adding a phosphorus binder to the food when appropriate can help control your cat's phosphorus levels.

On the left is a list of this page's contents.

Please also visit the Phosphorus Binders page.

Why High Phosphorus Levels Matter

Phosphorus is a mineral essential for good health which is contained in many foods. The body is very good at regulating its phosphorus levels by removing excess phosphorus via the kidneys. However, the kidneys of a CKD cat can no longer efficiently excrete excess phosphorus, so the vast majority of CKD cats will develop levels of phosphorus in their blood that are too high: this is known as hyperphosphataemia.

In contrast to the protein debate, there is no dispute about the importance of treating hyperphosphataemia because of the problems it causes. Indeed, an update on the Western Veterinary Conference 2017 reports that "phosphorus restriction may be the "single most powerful treatment," according to Dr. Chew."

High phosphorus levels may:

Make the Cat Feel Unwell

High phosphorus levels can cause a variety of unpleasant symptoms and generally make a CKD cat feel quite unwell. There is a list of possible symptoms below.

Cause CKD Progression

High phosphorus levels can make CKD progress more quickly. A human study, Serum phosphate and mortality in patients with chronic kidney disease (2010) Eddington H, Hoefield R, Sinha S, Chrysochou C, Lane B, Foley RN, Hegarty J, New J, O'Donoghue DJ, Middleton RJ & Kalra PA Clinical Journal of the American Society of Nephrology 5(12) pp2251-2257, found that the risk of dying for patients in stages 3 and 4 was higher if they had higher phosphorus levels, even if the phosphorus was technically within normal range. A study of cats, Survival in cats with naturally occurring chronic kidney disease (2000-2002) (2008) Boyd LM, Langston C, Thompson K, Zivin K & Imanishi M Journal of Veterinary Internal Medicine 22(5) pp1111-7, found that there was an 11.8% increase in the risk of death for every one mg/dl increase in phosphorus in the cat's blood.

Survival of cats with naturally occurring chronic renal failure: effect of dietary management (2000) Elliott J, Rawlings JM, Markwell PJ, Barber PJ Journal of Small Animal Practice 41(6) pp235-242, found that the cats who ate reduced phosphorus food or food with added phosphorus binders lived more than twice as long as those who did not.

The role of phosphorus in the pathophysiology of chronic kidney disease (2013) Geddes RF, Finch NC, Syme HM, Elliott J Journal of Veterinary Emergency & Critical Care (San Antonio) 23 pp122-133 reports that a phosphatonin called fibroblast growth factor 23 (FGF-23) "is now considered to be a key regulator of plasma phosphorus concentration in people" and states that the role of FGF-23 in companion animals with CKD is also being investigated. Early Detection discusses the role FGF-23 may play in the early detection of CKD.

Laboratory evaluation in dogs and cats with chronic kidney disease (2015) Grauer GF Clinician's Brief May 2015 pp65-69 states "Tighter control of hyperphosphatemia, renal proteinuria, and systolic hypertension may improve treatment outcome."

Cause Secondary Hyperparathyroidism

Elevated phosphorus levels can adversely affect calcium levels and eventually can trigger problems with a hormone called parathyroid hormone (PTH), causing a serious condition called secondary hyperparathyroidism (which is not the same thing as hyperthyroidism, although Cornell University College of Veterinary Medicine does mention that around 20% of cats with hyperthyroidism also have elevated phosphorus levels).

Chronic kidney disease (CKD) in dogs and cats — staging and management strategies (2015) Chew D Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference states that it is possible for a CKD cat to develop secondary hyperparathyroidism even if phosphorus levels and ionised calcium levels are normal. He explains "In the early stages of chronic kidney disease increased levels of parathyroid hormone (PTH) keep serum phosphorous within the normal range by increasing phosphate excretion into urine. This allows for normalization of serum phosphorous at the expense of hyperparathyroidism." Therefore, even a cat with normal phosphorus levels but with elevated PTH levels would benefit from phosphorus restriction.

Relationship among serum creatinine, serum gastrin, calcium-phosphorus product, and uremic gastropathy in cats with chronic kidney disease (2014) McLeland SM, Lunn KF, Duncan CG, Refsal KR & Quimby JM Journal of Veterinary Internal Medicine 28(3) pp827-37 compared CKD cats with healthy cats. They found that "The high incidence of fibrosis and mineralization among cats with CKD suggests that more aggressive management of hyperphosphatemia and renal secondary hyperparathyroidism should be considered."

Reduce Response to Anaemia Treatment

High phosphorus levels may reduce the cat's response to Erythropoiesis stimulating agents (ESAs), a treatment used for severe anaemia, which is relatively common in CKD cats.

Symptoms of Hyperphosphataemia

There are a number of different symptoms of high phosphorus levels. You may not see all of these symptoms, and some of them may have other causes as outlined under each symptom. However, if you see any of the symptoms described below, please ensure that your cat's phosphorus level is checked (via bloodtests) and is no higher than 6 mg/dl (US) or 1.9 mmol/L (international).

Loss of appetite

Itching

Lack of co-ordination in the limbs

Loss of Appetite

High phosphorus levels can make a cat feel bad and lead to a loss of appetite, particularly if secondary hyperparathyroidism develops.

Other causes include levels of toxins in the blood which may cause excess stomach acid, anaemia, crashing, metabolic acidosis, mouth ulcers, fluid build-up, the use of antibiotics, constipation or the use of medication for hyperthyroidism. Dental problems may also cause loss of appetite.

Cats who do not eat are at risk of developing a potentially life-threatening condition known as hepatic lipidosis; Mar Vista Vet has more information about this. Therefore, it is important to try to find the cause and treat it as quickly as possible.

Itching

Itching is fairly common in cats with high phosphorus levels, particularly if the high phosphorus levels go untreated, resulting in secondary hyperparathyroidism.

Itching may also be caused by general levels of toxins in the blood. Alternatively itching may indicate a vitamin B deficiency or be a sign of an essential fatty acids deficiency. Itching on the face in particular may be a side effect of medication for hyperthyroidism. Occasionally itching can be a sign of liver problems; if this is the case, your cat's bloodwork should show elevated liver values.

Lack of Co-ordination in the Limbs/Back Leg Weakness

This can be due to high phosphorus levels interfering with the nerve messages that control the limbs, a condition known as neuropathy. Some of the symptoms include "forgetting" where the hind legs are (getting up and leaving without them, for instance, or leaving them in the air after licking them), or stumbling and feet crossing over when walking.

Plantigrade Posture

You may also see a plantigrade posture (as demonstrated by Ollie to the left), where the cat walks on his/her hocks instead of his/her feet. This is most common in diabetic cats, but may sometimes be seen in cats with high phosphorus levels, or with neurological problems from other causes. In Ollie's case, the cause was low potassium levels.

Long Beach Animal Hospital has a photograph of a cat with diabetic neuropathy doing this (scroll down to Symptoms).

Newman Veterinary has a good before and after photo of a diabetic cat with this problem, scroll down a little to Other Common Consequences, then click on Plantigrade Stance (in red font).

Teeth Grinding

If your cat's high phosphorus levels go untreated, your cat may eventually develop a condition called secondary hyperparathyroidism. One symptom of secondary hyperparathyroidism is "rubber jaw", which may present as teeth grinding.

Other more common causes of teeth grinding include excess stomach acid, dental problems and dehydration.

Youtube has a video of a cat grinding his/her teeth.

Knuckling

"Knuckling" may also be seen, where the cat walks on the top of the foot with the toes tucked underneath, appearing almost to be dragging the toes behind. I haven't been able to find a photo of a cat with high phosphorus levels doing this, but there is a video on youtube of a dog with a similar problem (but with a different cause).

Weakness

Weakness and muscle wasting may be seen, especially in the back legs. This can be caused by high phosphorus levels leading to secondary hyperparathyroidism.

Weakness in the back legs is often caused by low potassium levels or occasionally by low magnesium or low calcium levels; while muscle wasting may be caused by metabolic acidosis. General weakness may be caused by anaemia. If your cat no longer jumps, this may be thought to be weakness when in fact it is an unwillingness to jump because of blindness caused by hypertension. An inability to jump or climb may also be caused by arthritis.

If your cat suddenly cannot walk properly on one leg, particularly a back leg, and the leg feels cold to the touch, this may indicate a heart-related problem known as an arterial thromboembolism. This is a medical emergency, and you need to contact your vet as soon as possible.

Weight Loss

This may be caused by high phosphorus levels.

Other causes include proteinuria or metabolic acidosis. Weight loss may also be a symptom of other diseases such as diabetes or hyperthyroidism. Other possible causes include IBD (inflammatory bowel disease) or cancer.

The World Small Animal Veterinary Association has a body condition score chart showing how to gauge your cat's physical condition.

Nausea

High phosphorus levels and secondary hyperparathyroidism it may cause may result in nausea, which may be manifested as a lack of appetite.

Anaemia or metabolic acidosis may also cause nausea.

Twitching, Trembling or Shaking

Twitching may be caused by high phosphorus levels.

Other possible causes of twitching in CKD cats include:

low or high potassium levels

high toxin levels (uraemia)

high blood pressure

calcium imbalances (especially head twitching)

hyperthyroidism

Vitamin B deficiency

maropitant (Cerenia) for nausea

If your cat only twitches while you are giving fluids, it is probably caused by either the type of fluid used or by giving cold (room temperature) fluids but make sure your cat is not being overhydrated.

Pharaoh's Shakes is a video showing a CKD cat twitching.

Pet MD mentions that twitching may be caused by kidney disease.

Guide to tremor and twitch syndromes in dogs and cats (2021) Lowrie M InPractice Jan/Feb 21 pp4-17 discusses possible causes of twitching.

Measuring Phosphorus and Target Levels

Your vet can determine if your cat's phosphorus levels are too high by checking your cat's bloodwork for phos, P or Pi (these are all abbreviations for phosphorus). Most vets do this routinely, but if yours does not, ask for it to be done.

Ranges for phosphorus levels

Goal for phosphorus levels in CKD cats

When to take action to reduce phosphorus levels

Veterinary references for phosphorus goals

Monitoring phosphorus levels

Summary of phosphorus goals

Ranges for Phosphorus Levels

A typical laboratory range for phosphorus levels in cats is:

USA: 2.5 - 7.5 mg/dl (USA)

Internationa: 0.81 - 1.61 mmol/L

Unfortunately, many vets see that a CKD cat's phosphorus level falls within the normal range for their laboratory and are satisfied, forgetting that the higher end of the range only applies to healthy growing kittens.

Even a healthy older cat should have phosphorus levels in the bottom half of most laboratory ranges. The role of phosphorus in feline chronic renal disease (2010) Chew D & Kidder AC CVC in San Diego Proceedings states "Levels of serum phosphate are often higher in young growing animals than in adults...The normal range for many laboratories unfortunately includes that of adult and growing animals, making it difficult or impossible to detect early rises in serum phosphorus above normal. Serum phosphate concentration is less than 5.5 mg/dl in most healthy adults."

If a healthy adult cat needs phosphorus to be in the lower part of most laboratories' reference ranges, it is even more important for a CKD cat. Chronic kidney disease (CKD) in dogs and cats — staging and management strategies (2015) Chew D Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference states "Control of total body phosphate burden is an overarching goal in the management of CKD. Increasing phosphate retention during CKD occurs even as serum phosphorus values remain within the reference range."

Phosphorus control is also important in CKD because if the cat's phosphorus multiplied by calcium is higher than 70 (US) or 5 (international), the cat is at risk of tissue calcification (see secondary hyperparathyroidism).

So if your vet tells you that your cat's phosphorus level is fine, don't just take that at face value, ask for the exact level. Chances are it is too high for a CKD cat.

Goal for Phosphorus Level in CKD Cats

So how low do you want your cat's phosphorus level to be?

I used to recommend (as did many CKD specialists) that you try to get the level down to 4.0 mg/dl (US) or 1.3 mmol/L (international).

Chronic kidney disease (CKD) in dogs and cats — staging and management strategies (2015) Chew D A Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference gives a slightly less ambitious target: "A targeted serum phosphate of less than 4.5 mg/dl [1.45 mmol/L] is recommended for CKD patients to ensure less phosphate burden in the bodies of these dogs and cats."

There is nothing wrong with these goals per se, and if you can get your cat's phosphorus level down to 4-4.5 mg/dl or 1.30-1.45 µmol/L, that is great; but as the CKD progresses, achieving such levels can become more challenging. IRIS treatment recommendations for CKD in cats (2023) International Renal Interest Society therefore recommend a staged approach to phosphorus control, as shown in the table below.

These values apply to adult cats. If your cat is very young (6-12 months old), then a higher phosphorus target is appropriate, because kittens need phosphorus for their bones to grow properly. Check out the table above and discuss with your vet what would be an acceptable level.

Don't forget, these stages apply to stable cats; if your cat is dehydrated, for example, the bloodwork will not be accurate.

Phosphorus Goal: US Values

Stage of CKD	Creatinine Level in mg/dl	Phosphorus Goal
IRIS Stage 2	Between 1.6 and 2.8	2.7 - 4.59 mg/dl
IRIS Stage 3	Between 2.9 and 5.0	Below 5.0 mg/dl
IRIS Stage 4	Over 5.0	Below 6.0 mg/dl

Phosphorus Goal: International Values

Stage of CKD	Creatinine Level in mmol/L	Phosphorus Goal
IRIS Stage 2	Between 140 and 249	0.9 - 1.49 mmol/L
IRIS Stage 3	Between 250 and 439	Below 1.60 mmol/L
IRIS Stage 4	Over 440	Below 1.90 mmol/L

When to Act to Reduce Phosphorus Levels

You must definitely take action if your cat's blood phosphorus level is above 6 mg/dl (US) or 1.9 mmol/L (international). This applies whatever stage of CKD your cat is in.

Ideally you want your cat's phosphorus level to be no more than 4.0 mg/dl (US) or 1.3 mmol/L (international).,

This ideal target can be harder to achieve as the CKD progresses, which is why IRIS treatment recommendations for CKD in cats (2023) International Renal Interest Society state that you should try to control high phosphorus levels depending upon the CKD stage, as follows:

Stage 2: phosphorus should be no higher than 4.6 mg/dl or 1.5 mmol/L

Stage 3: phosphorus should be no higher than 5.0 mg/dl, or 1.6 mmol/L

Stage 4: phosphorus should be no higher than 6.0 mg/dl, or 1.9 mmol/L

So if, for example, your cat is in Stage 2 with a phosphorus level of 5.5 mg/dl, you should definitely be treating it.

Additional Guidelines for Cats in Stages 1 or 2

Following the introduction in 2022 of a new test for a renal biomarker called fibroblast growth factor 23 (FGF-23), the International Renal Interest Society issued new guidelines in 2023. IRIS treatment recommendations for CKD in cats (2023) International Renal Interest Society state that for cats in CKD Stages 1 or 2, where phosphorus levels may appear normal, "Measurement of FGF23 may help to identify cats which would benefit from dietary phosphate restriction where plasma phosphate is in the target range. FGF23 >400 pg/ml in the absence of hypercalcaemia, anemia or marked inflammatory disease is an indication to start dietary phosphate restriction. Monitor serum calcium and if total calcium exceeds 12 mg/dl (3 mmol/l) switch the cat to a less phosphate restricted diet."

What this is saying is that if your cat is in early stage CKD, with phosphorus level apparently in the normal range, but your cat's FGF-23 level is above 400 pg/ml, you need to take steps to control your cat's phosphorus levels.

Additional Guidelines for Cats in Stage 4

For cats in Stage 4 CKD, IRIS treatment recommendations for CKD in cats (2023) International Renal Interest Society state "FGF23 may assist in determining whether further phosphate restriction would be beneficial to the cat. If FGF23 is >700 pg/ml, further phosphate restriction should be applied (e.g. by increasing the dose of phosphate binder) provided there is no evidence of hypercalcaemia, marked anemia or severe inflammatory disease (all of which can increase FGF23 independently of mineral bone disturbance)."

Additional Guidelines for Cats with High Phosphorus x Calcium

Even for cats with lower phosphorus levels, action is necessary if:

phosphorus multiplied by total calcium is higher than 70 in US values; or
phosphorus multiplied by total calcium is higher than 5 in international values

This is because the cat is at risk of tissue calcification (see secondary hyperparathyroidism), so action is necessary.

Veterinary References for Phosphorus Goals

If your vet refuses to accept that a phosphorus level above 6 mg/dl (US) or 1.9 mmol/L (international) is too high for your cat, print out and show him/her some of these references:

IRIS treatment recommendations for CKD in cats (2023) International Renal Interest Society provide detailed guidelines for each stage of CKD.

ISFM consensus guidelines on the diagnosis and management of feline chronic kidney disease (2016) Sparkes AH, Caney S, Chalhoub S, Elliott J, Finch N, Gajanayake I, Langston C, Lefebvre H, White J & Quimby J Journal of Feline Medicine & Surgery 18 pp219-239 make similar recommendations to IRIS.

Proteinuria and renal disease: a round table discussion (2005) - on page 14 Professor J Elliott of the Royal Veterinary College states that "the goal should be to keep the serum phosphorus concentration at the lower end of the reference range."

Phosphatemia management in the treatment of chronic renal disease: a round table discussion (2006) Elliott J, Brown SA, Cowgill LD, Grauer GF, Polzin DJ, Fernandez del Palacio MJ & Lefebvre H states "practitioners need to realise that a phosphate in the normal range could still be abnormal in renal patients." Page 6 provides detailed recommendations on phosphorus control.

Monitoring Phosphorus Levels

If your cat is eating a therapeutic kidney diet, it may take up to a month for you to see a difference in your cat's blood phosphorus levels.

If you are using phosphorus binders, you should start to see a difference in blood phosphorus levels around 7-10 days after starting binders.

IRIS treatment recommendations for CKD in cats (2023) International Renal Interest Society recommend checking phosphorus and calcium levels every 4-6 weeks to start with, reducing to every 12 weeks once your cat is stable. Dr Chew makes similar recommendations.

Since in my experience most people will be visiting the vet fairly frequently in the first month after diagnosis, I suggest checking your cat's blood phosphorus level every two weeks until the level is acceptable, then checking it every 2-3 months to see if any adjustments to your treatment plan are necessary. This is worth doing because, according to Treatment options for hyperphosphatemia in feline CKD: what's out there (2009) Kidder AC & Chew D Journal of Feline Medicine and Surgery 11(11) pp913–924, "Serum phosphorus concentration may increase in CKD cats that increase their food intake following other supportive CKD treatments."

Summary of Phosphorus Goals

USA

You must definitely take action as soon as your cat's phosphorus level goes above 6 mg/dl, though in some cases you should take action sooner (see above).

You must also take action if your cat is in CKD Stage 1 or 2 with normal phosphorus levels, but with a fibroblast growth factor 23 (FGF-23) level above 400 pg/ml.

If your cat's level is below 6 mg/dl, but multiplying it by your cat's calcium level gives you a reading of 60-70 or over, take action now.

Your aim is to reduce your cat's phosphorus to a level of below 4.6 mg/dl. You may not be able to get it this low if your cat is in IRIS Stage 3 or 4, but aim never to let it go above 6 mg/dl.

You can read below about ways to reduce the level. At first check phosphorus levels every 2-4 weeks to see if any adjustments to your treatment plan are necessary.

If your cat is very young (6-12 months old), then a higher phosphorus level is appropriate, because kittens need phosphorus for their bones to grow properly. Check out the information above and discuss with your vet what would be an acceptable level.

Low phosphorus levels (below 4 mg/dl) are extremely rare in CKD cats, but are generally not considered to be a problem as long as they do not fall below 2.7 mg/dl. I would try not to go below 3 mg/dl. See Diagnosis for more information.

International

You must definitely take action as soon as your cat's phosphorus level goes above 1.9 mmol/L, though in some cases you should take action sooner (see above).

You must also take action if your cat is in CKD Stage 1 or 2 with normal phosphorus levels, but with a fibroblast growth factor 23 (FGF-23) level above 400 pg/ml.

If your cat's level is below 1.9 mmol/L, but multiplying it by your cat's calcium level gives you a reading of 5 or over, take action now.

Your aim is to reduce it to a level of below 1.5 mmol/L. You may not be able to get it this low if your cat is in IRIS Stage 3 or 4, but aim never to let it go above 1.9 mmol/L.

You can read below about ways to reduce the level. Check phosphorus levels every month to see if any adjustments to your treatment plan are necessary.

If your cat is very young (6-12 months old), then a higher phosphorus level is appropriate, because kittens need phosphorus for their bones to grow properly. Check out the information above and discuss with your vet what would be an acceptable level.

Low phosphorus levels (below 1.3 mmol/L) are not normally present in CKD cats, but are generally not considered to be a problem as long as they do not fall below 0.9 mmol/L. I would try not to go below 1 mmol/L. See Diagnosis for more information.

Ways to Control High Phosphorus Levels

As discussed above, focusing on phosphorus control is one of the most effective ways to help your CKD cat feel better and live longer. The good news is, it is usually not too difficult or expensive to do this, though it may become harder to keep your cat's phosphorus levels as low as you would like as the CKD progresses..

There are two main ways to control phosphorus levels, in order of preference:

Controlling phosphorus levels with diet

Controlling phosphorus levels with phosphorus binders

Controlling Phosphorus Levels with Diet

How high your cat's blood phosphorus level should be depends upon the stage of the disease (see above for more information) but it should always be below 6 mg/dl (US) or 1.9 mmol/L (international).

The first line of phosphorus control is to feed foods low in phosphorus. If your cat's phosphorus level is currently below the level mentioned above, it is still wise to feed a food as low in phosphorus as you can, because the less phosphorus your cat eats, the less work there is for the kidneys to do trying to process it. Also, phosphorus levels tend to rise as the CKD progresses, and usually it is easier to keep them low rather than have to work on reducing them.

Target Phosphorus Content in Food

What do I mean by foods low in phosphorus?

Small Animal Clinical Nutrition - 5th Edition, Chapter 37 Chronic Kidney Disease (2010) Hand MS (Ed.) Mark Morris Institute states that "To achieve beneficial effects, the recommended phosphorus levels for foods used to manage CKD are 0.3 to 0.6% DM for cats." DM means dry matter analysis (DMA), which is different to the percentage given on the can.

Management of feline chronic failure (1998) Brown S Waltham Focus 8(3) pp27-31 states that ideally you want to feed a food with a phosphorus level under 0.5% on a dry matter analysis basis.

Feeding a Therapeutic Kidney Diet

A therapeutic kidney diet has many characteristics, but one of the most important is the reduced phosphorus content. Many therapeutic kidney diets have phosphorus levels below 0.5%.

Most vets do recommend feeding these foods. Chronic kidney disease (CKD) in dogs and cats — staging and management strategies (2015) Chew D Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference says "An initial goal is to attempt to return high serum phosphorus concentrations to within the normal range by the feeding of a phosphate-restricted renal diet."

Fibroblast growth factor 23 (FGF-23) is important for the regulation of phosphorus levels in the body. It is possible that FGF-23 levels may in fact rise before other signs of CKD appear. The effect of feeding a renal diet on plasma fibroblast growth factor 23 concentrations in cats with stable azotemic chronic kidney disease (2013) Geddes RF, Elliott J & Syme HM Journal of Veterinary Internal Medicine 27(6) pp1354-61 states "Feeding renal diet is associated with reductions in plasma FGF-23 concentrations in hyper- and normophosphatemic cats with stable azotemic CKD, suggesting that dietary phosphate restriction may enable cats with CKD to maintain normal plasma phosphate concentrations in association with lower plasma FGF-23 concentrations."

Survival of cats with naturally occurring chronic renal failure: effect of dietary management (2000) Elliott J, Rawlings JM, Markwell PJ, Barber PJ Journal of Small Animal Practice 41(6) pp235-242 found that feeding a reduced phosphorus therapeutic kidney diet provided effective phosphorus control in two thirds of the cats in Stage 2 and 3 CKD. In addition, the cats eating the therapeutic kidney diet lived more than twice as long as the cats in the study who ate normal food with no phosphorus binder.

See Which Foods to Feed for more information about therapeutic kidney diets.

Feeding Other Commercial Foods

If your cat won't eat the therapeutic kidney foods (see Which Foods to Feed for tips on how to increase your chances of success), you still need your cat to eat, so you will need to look at feeding commercial or homemade foods with a lower phosphorus content.

In the USA, the minimum level of phosphorus in a non-therapeutic adult maintenance food is 0.5%, so you are not going to find a complete commercial food with phosphorus below this level; in fact, most non-therapeutic adult maintenance foods unfortunately contain far more phosphorus than this.

Many people seem to think that a food below 1% phosphorus on a dry matter analysis (DMA) basis is low phosphorus but that is not the case; you want to get as close to a level of below 0.5% on a DMA basis as possible. If you check the food data tables in the Diet and Nutrition section, you can try to find a food which your cat will eat with the lowest possible level of phosphorus.

I sometimes receive excited e-mails from people who are thrilled because they have apparently found a food that does not contain any phosphorus at all, because it is not mentioned on the label. It is virtually impossible to find a cat food containing little or no phosphorus, especially if the food contains animal-based protein, as most cat foods do (phosphorus is found in meat, particularly bones). It is simply because US law does not require phosphorus to be listed on the label.

Food Choices

Canned foods are generally a better choice for CKD cats than dry foods, but phosphorus control is so important that if I had a cat who was prepared to eat a dry therapeutic kidney diet, I would feed that rather than feed a non-therapeutic commercial canned diet with added binders.

Effect of boiling on dietary phosphate and nitrogen intake (2006) Cupisti A, Comar F, Benini 0, Lupetti S, D’Alessandro C, Barsotti G & Gianfaldoni D Journal of Renal Nutrition 16(1) pp36–40 is an interesting study which assessed the effect of boiling beef and chicken breast for 10, 20 and 30 minutes. The study found that this reduced phosphorus levels, and that soaking the meat in cold water for an hour before boiling it reduced the phosphorus content even further. The study concludes "Our results show that consuming boiled foods can significantly reduce dietary phosphate while preserving protein intake, namely reducing the effective phosphate intake per gram of dietary protein. This can represent additional advice to the patient for limiting the dietary phosphorus load at the same protein intake, leading to a better control of phosphate balance together with a lower risk of protein malnutrition." This may be worth bearing in mind if you make your own cat food. However, you cannot simply boil a chicken breast and give it to your cat, that would not consitute a balanced meal. See Which Foods to Feed for more information on homemade diets.

See above for information on how to monitor your cat's phosphorus levels.

Type of Phosphorus in Food: Organic and Inorganic

In the last few years research has indicated that the type of phosphorus contained in cat foods may be an issue.

All cat foods contain some phosphorus, because it is essential for a healthy body, particularly the skeleton. AAFCO guidelines in the USA require a minimum level of phosphorus (0.5% on a dry matter basis) in foods for adult maintenance, but unfortunately do not stipulate a maximum, and most foods have a much higher level than the minimum. Manufacturers are also not required under AAFCO guidelines to state the amount of phosphorus in a cat food, let alone which type of phosphorus it contains, but foods will contain organic or inorganic phosphorus, or possibly both.

Organic phosphorus comes from meat and bonemeal. It is bound to protein and released more slowly than inorganic phosphorus.

Inorganic phosphorus is a food additive. It comes in a number of different forms, including:

dicalcium phosphate
tricalcium phosphate
monosodium phosphate
monopotassium phosphate
phosphoric acid

Most commercial cat foods contain inorganic phosphates, not least because they improve the texture and increase palatability. They are often used in foods designed to support dental health because some of them may help reduce tartar formation. A review of phosphorus homeostasis and the impact of different types and amounts of dietary phosphate on metabolism and renal health in cats (2020) Laflamme D, Backus R, Brown S, Butterwick R, Czarnecki-Maulden G, Elliott J, Fascetti A & Polzin D Journal of Veterinary Internal Medicine 34(6) pp2187-2196 states "Among commercial cat foods in the United States, the most commonly added phosphorus sources for wet foods include tricalcium phosphate, dicalcium phosphate, and sodium tripolyphosphate, while phosphoric acid and dicalcium phosphate are the most commonly added sources of phosphate in dry cat foods."

Downsides of Inorganic Phosphate

Research indicates that inorganic phosphates are absorbed at a higher level than organic phosphates. Effect of a high phosphorus diet on indicators of renal health in cats (2018) Dobenecker B, Webel A, Reese S & Kienzle E Journal of Feline Medicine and Surgery 20(4) pp339-343 found an "apparent phosphorus digestibility of about 60% in the HP [inorganic phosphorus] group vs a digestibility of around 40% in the controls [organic phosphorus]."

This is not ideal for CKD cats, who tend to have high phosphorus levels anyway. New concepts in phosphorus homeostasis and its impact on renal health with particular reference to the cat (2022) Elliott J & Geddes RF The Veterinary Journal 283–284 105842 concludes "Rapidly absorbed inorganic forms of phosphorus added to the diet are potentially harmful to the kidney if the entry rate of phosphorus into the body exceeds the body’s ability to dispose of it."

Effect of a high phosphorus diet on indicators of renal health in cats (2018) Dobenecker B, Webel A, Reese S & Kienzle E Journal of Feline Medicine and Surgery 20(4) pp339-343 fed thirteen healthy cats a diet containing around five times the maintenance requirement for phosphorus for 29 days. This diet contained inorganic phosphorus, whereas the cats in the control group (CON) were given food containing organic phosphorus. The study found that "Renal phosphorus excretion was significantly increased in the HP group (115 mg/kg body weight/d vs 16 mg/kg body weight/d in the CON group)." The study concludes "The intake of a diet with an excessive content of highly available phosphorus may have adverse effects on parameters of kidney function in healthy cats."

Effects of the longterm feeding of diets enriched with inorganic phosphorus on the adult feline kidney and phosphorus metabolism (2018) Alexander J, Stockman J, Atwal J, Butterwick R, Colyer A, Elliott D, Gilham M, Morris P, Staunton R, Renfrew H, Elliott J & Watson P British Journal of Nutrition 21 pp1-21 looked at varying levels of phosphorus and calcium intake in cats. In the first part of the study, diets providing 4.8 g/1000 kcal of phosphorus (which is approximately 3.6 g/1000 kcal inorganic phosphorus), and a calcium:phosphorus ratio of 1:2 and 0:6 respectively were fed. This is a high level of inorganic phosphorus but nevertheless possible in some commercial cat foods. The study was halted prematurely because after four weeks GFR had reduced and changes in the cats' kidneys were also visible on ultrasound. A second study was then undertaken which fed lower (3.6 g/1000 kcal) levels of phosphorus. Bloodwork showed no changes but some kidney changes were visible on ultrasound and three of the cats developed CKD.

Dietary phosphorus and renal disease in cats: where are we? (2024) Stockman J Journal of Feline Medicine and Surgery 26(10) states "There are currently no stipulations regarding a maximum level of phosphorus in European or American guidelines (for total or inorganic phosphorus), although the European guidelines (FEDIAF) allow for the calcium:phosphorus to be between 1 and 2, and in a footnote explain that there are changes in renal function parameters when cats are fed diets with high inorganic phosphorus."

Inorganic Phosphorus: What to Do

It is largely impossible to avoid inorganic phosphates if you feed a commercial diet. However, there are two things you can do to minimise any possible risks:

Try to avoid foods containing sodium phosphates or phosphoric acid. The phosphate additives phosphoric acid and sodium phosphate lead to hyperphosphatemia as well as increased FGF23 and renal phosphate excretion in healthy cats (2023) Steffen C & Dobenecker B Journal of Urology and Renal Diseases 08 1338 concludes "Adding phosphoric acid in combination with a soluble sodium phosphate to the diet of healthy cats caused a significant increase of serum FGF23, phosphate concentrations, and the serum calcium by phosphorus product as well as renal phosphorus excretion. In this study, we proposed the cat as translational model for human medicine, due to its high sensitivity to dietary phosphate. These findings raise serious concerns regarding the safety of using highly available phosphate sources in processing of food products for feline and human consumption."

Check the calcium:phosphorus ratio of the food (which is not easy, since manufacturers are not obliged to state the phosphorus values of foods). Aim to feed a food with a calcium:phosphorus ratio of 1:1 or greater. Diets for cats with chronic kidney disease (2024) Quimby J & Ross International Renal Interest Society states "Phosphorus from inorganic sources (e.g., sodium or potassium phosphate salts) is more bioavailable that phosphorus from organic sources (e.g., meat, bone meal, grains). Additionally, the dietary calcium:phosphorus ratio is important as feeding highly bioavailable phosphorus salts in combination with too little calcium (leading to a calcium to phosphorus ratio of <1) has been documented to result in renal damage in previously healthy cats."

In light of the recent research into the issue of inorganic phosphorus, many researchers are pushing the food manufacturers for changes to their formulations. Dietary phosphorus and renal disease in cats: where are we? (2024) Stockman J Journal of Feline Medicine and Surgery 26(10) says "While the total amount of phosphorus often does not reflect the total amount absorbed, the author suggests that new guidelines include a limit on total dietary phosphorus (eg, 4 g/1000 kcal), as well as a calcium:phosphorus above 1 and a limit on soluble phosphorus of up to 1 g/1000 kcal. This could be controlled during the diet formulation."

Controlling Phosphorus Levels with Phosphorus Binders

If your cat will not eat a therapeutic kidney diet, or if his/her phosphorus levels are still too high (over 6 mg/dl (US) or 1.9 mmol/L (international)), despite feeding such a diet for a month, products called phosphorus binders are used.

Unfortunately, even if your cat is eating a low phosphorus diet, at some point it is quite possible that this will become insufficient to control your cat's phosphorus levels. The role of phosphorus in feline chronic renal disease (2010) Chew D & Kidder AC CVC in San Diego Proceedings states "Diet alone is not successful in adequate phosphorus control as chronic renal disease becomes more advanced. In these instances serum phosphorus concentration increases above the normal range or stays in the upper half of the normal range." In this situation, you will also have to consider the addition of phosphorus binders.

IRIS treatment recommendations for CKD in cats (2023) International Renal Interest Society recommend using phosphorus binders if dietary changes are not sufficient to control phosphorus levels.

Phosphorus binders are simply added to the cat's food: they bind with some of the phosphorus in the food in the intestine, thus preventing it from being absorbed and thereby reducing levels of phosphorus in the cat's body.

Survey of dietary and medication practices of owners of cats with chronic kidney disease (2014) Markovich JE, Freeman LM, Labato MA and Heize CR Journal of Feline Medicine and Surgery 22 found that only 22% of the 1080 cats in the survey were receiving phosphorus binders. Not all of the cats may have required binders at the time of the survey but this is still a very low percentage considering the consequences of not controlling phosphorus levels in CKD.

There is a separate page all about phosphorus binders.

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This page last updated: 04 May 2025

Links on this page last checked: 04 May 2025

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TREATING YOUR CAT WITHOUT VETERINARY ADVICE CAN BE EXTREMELY DANGEROUS.

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