Dietary management of the cat with chronic kidney disease

Healthy kidneys are required for many vital functions including elimination of nitrogenous waste products, hormones and drugs from the body via the urine. In addition they are involved in:

• Production of hormones such as erythropoeitin (required to stimulate production of red blood cells by the bone marrow) and renin (important in water and electrolyte homeostasis)
• Activation of vitamin D (important in calcium and phosphate homeostasis)
• Regulation of acid–base balance
• Maintaining normal electrolyte levels (calcium, phosphate, potassium, sodium and chloride)
• Maintaining normal hydration status.

A range of complications can therefore be seen in patients with reduced kidney function (Table 1).

Chronic kidney disease (CKD) is one of the most common diagnoses made in clinical practice — it is estimated to affect at least a third of cats over the age of ten (Lulich et al, 1992; Polzin et al, 1992). Typically this is a condition which affects middle aged and older cats. In many cases, the cause of the renal disease is not known although acknowledged causes include:

• Congenital problems such as polycystic kidney disease which is especially common in Persian and related cats. Prevalence rates of up to 50% have been reported for Persian and exotic cats around the world (Barrs et al, 2001; Beck et al, 2001; Cannon et al, 2001; Barthez et al, 2003; Helps et al, 2007; Domanjiko-Petric et al, 2008; Bonazzi et al, 2007; Bonazzi et al, 2009) (Figure 1)

  

• Acquired causes such as intoxications (e.g. lilies, antifreeze, grapes and raisins), infections (e.g. pyelonephritis, feline infectious peritonitis) and neoplasia (e.g. lymphoma, carcinomas).

For the vast majority of cases, the cause of renal disease is not identifiable at the time of diagnosis. Diagnosis currently relies on assessment of blood urea and creatinine in conjunction with urine specific gravity. Patients can be classified according to the International Renal Interest Society (IRIS) guidelines which grades the severity of renal disease according to blood creatinine levels (Table 2). It is important to use creatinine values obtained after the cat has been rehydrated since dehydration increases creatinine levels and may give a false impression that the kidney disease is worse than it actually is. Some cats with CKD will pass through all of the IRIS stages as their kidney disease progresses; other cats will remain stable for many years in the same stage. Patients can also be sub classified according to presence/severity of proteinuria and systemic hypertension.

In recent years there have been many advances in treatment options and long-term home care can be very rewarding for all involved. Treatment aims to:

• Identify and treat any underlying cause of the disease such as bacterial pyelonephritis
• Provide ‘proven’ general therapies for CKD such as prescribing renal diets and ensuring that the cat maintains normal hydration as far as possible. These measures help the patient to compensate for their renal disease and live for as long as possible with as good a quality of life as possible
• Provide additional treatments according to the individual's specific needs. For example, this might include specific treatment for systemic hypertension or proteinuria.

This article will concentrate on the effect that dietary therapy can have on the quality and length of life of a cat with CKD. Dietary therapy is the single most beneficial treatment for cats with renal disease (Roudebush et al, 2009). Several studies have now shown that cats with CKD that will eat renal prescription diets will live much longer, healthier lives (Elliott et al, 2000; Plantinga et al, 2005; Ross et al, 2006). Typical median survival times for cats with CKD receiving a standard diet are around 7 months compared with 16 months for those receiving a renal prescription diet (Plantinga et al, 2005). Renal prescription diets are especially proven for cats with IRIS Stage 3 and 4 renal disease. Renal diets are modified in many ways to assist cats with renal failure as is discussed in more detail below.

Characteristics of an ‘ideal’ renal diet

The ideal diet for a cat with renal disease should have a number of basic properties. It should be:

• Highly palatable since most cats with CKD have a reduced appetite
• Formulated to help compensate for the cat's reduced renal function
• Formulated to prevent and/or treat common and important complications that are seen in patients with CKD (common complications are itemized in Table 1).

Modifications routinely made to renal prescription diets

Over the years there has been a large amount of research conducted both on naturally occurring cases of renal disease as well as experimental models. As a result of this research, several modifications are routinely made to create renal prescription diets. Each of these will be discussed separately but are itemized below:

• Phosphate restriction
• Protein restriction
• Non-acidifying
• Potassium and B vitamin supplementation
• High palatability and calorie content

The diet should be introduced gradually to encourage acceptance by the cat.

Phosphate restriction

Cats with CKD are vulnerable to hyperphosphataemia since normal regulation of blood phosphate levels requires renal excretion. Hyperphosphataemia is a major contributor to the development of renal secondary hyperparathyroidism and is believed to be damaging to the kidneys, contributing to continued renal injury. Hyperphosphataemia is commonly en-countered in CKD and is associated with a poor prognosis (King et al, 2007; Boyd et al, 2008).

IRIS (2011) has defined ‘target levels’ for blood phosphate; ideally the phosphate should be kept below 1.45 mmol/l but as the renal disease worsens, this may not be possible. IRIS therefore have recommended the following target levels for blood phosphate according to the IRIS stage of disease (Elliott et al, 2006):

• IRIS Stage 2: aim for blood phosphate levels to be between 0.81 and 1.45 mmol/l
• IRIS Stage 3: aim for blood phosphate levels to be between 0.81 and 1.61 mmol/l
• IRIS Stage 4: aim for blood phosphate levels to be between 0.81 and 1.94 mmol/l.

Feeding a low phosphate diet has been shown to produce the most significant benefits to cats with CKD (Barber et al, 1999; Elliott et al, 2000). The use of low protein diets will achieve a reduced phosphorus intake, but this will not always be sufficient to prevent the hyperphosphataemia encountered in CKD (Barber et al, 1999; Elliott et al, 2000; Elliott et al, 2006). If hyperphosphataemia persists (i.e. exceeds the target levels cited by IRIS) despite dietary restriction and in those cats that will not eat a prescription diet, oral phosphate binders may be given with meals. Oral phosphate binders are discussed in more detail later in this article.

Management of phosphate levels through use of renal prescription diets with additional phosphate binders, if needed, reduces renal secondary hyperparathyroidism and survival times can be more than doubled (Barber et al, 1999; Elliott et al, 2000). Indeed feeding a phosphate restricted diet can also reverse consequences of renal secondary hyperparathyroidism such as soft tissue metastatic calcification in some patients (Jackson and Barber, 1998). An experimental study has shown that cats with renal disease fed a low phosphate diet are less vulnerable to progression of their renal disease — in contrast to CKD cats fed a standard diet, these cats showed no or greatly reduced renal mineralization, fibrosis or mononuclear cell infiltration (Ross et al, 1982).

Protein restriction

Protein restriction is commonly advocated for management of renal disease. In some species, protein restriction has beneficial effects on the rate of progression of renal disease, however, this has not been demonstrated to be the case in cats. The main benefit of protein restriction and feeding a high biological value protein to cats with CKD is management of clinical signs (Polzin et al, 1991; Finco et al, 1998; Levey et al, 1999).

Accumulation of protein breakdown products is one of the causes of uraemic clinical signs. Hence feeding a protein restricted diet helps to reduce the severity of azotaemia and clinical signs associated with uraemia such as nausea/vomiting, anorexia and lethargy. A double-blinded controlled study showed that cats with IRIS Stage 2 or 3 renal disease had a much better quality of life when fed a renal prescription diet compared with a standard food (Ross et al 2006). None of the cats receiving the renal diet suffered uraemic episodes compared with 26% of the cats receiving a standard adult maintenance diet. None of the cats receiving the renal diet died during the 2-year study period compared with 22% of the cats receiving the standard diet. The authors of the study also reported excellent compliance with the renal prescription diet which was Hill's k/d (dry and moist) in this study.

Blood urea and phosphate levels may decrease in cats transitioned to a protein and phosphate restricted diet (Elliott et al, 2000).

Non-acidifying

Renal diets are formulated to help prevent development of metabolic acidosis, one of the most common complications associated with CKD (Table 1). Maintenance of normal acid–base status also helps to protect CKD cats from hypokalaemia.

Potassium and B vitamin supplementation

Cats with CKD are vulnerable to losing excessive amounts of potassium and water soluble vitamins in the urine. Mild hypokalaemia is often associated with only vague clinical signs such as malaise and inappetence, while blood potassium less than 2.5 mmol/l can result in severe muscle weakness and ventroflexion of the neck (Figure 2). Hypokalaemia worsens renal function and can cause renal failure (Dow et al, 1990; DiBartola et al, 1993). Potassium supplementation of hypokalaemic cats with CKD often results in improved renal function (Dow et al, 1987).

Palatability and calorie density

Most cats with CKD have a poor appetite and reduced bodyweight (Lulich et al, 1992). A vital aim of dietary therapy is to promote achievement of normal body condition score. Renal prescription diets are often high in fat and calories aiding palatability and helping CKD cats with a poor appetite to maintain a normal bodyweight. For example, Hill's k/d paté contains about double the number of calories per gram that normal Whiskas does. Although field encountered compliance is anecdotally often reported as poor, several clinical studies have reported excellent compliance when complimentary supplies of renal diets were provided to owners of CKD cats (King et al, 2006; Ross et al, 2006).

Other modifications

Antioxidants are added to many renal prescription diets with the intention of reducing renal oxidative stress — a factor believed to contribute to the progression of renal disease. Cats with CKD have a tendency towards oxidative stress and in one study, this tendency was benefitted by a 4 week programme of supplementation with the antioxidants vitamins E and C and beta carotene (Yu and Paetau-Robinson, 2006).

Many renal prescription diets include a source of soluble fibre. The aim of this is that nitrogenous waste products, such as urea, are bound in the bowel and eliminated with the faeces rather than being absorbed into the body.

Omega 3 fatty acid supplementation (e.g. eicosapentanoic acid) is often included since these have been shown to have renoprotective properties in dogs (Brown et al, 1996a; Brown et al, 1996b; Finco et al, 2000).

Moist or dry?

All renal prescription diets are formulated in dry and moist forms. In an ideal world, a moist diet is preferable for a cat with renal disease because it is vulnerable to dehydration which worsens renal function and can precipitate a renal crisis requiring hospitalization. However, in the author's personal experience many CKD patients prefer dry renal prescription diets. The many modifications present in all renal prescription diets make these preferable to feeding standard cat food. Ultimately, a renal diet is the best diet for a cat with IRIS Stage 3 or 4 renal disease (Roudebush et al, 2009).

Goals of dietary therapy

Given the enormous difference that renal prescription diets can make to a patient's quality of life and survival, a major goal is succeeding in persuading owner and cat that this is worthwhile. Compliance rates for dietary success are rarely published but anecdotally often reported as around 25% of patients where a recommendation was made to pursue specific dietary care. In the author's experience, compliance can be improved by:

• Educating the owner as to the benefits of dietary management and the very real differences this will make to their cat's quality and length of life (Caney, 2008)
• Aiming for a gradual transition to a renal diet. It is important to reassure owners that this may take weeks or months to achieve in full.

Recommendations for cats with persistently poor appetites

Poor appetite is a common problem in CKD patients and may need specific treatment. In the first instance, care should be taken to look for causes of poor appetite that can be addressed, for example, nausea related to uraemic gastritis, hypokalaemia and dental disease. If no identifiable reason can be found then tactics which may be of benefit include:

• Nursing techniques such as hand feeding, offering slightly warmed food and sitting with the cat, and stroking it while it eats (Figure 3)

  

• Use of appetite stimulants such as cyproheptadine (1 mg per cat once or twice a day as needed) or mirtazapine (1.875 mg per cat every 3 to 4 days)

• Treatment with H2-blockers such as famotidine (0.5 mg/kg once a day) in case gastric hyperacidity is causing some loss of appetite

• Use of anti-emetics such as maropitant (0.5–1.0 mg/kg/day).

It is important to note that none of the treatments quoted above are currently licensed for the treatment of cats.

Dietary treatment of ‘problem cases’

In those situations where either cat or owner (or both) are unwilling to comply fully with ideal dietary recommendations, there is still much that can be done to improve the diet of a cat with CKD. A senior diet is preferable to standard adult maintenance cat food in most cases since this will typically contain lower levels of phosphate. The diet can be further modified by:

• Adding a phosphate binder such as lantharenal, calcium carbonate or aluminium hydroxide to the food. Oral phosphate binders bind to phosphate present in the diet and limit what is absorbed by the bowel (Wagner et al, 2004; Schmidt et al, 2006; Brown et al, 2008; Kidder and Chew, 2009). It can take several months for the total body excess phosphate levels and hence blood phosphate levels to normalize
• Avoiding acidifying diets which increase the risk of metabolic acidosis and hypokalaemia in cats with CKD. Where indicated, sodium bicarbonate can be added to standard cat food as an alkalinizing agent
• Other dietary supplements: potassium, B vitamins and fatty acids can be obtained and added to the diet — a variety of preparations are available from veterinary wholesalers.

Conclusions

Dietary therapy can have an enormously positive influence on the health and survival of cats with CKD. Cats that will eat renal prescription diets live on average more than twice as long, following their diagnosis, as those cats that receive standard adult maintenance diets. Where possible, all CKD patients should be transitioned to renal prescription diets — especially those in IRIS Stages 3 or 4. Phosphate restriction through use of renal prescription diets and/or phosphate binders is indicated for cats in IRIS Stages 2, 3 and 4. For those situations where feeding a renal diet is not possible, a standard diet can be modified to more closely resemble a renal diet.

• To answer the CPD questions on this article visit www.theveterinarynurse.com Questions have been approved by Harper Adams University College

Key Points

• Chronic kidney disease (CKD) is one of the most common diagnoses made in middle aged and elderly cats.
• Renal prescription diets are modified in a number of ways to support cats with CKD.
• CKD patients that will eat a renal prescription diet live, on average, much longer and healthier lives than CKD patients eating standard cat food.
• Management of renal secondary hyperparathyroidism through use of prescription renal diets and phosphate binders is proven to more than double survival times of cats with CKD.