Feeding pets with renal disease (Proceedings)

Renal disease is one of the medical conditions in veterinary medicine that is most amenable to dietary management. Research in both dogs and cats has demonstrated that specially formulated diets for renal disease help prevent uremic episodes, slow disease progression, and can double survival time1,2. Dietary modifications for chronic kidney disease (CKD) are designed to help mitigate many of the metabolic changes that occur secondary to decreased renal function. Most "renal diets" are restricted in phosphorus, reduced in protein and sodium, +/- supplemented with potassium, alkalinizing, and enriched with B-vitamins and often omega-3 fatty acids.

Phosphorus

The most critical nutritional modification in renal disease is phosphorus restriction. Numerous well-designed studies have shown that controlling blood phosphorus concentration through dietary modification slows the progression of CKD. Reduced glomerular filtration rate (GFR) in CKD leads to decreased renal excretion of phosphorus. Hyperphosphatemia in turn leads to increased secretion of parathyroid hormone, leading to increased release of calcium and phosphorus from bone. This combination can cause mineralization of soft tissue, advances renal damage, and may lead to marked bone loss ("rubber jaw").

The International Renal Interest Society (IRIS) has published guidelines on phosphorus management for renal patients at www.iris-kidney.com/pdf/IRIS_2009_Treatment_Recommendations_Summary.pdf. It is important to note that the target serum phosphorous concentrations are all in the lower end to middle of most laboratory reference intervals. This is because increased concentrations of parathyroid hormone can occur when serum phosphorus is still within the reference intervals3. As renal disease advances, it often becomes impossible to keep phosphorus concentrations within the IRIS guidelines using diet alone. In these cases, phosphate binders should be used in addition to the lowest phosphorus diet that is appropriate for the patient. Commercial renal diets range from 0.48 – 1.0 and 0.8 -1.28 g/Mcal (1000 kcal = 1 Mcal) phosphorus in dog and cat diets, respectively. The Association of American Feed Control Officials (AAFCO) minimums for dog and cat maintenance are 1.4 and 1.25 g/Mcal phosphorus, respectively. Therefore, all diets with a "formulated to meet AAFCO nutrient profiles" statement on the bag (predominantly OTC diets) will be inappropriate for dogs with renal disease, while the vast majority will be inappropriate for cats with renal disease, by definition.

Protein

Reduction of dietary protein is probably the best known and most controversial nutritional modification for patients with renal disease. As many publications point out, there is no evidence that high protein diets are harmful to the kidneys per se. However, many of the toxins that build up in the bloodstream when GFR is markedly compromised by advanced tuberulointerstitial disease are nitrogenous compounds. Reducing dietary protein can improve quality of life in these patients by reducing the effects of these toxins on other systems. Additionally, the limiting factor in phosphorus restriction in diets is often the animal protein content as most meats are high in phosphorus. It is best to determine the appropriate degree of protein reduction necessary based on the individual patient's laboratory values and the need to balance protein intake with adequate phosphorus restriction and other nutritional modifications. Protein reduction, when appropriate, should be done by maximizing the quality of the protein to ensure that the physiologic requirements for protein are being met without adding excess amounts that will quickly be metabolized into uremic toxins.

In contrast to the situation for CKD without proteinuria, for animals with evidence of a significant protein losing nephropathy (with or without concurrent tubular disease), protein reduction may be as critical or even more critical to slowing disease progression than phosphorus restriction. It has been shown in many species that increasing dietary protein exacerbates glomerular protein loss. Albumin is toxic to the tubules, leading to accelerated degradation of the entire kidney. Therefore, although it may seem counterintuitive, most animals with PLN respond to lower dietary protein with lower urine protein:creatinine ratios (UPC) and higher serum albumin. Unfortunately, there is no known dose-response data to serve as a guideline for what amount of protein should be used. For most patients, it is likely appropriate to switch to a commercial renal diet (12-15% and 20-27% protein calories (metabolizable energy, ME) for dogs and cats, respectively), even if azotemia is not present. For dogs or cats that were consuming particularly high protein/phosphorus diets (such as many grain free and low carbohydrate diets, raw diets and/or lots of meat based treats) at diagnosis, reduction of protein and phosphorus down to around AAFCO minimums (~18 and 23% protein ME and 1.4 and 1.25 g/Mcal phosphorus, respectively for dogs and cats) may be a good initial step. Further dietary modification can then be based on laboratory and clinical response.

Sodium

The vast majority of commercial pet foods contain sodium well in excess of physiological requirements. Due to mainly theoretical concerns regarding blood pressure and water balance, excess sodium is generally avoided in diets for patients with renal disease. The sodium concentration in all commercial renal diets for both dogs and cats is well above AAFCO minimums, but less than the sodium concentrations of most OTC diets.

Potassium

Serum potassium values can vary dramatically both among patients and between dogs and cats. Dogs with CKD are more likely to have hyperkalemia, especially those on ACE-inhibitors for glomerular disease, while cats with CKD are more likely to have hypokalemia. Commercial renal diets also vary in their potassium concentrations, with feline diets generally being higher than canine diets. The wide range of potassium concentrations in canine diets allows for selection of the diet most appropriate for the individual patient. For the rare cases that are hyperkalemic on the lowest potassium commercial renal diet, a home-cooked diet may be required if medications cannot be altered.

Acid-base

One of the many roles of the kidney is acid-base balance. The kidney regulates the excretion of hydrogen ions and bicarbonate regeneration and animals with renal impairment often become acidemic. Commercial renal diets are thus designed to be relatively alkalinizing to help counteract these changes. In cats, this is significantly different from OTC maintenance diets which are generally acidifying due to both ingredient composition and specific intention to help prevent struvite-related urinary problems.

B-vitamins

B-vitamins are water-soluble and with rare exception (B-12) are not stored in the body. They are, however, needed daily for nearly all metabolic pathways. Most commercial renal diets for both dogs and cats are fortified with additional B vitamins to ensure adequate amounts despite the potential for increased renal losses secondary to polyuria.

Omega-3 fatty acids

Research in dogs has shown potential reno-protective effects of supplementation with long chain omega-3 fatty acids from fish oil (eicosapentaenoic acid; EPA; and docosahexaenoic acid; DHA). However, there is some conflicting evidence and a clear dose response has not been determined for dogs. There is no published information regarding omega-3 fatty acid supplementation in cats with renal disease. Flax, which is a good source of the short chain omega-3 fatty acid alpha-linoleic acid (ALA), has not been investigated to see whether it is beneficial. As its endogenous conversion to DHA and EPA is poor in dogs and essentially non-existent in cats, it should only be used as a last resort when supplementation with fish oil is not feasible.

Many commercial renal diets already contain supplemental fish oil, but the amounts vary. The author tends to use a dose of around 300 mg total DHA + EPA per 10 pounds of body weight. This amount can be supplemented on top of a commercial diet that is not already supplemented, or the total intake from the diet alone can be calculated and fish oil added to attain the final desired dose. For many patients, liquid fish oil may also be a palatability enhancer.

When should a renal diet be started?

In the absence of proteinuria, initial dietary modifications for early asymptomatic CKD (IRIS Stage 1 or when CKD is suspected but not confirmed) should be geared towards reducing phosphorus. There are a handful of commercial diets (a few OTC and veterinary therapeutic diets made by Hill's and Iams Veterinary Formulas Early Stage Renal canine) with phosphorus below 1.5 g/Mcal and moderate protein which can be considered. These diets will be higher in protein than renal diets and may not have the other modifications that have been previously discussed. Once CKD progresses to stage 2 or if significant proteinuria is evident, most patients should be fed only a commercial renal diet.

What about concurrent diseases?

Common concurrent disease processes that can make it difficult to find an appropriate renal diet include food allergies and fat intolerance (hyperlipidemia, history of pancreatitis). Some patients with early CKD and confirmed or suspected food allergies/intolerances may be able to be fed a commercial lower protein and phosphorus limited antigen diet, such as some of the Hill's Prescription Diet d/d line. For suspected food allergies or intolerances in later stage CKD, the best option is generally to trial a few of the renal diets to see if they are tolerated. For confirmed allergies to ingredients that cannot be avoided in the renal diets or for fat intolerance, home-cooked diet formulations may be required.

What if the pet will not eat a renal diet?

There are at least a dozen reduced phosphorus and protein diets (including canned and dry options) that may be options for dogs with various stages of CKD. A slow transition from the previous diet is recommended whenever possible. Low protein palatability enhancers such as fish oil, home-made low sodium meat broths, honey, pancake syrup, applesauce and some human enteral products can be added to the diet to increase interest. Meats and other foods high in protein, phosphorus, and sodium should be avoided as they have the potential to negate the benefits of the diet and actually make the patient feel worse in the short term. It is common for pets with later stage disease to have cyclical appetites where they may not be interested in eating the same food every day. Rotating between several appropriate diets may help overcome this issue. Appetite stimulants rarely result in consistent consumption of enough food to meet energy requirements.

Home-cooked diets may be more palatable to dogs with CKD than commercial diets. However, the majority of the recipes in books and online are unbalanced and may not be appropriate for renal patients despite assertions to the contrary. Clients wishing to try a home-cooked diet should obtain a custom recipe from a board-certified veterinary nutritionist, usually through a veterinary teaching hospital nutrition service. Some balanced (but not customized) recipes for home-cooked renal diets can also be obtained by veterinarians for their clients from BalanceIT.com.

There are at least a dozen potential commercial options for cats with various stages of renal disease (including canned and dry options). Unfortunately, cats tend to be notoriously picky about switching foods and transitioning them to a renal diet can be very challenging. However, the potential to double their survival time and improve their quality of life should serve as a powerful motivator to clients to persevere. Cats also seem more likely than dogs to go through a prolonged period of wasting as their appetite decreases and their intake does not meet their energy requirements. Unlike in dogs, appropriate home-cooked diets are rarely more appealing to cats than commercial diets. Palatability enhancers such as homemade, low sodium meat broths, fish oil and animal fats can be used to encourage intake. Clinicare RF may be palatable to some cats and can be used to supplement their food consumption. Appetite stimulants rarely result in consistent consumption of enough food to meet energy requirements.

Assisted feeding

Assisted (tube) feeding should be considered for patients with renal disease that are not able to maintain an appropriate body condition via voluntary intake of an *appropriate* diet for their stage of disease. Esophagostomy (E) tubes are the most commonly used and can make a huge difference in the quality of life for both owner and pet. An E-tube allows for an appropriate diet (i.e. a blenderized canned renal diet) to be fed in appropriate amounts to maintain body weight and often reduces the stress surrounding mealtimes for both pet and owner. Many medications as well as oral fluids can be administered via the tube, which can also enhance quality of life for the entire family.

Feeding tubes are best placed prior to the animal becoming significantly debilitated, rather than as a last-ditch attempt to prolong the life of an emaciated, anorexic, severely uremic animal. For this reason, the author often discusses feeding tubes as an option with clients long before the pet requires it, even at the first or second visit after the renal disease is diagnosed. Clients are advised to consider that at some point a feeding tube may be appropriate and encouraged to decide as a family how they would handle the situation if it arose. In the authors' experience, these types of emotional decisions are best made when the pet is not in immediate danger and the clients have ample time to consider all the implications. It is often helpful to keep a list of good clients who have successfully used feeding tubes and are willing to talk to other clients about their experiences.

References

Ross SJ, Osborne CA, Kirk CA, et al. Clinical evaluation of dietary modification for treatment of spontaneous chronic kidney disease in cats. J Am Vet Med Assoc 2006;229:949-957.

Jacob F, Polzin DJ, Osborne CA, et al. Clinical evaluation of dietary modification for treatment of spontaneous chronic renal failure in dogs. J Am Vet Med Assoc 2002;220:1163-1170.

Cortadellas O, Fernandez del Palacio MJ, Talavera J, et al. Calcium and phosphorus homeostasis in dogs with spontaneous chronic kidney disease at different stages of severity. J Vet Intern Med 2010;24:73-79.