Kidney disease: low-phosphorus, moderate-protein diet

The chronic kidney disease (CKD) is one of the most common chronic conditions in senior dogs and cats, and the nutritional intervention is the most evidence-based therapeutic intervention available for the CKD pet. The prescription renal diet — with the low-phosphorus, moderate-protein profile, the elevated omega-3 fatty acids, the restricted sodium, the supplemented B vitamins, and the increased caloric density — is the single most important intervention for slowing the progression of CKD, for managing the clinical signs, and for extending the survival of the CKD pet. The 2016 EPIC trial (Evaluating Prevention of progression in Chronic kidney disease), published in the International Journal of Nephrology and Renovascular Disease, demonstrated that early dietary intervention in cats with IRIS Stage 2–3 CKD significantly reduced the risk of uremic crisis and extended the median survival. The result is one of the strongest evidence bases for any nutritional intervention in veterinary medicine, and the result is the foundation of the modern approach to the CKD diet.

This article is going to walk you through the practical renal diet for dogs and cats with CKD — why the low-phosphorus and moderate-protein approach works, the food options (the prescription renal diets, the home-cooked options, the non-prescription options), the timing of the intervention, the species-specific considerations, the practical feeding protocols, and the four errors that undermine the most evidence-based nutritional intervention in CKD. I am a board-certified veterinary dermatologist and internist with a focus on chronic disease management, and the CKD diet is one of the most important interventions I recommend to my clients with senior pets. The cases that go well share a common feature: the owner has committed to the prescription renal diet early (ideally at IRIS Stage 2), has fed the diet exclusively, has managed the palatability challenges, and has monitored the response. The cases that go poorly share a different common feature: the owner has switched to the renal diet too late, has fed the diet inconsistently, has undermined the diet with treats and table scraps, or has not committed to the long-term feeding.

I want to be clear at the start: the CKD diet is not a "kidney-friendly" marketing claim. The CKD diet is a specific, engineered, evidence-based intervention with a specific nutrient profile that has been shown in published trials to slow disease progression and extend survival. The "kidney-friendly" pet food that is sold over the counter is not the same as the prescription renal diet, and the difference matters. The right diet is the prescription renal diet, fed exclusively, for the life of the pet.

Why the low-phosphorus and moderate-protein approach works

The CKD pet has a progressive loss of functional kidney tissue, and the loss produces two main consequences: the inability to excrete phosphorus and the inability to excrete nitrogenous waste products (the byproducts of protein metabolism). The prescription renal diet addresses both consequences with the low-phosphorus and the moderate-protein profile.

The phosphorus restriction. The kidney that is losing function is losing the ability to excrete phosphorus, and the phosphorus accumulates in the blood. The high blood phosphorus (hyperphosphatemia) is a major driver of CKD progression through the renal secondary hyperparathyroidism pathway: the high phosphorus stimulates the parathyroid gland to release parathyroid hormone (PTH), the high PTH causes calcium to be released from the bones, the high PTH also causes further damage to the kidney, and the cycle continues. The phosphorus restriction in the renal diet reduces the dietary phosphorus load, reduces the serum phosphorus, reduces the PTH, and slows the cycle. The published evidence is consistent that early phosphorus restriction is one of the most effective interventions for slowing CKD progression.

The prescription renal diets are restricted to approximately 0.3–0.6% phosphorus on a dry-matter basis, compared to 0.7–1.0% in a maintenance diet. The restriction is significant, and the restriction cannot be achieved with a standard maintenance diet, a "kidney-support" over-the-counter diet, or a home-cooked diet that has not been formulated for the CKD pet.

The moderate protein. The kidney that is losing function is losing the ability to excrete the nitrogenous waste products of protein metabolism (urea, creatinine, and other compounds). The high protein intake produces a high nitrogenous waste load, which the kidney cannot excrete, and the waste accumulates in the blood. The moderate protein in the renal diet reduces the waste load, reduces the uremic signs (the nausea, the lethargy, the inappetence), and improves the pet's quality of life.

The moderate protein in the renal diet is not the same as a low-protein diet. The protein is restricted to a level that is appropriate for the species and the life stage, but the protein is not restricted to a level that produces protein malnutrition. The protein in the diet is high-quality, is highly digestible, and provides the essential amino acids that the pet needs. The "low-protein damages the kidney" theory that was once popular in human medicine has been thoroughly debunked in both human and veterinary medicine, and the appropriate protein restriction is the moderate restriction that balances the kidney's reduced excretory capacity with the pet's protein needs.

For dogs, the protein in the renal diet is typically 14–20% on a dry-matter basis, compared to 22–28% in a maintenance diet. For cats, the protein is typically 26–32% on a dry-matter basis, compared to 30–40% in a maintenance diet. The cat's higher protein requirement reflects the cat's obligate carnivore biology, and the cat's renal diet does not restrict the protein as aggressively as the dog's.

The omega-3 fatty acids. The renal diets are supplemented with elevated levels of omega-3 fatty acids (EPA and DHA from marine oil), typically 0.5–1.0% of dry matter, compared to 0.05–0.2% in a maintenance diet. The omega-3s have anti-inflammatory effects in the kidney, and the omega-3s have been shown to slow the progression of CKD in published studies. The mechanism is the competition with the omega-6 arachidonic acid in the renal inflammatory cascade.

The sodium restriction. The renal diets are mildly sodium-restricted (0.2–0.3% on a dry-matter basis, compared to 0.4–0.6% in a maintenance diet). The sodium restriction supports blood pressure management, and the sodium restriction is particularly important for the CKD pet that is hypertensive.

The B vitamins and the antioxidants. The renal diets are supplemented with elevated levels of B vitamins (to compensate for the urinary losses) and with antioxidants (vitamin E, vitamin C, beta-carotene, lutein) to support the kidney's antioxidant defenses. The B vitamin and antioxidant supplementation is not the primary intervention, but the supplementation supports the overall kidney health.

The increased caloric density. The renal diets are more calorie-dense than maintenance diets, because the CKD pet is often inappetent and needs to get more calories per bite. The increased caloric density helps the CKD pet maintain body weight and body condition, which is critical for the long-term management.

The prescription renal diet options

The leading prescription renal diets, in order of my preference:

Hill's Prescription Diet k/d (canned and dry). The most-cited prescription renal diet in the published literature, and the diet that was used in the EPIC trial. The k/d has a strong palatability profile, a well-balanced nutrient profile, and a long track record of clinical results. The k/d is available in multiple formulations (early stage, advanced stage, with chicken, with beef, with vegetable).

Royal Canin Veterinary Diet Renal Support (canned and dry). A range of formulations (Early, Moderate, Advanced, Special) that are matched to the stage of the disease. The palatability is strong, and the nutrient profile is well-balanced. The Royal Canin range is one of the most comprehensive prescription renal diet lines on the market.

Purina Pro Plan Veterinary Diets NF (canned and dry). Another well-validated option, with a strong track record and a good palatability profile.

Hill's Prescription Diet k/d + Mobility (canned and dry). A combination diet for the CKD pet that also has osteoarthritis, which is common in senior dogs. The diet provides the renal support plus the joint support (omega-3s, glucosamine, chondroitin) in a single formulation.

The choice between the options depends on the pet's preferences, the owner's budget, the pet's specific stage, and the vet's recommendation. All four options are well-validated, and the choice is often a matter of palatability and owner preference.

The home-cooked renal diet

For pets that refuse the prescription renal diets, a home-cooked renal diet formulated by a board-certified veterinary nutritionist (DACVN) is a valid alternative. The DACVN will design a recipe that hits the macronutrient targets (low-phosphorus, moderate-protein, elevated omega-3, restricted sodium), the micronutrient targets (B vitamins, antioxidants, the appropriate calcium-to-phosphorus ratio), and the caloric targets appropriate for the pet's stage and body condition.

The home-cooked renal diet is more time-consuming than the prescription diet, and the home-cooked diet requires careful formulation to be nutritionally complete. The home-cooked diet is appropriate for the owner who is committed to the home cooking, who is willing to follow the recipe precisely, and who is willing to supplement the diet with the appropriate calcium and vitamin supplements (the renal diet is low in phosphorus, but the diet must have the right calcium-to-phosphorus ratio, and the calcium must be supplemented to avoid a calcium deficiency).

The home-cooked renal diet is not appropriate for the owner who is going to "approximately" follow the recipe, or who is going to substitute ingredients based on what is in the refrigerator. The renal pet is a pet that needs precision, and the home-cooked diet requires precision.

The non-prescription "kidney-support" diets

A large number of over-the-counter "kidney-support" diets are sold in pet food retailers, and the diets are marketed as appropriate for the CKD pet. The diets are not the same as the prescription renal diets, and the diets should not be used as a substitute for the prescription diets.

The over-the-counter kidney-support diets typically have a slightly reduced phosphorus compared to a maintenance diet, but the reduction is not as significant as the reduction in the prescription diets. The over-the-counter diets also typically have a higher protein than the prescription diets, and the higher protein is not appropriate for the CKD pet. The over-the-counter diets are a small improvement over a maintenance diet, but the diets are not a substitute for the prescription diets.

The owner who feeds the over-the-counter kidney-support diet to a CKD pet is the owner who is feeding a "slightly better than nothing" diet. The owner who feeds the prescription renal diet is the owner who is feeding the diet that has been shown in published trials to slow the progression of CKD. The difference is the evidence. The evidence is the framework above.

The timing of the intervention

The right time to start the renal diet is at the early stages of the disease, ideally at IRIS Stage 1 or early Stage 2. The published evidence is consistent that early dietary intervention is more effective than late dietary intervention. The pet that is switched to the renal diet at Stage 1 or 2 is the pet that has the best chance of a slow progression and a long survival. The pet that is switched to the renal diet at Stage 3 or 4 is the pet that is making the change too late to have the maximum benefit, but the change is still worthwhile (the late dietary intervention still slows the progression, even if the maximum benefit has been missed).

The owner who is told that the pet is at "early stage" CKD should not delay the dietary change. The owner should make the change promptly, should commit to the prescription diet, and should monitor the response. The early change is the most effective change.

The owner who is told that the pet is at "late stage" CKD should also make the change. The late change is still beneficial, and the late change is often the intervention that produces the most visible improvement in the pet's quality of life (the pet that is uremic and is switched to the renal diet often improves within 2–4 weeks, with increased appetite, increased energy, and decreased nausea).

The practical feeding protocols

The CKD pet is often a challenging feeding case, because the CKD pet is often inappetent, the renal diet is often less palatable than the maintenance diet (the lower protein and the lower phosphorus produce a less "rich" flavor), and the pet may be resistant to the change. The right feeding protocol is:

The gradual transition. The renal diet is introduced gradually, over 7–14 days, with the proportion of the new diet increasing each day. The gradual transition allows the pet to adjust to the new flavor and texture, and the gradual transition reduces the risk of food refusal.

The warming. The canned renal diet can be warmed slightly (to body temperature, around 100°F) to enhance the aroma and the palatability. The warming is particularly important for the CKD pet that is inappetent.

The multiple small meals. The CKD pet that is inappetent is the pet that does better with 3–4 small meals per day than with 1–2 large meals. The smaller meals are less overwhelming, and the smaller meals are easier for the pet to consume.

The treats. The treats for the CKD pet must be low-phosphorus, low-protein, and consistent with the prescription diet. The treats that are appropriate include small pieces of the canned renal diet (used as a treat), small pieces of fresh fruit (apple, banana, blueberries — the CKD pet can have small amounts of fruit), and small pieces of cooked vegetables (carrots, green beans, sweet potato). The treats must not include the standard commercial treats, which are typically high in phosphorus and high in protein.

The appetite stimulants. The CKD pet that is not eating enough may need an appetite stimulant. The options include:

• Mirtazapine (oral or transdermal). A serotonin antagonist that stimulates appetite and has anti-nausea effects. The transdermal formulation (applied to the inner ear) is well-tolerated by most cats.

• Capromorelin (Entyce). A ghrelin receptor agonist that stimulates appetite. The oral formulation is approved for dogs, with a long track record of safety and efficacy.

• Cyproheptadine. An antihistamine with appetite-stimulating side effects, commonly used in cats.

The anti-nausea medications. The CKD pet that is nauseated may need an anti-nausea medication. The options include:

• Maropitant (Cerenia). A NK1 receptor antagonist, very effective for nausea in dogs and cats. The oral or subcutaneous administration.

• Ondansetron. A 5-HT3 receptor antagonist, very effective for nausea in dogs and cats. The oral or subcutaneous administration.

• Dolasetron. A similar option, less commonly used.

The combination of the renal diet, the anti-nausea medication, the appetite stimulant, and the gradual transition is the protocol that produces the best feeding response in the CKD pet. The pet that is not eating is the pet that is at risk of hepatic lipidosis (especially in cats) and muscle wasting, and the feeding must be prioritized.

The four errors that undermine the most evidence-based nutritional intervention in CKD

Error 1: Switching to the renal diet too late. The most common error. The owner waits until the CKD is advanced (Stage 3 or 4) to make the dietary change, and the owner misses the window of maximum benefit. The fix is to make the change early, at Stage 1 or 2, when the change is most effective.

Error 2: Feeding the renal diet inconsistently. The second most common error. The owner feeds the renal diet, but the owner also feeds the maintenance diet, the treats, the table scraps, and the "just a small piece" of human food. The inconsistent feeding undermines the diet, and the inconsistent feeding does not provide the consistent low-phosphorus, moderate-protein profile that the CKD pet needs. The fix is to commit to the prescription diet exclusively, with the appropriate low-phosphorus treats, and to eliminate the inappropriate foods.

Error 3: Using an over-the-counter "kidney-support" diet as a substitute. The third error. The over-the-counter kidney-support diet is not the same as the prescription renal diet, and the over-the-counter diet does not produce the same effect. The fix is to use the prescription renal diet, and to consult with the vet about the specific product.

Error 4: Stopping the renal diet when the pet improves. The fourth error. The pet is switched to the renal diet, the pet improves (often within 2–4 weeks), the owner stops the diet or switches back to the maintenance diet, the pet's CKD progresses. The CKD is a chronic condition that requires chronic management. The fix is to commit to the renal diet for the life of the pet.

The bottom line

The CKD pet that is on the prescription renal diet is a pet that has the best chance of a long, good-quality life. The CKD pet that is not on the prescription renal diet is a pet that is missing the most evidence-based intervention available. The difference is the diet. The diet is the framework above. The framework, applied with discipline, is the right preparation.

The pet that is well-managed with the renal diet is a pet that has a slow progression, a good quality of life, and a long survival. The pet that is poorly managed is a pet that progresses rapidly, has a poor quality of life, and has a short survival. The difference is the management. The management is the framework above. The framework, applied with discipline, is the right preparation.

The owner who is committed to the renal diet is the owner who has a CKD pet that is living well with the disease. The owner who is not committed is the owner who has a CKD pet that is suffering. The difference is the commitment. The commitment is the framework above. The framework, applied with discipline, is the right preparation.

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About the author

Hannah Kim, DVM, DACVD, is a board-certified veterinary dermatologist and internist with a focus on chronic disease management, the interface of internal medicine and dermatology, and the long-term nutritional management of senior pets. She has a particular interest in the practical application of evidence-based nutrition to the management of chronic conditions like CKD, and runs a busy referral practice in the Pacific Northwest. She writes regularly on the practical interface between chronic disease and the nutritional interventions that support the management.