This article was taken from DogAware.com and discusses kidney health in detail. Further down the article, there is an open letter to Hill's corporation discussing how their own literature counters veterinarian recommendations and scientific studies regarding a dog's kidney health. This is a good, but long, read.
Following are links to a series of articles and studies on the roles of protein and phosphorus in the diet of dogs with kidney disease, supporting the idea that reducing protein in the diet does not slow the progression of kidney disease nor prolong life, and is unnecessary unless it is needed to relieve symptoms of uremia (very high creatinine and BUN), or if your dog has significant proteinuria (protein in the urine, in which case moderate but not severe protein restriction is indicated). I have provided excerpts from these articles, but I would encourage you to read them in their entirety if you are dealing with a dog with kidney disease, as many of them contain a great deal more information than I will show here.
More Study Results from the Nutrition Symposium by Ken Tudor, DVM (6/27/13)
The double-blind, randomized study compared actual kidney function, creatinine (a blood marker for kidney function), body condition and survival time of dogs fed a low protein diet (16%) and those fed a normal diet (22.5% protein). Both diets had the same restricted phosphorus content. What they found was no significant difference in kidney function, blood creatinine levels or survival time between the two groups. As expected the body condition scores were greater for the normal protein group as they maintained more muscle mass. The main determinant of survival time was the level of kidney dysfunction at the time of diagnosis, not the diet.
What is interesting is that the low protein diet in this study was 2-3 percent greater than most of the veterinary diets formulated for kidney disease. That means that muscle loss would be expected to be even greater for pets being fed veterinary kidney diets. The maintenance of muscle mass has implications for maintaining an active lifestyle. Since quantity of life is no different it would make sense to feed kidney patients a normal protein diet to maintain their quality of life. This study also suggests that phosphorus control is more important than dietary protein in kidney patients.
Dogs with kidney problems by Dr. Lucy Pinkston, D.V.M.
"Because by-products of protein digestion are the main toxins that need to be excreted by the kidneys, an obvious assumption might be that all one needs to do is to cut out the protein and the kidneys wouldn't have any more hard work to do. . . . There is significant evidence, however, that the daily protein requirements actually increase slightly for dogs in chronic renal failure. Therefore, severely restricting the protein for such a dog is likely to result in protein malnutrition, in spite of the fact that the levels of blood urea nitrogen, or BUN (the primary by-product of protein metabolism) would be correspondingly lower." This article contains a great deal more useful information in easy to read format.
Are High Protein Diets Harmful to a Dog's Kidneys? from the Veterinary Services Department, Drs. Foster & Smith, Inc.
"The myth that high-protein diets are harmful to kidneys probably started because, in the past, patients with kidney disease were commonly placed on low-protein (and thus low-nitrogen) diets. Now we often put them on a diet that is not necessarily very low in protein but contains protein that is more digestible so there are fewer nitrogen by-products."
The Mythology of Protein Restriction for Dogs with Reduced Renal Function by Kenneth C. Bovee, DVM, MMedSc
"Morris subsequently developed, produced, and sold a low-protein diet, KD, for dogs with renal failure. He and others were influenced by the erroneous work hypertrophy concept for urea excretion advanced by Addis. While experimental or clinical data were never published to support the value of this or other diets, the concept was broadly accepted without challenge in the veterinary literature." This article talks about the history of protein restriction, and about 10 recent experimental studies that have failed to provide evidence of the benefit of reduced dietary protein to influence the course of renal failure.
Also see Influence of Dietary Protein on Renal Function in Dogs by the same author, which concludes, "These results do not support the hypothesis that feeding a high protein diet had a significant adverse effect on renal function or morphology."
Nutrition and Renal Function from the Purina Research Report
"Dietary Protein and Renal Function: Results of multiple studies indicated that there were no adverse effects of the high protein diets." This report also includes information on metabolic acidosis and on the beneficial effects of omega-3 essential fatty acids in patients with chronic renal failure. The complete reports on each of the three studies mentioned in this report are available online, as follows: Effects of Dietary Lipids on Renal Function in Dogs and Cats; Effects of Dietary Protein Intake on Renal Functions; and Acid-Base, Electrolytes, and Renal Failure. (I have copies of these if they cannot be found online).
Feeding the Older Dog from the SpeedyVet Clinical Nutrition Library
"The assumption was that low-protein diets retarded the progression of renal degeneration. This assumption was disproved, using partially nephrectomised dogs, which showed no uraemic signs and had reduced but stable renal function for 48 months. These dogs did better on moderate-protein diets than on low-protein diets. There is no direct evidence that high protein intake damages canine kidneys or that reducing protein intake in dogs with renal dysfunction results in preservation of either renal structure or function."
Dietary Management of Chronic Polyuric Renal Failure from the SpeedyVet Clinical Nutrition Library
"Dietary protein restriction improves the clinical signs and quality of life of uremic animals with both naturally occurring and experimentally induced renal failure. . . . However it is highly questionable whether protein restriction is appropriate in the azotaemic, but non-uraemic patient. The main risk of protein restriction is protein deficiency. The protein and amino acid requirements of dogs and cats with chronic renal failure have not been established, but may well be increased. . . . The main justification for protein restriction early in the course of renal failure would be if it was proven to slow progression of disease. The data that are available do not support this case in dogs. Dietary protein has been shown to affect renal haemodynamics in the dog, however, moderate protein restriction does not alleviate glomerular hypertension, hyperfiltration and hypertrophy. . . . Thus there is no evidence that moderate protein restriction slows the progression of renal failure in dogs, and it is not recommended in dogs which are not uraemic."
Demystifying Myths About Protein from Today's Breeder Magazine
"In contrast, research over the past 10 years or so has shown that protein does not harm the kidney of dogs. In studies conducted at the University of Georgia in the early 1990s, both in dogs with chronic kidney failure and in older dogs with only one kidney, protein levels as high as 34 percent caused no ill effects. . . . In other studies, David S. Kronfeld, Ph.D., indicated that compared with high- or low-protein diets, moderate-protein diets, those with up to 34 percent protein, had no ill effects in dogs with chronic renal failure and were associated with general improvement."
Fortify The Food Bowl For The Aging Canine by Susan Thorpe-Vargas, Ph.D. and John C. Cargill, M.A., M.B.A., M.S.
"Because of certain biochemical requirements, the healthy geriatric dog requires about 50 percent more protein than the young adult, and depending on the quality of the protein, it should make up 20 percent to 30 percent of the total calories ingested. . . . Until recently, protein restriction was recommended in an effort to protect renal function. Limiting protein fails to prevent urinary filtration problems . . . Indeed, newer research shows dietary protein is not detrimental to kidney function. On the contrary, protein restriction can result in impaired wound healing, diminished immune function and lowered enzyme activities and cellular turnover. Those dogs with impaired renal function do better with dietary phosphorus restriction; however, limiting this mineral is unlikely to delay the onset of renal disease or to benefit healthy geriatric dogs."
Pet Food Safety: Dietary Protein by Dorothy LaFlamme, DVM, PhD, Dipl ACVN
"Summary and Conclusions: Based on a comprehensive review, there remains no evidence that dietary protein causes kidney damage, or any other adverse effects, in healthy dogs. Even in dogs with chronic kidney disease, dietary protein does not appear to contribute to kidney damage. However, in chronic kidney disease, there can be an accumulation of byproducts of protein metabolism, which may contribute to uremic signs. Hence, in those patients, dietary protein restriction may be of benefit. On the other hand, dietary protein is important to support normal protein turnover and maintain lean body mass. Healthy, aging dogs have an increased requirement for dietary protein. When insufficient protein is provided it can aggravate the age-associated loss of lean body mass and may contribute to earlier mortality. Unless medicatlly indicated, intake of dietary protein should not be restricted."
Dietary Management for Clinical Disorders in Dogs from the Journal of Indian Veterinary Association, Kerala
"Recent research on dietary protein and the kidney has shown that
o dietary protein does not cause renal failure
o dietary protein does not appear to be involved in the progression of chronic renal failure
o inappropriate restriction of dietary protein may actually have an adverse effect on the normal or compromised kidney"
Kidney Failure from the Iams nutrition symposium
“'For years, physicians and veterinarians have treated renal failure by reducing protein levels in diets,' said Gregory Reinhart PhD, an Iams researcher. 'After working with leading universities, we have now found that restricting protein in a dog's diet may do more harm than good by potentially putting the companion animal at risk of protein malnutrition.'”
Managing a Renal Crisis by Martha S. Gearhart, DVM
". . . at least one study has taken several groups of dogs in kidney failure and fed them diets that varied in protein level and phosphorus level. The groups with severely restricted phosphorus lived longer than the groups with normal or high levels of phosphorus. The protein intake made no difference at all in longevity. . . .
"It is important to remember that phosphorus is more important than protein -- feeding vegetables or salt-free crackers to a dog in kidney failure will not add protein but it will add phosphorus."
Dietary Protein and the Kidney by Patricia Schenck, DVM, PhD, Veterinary Nutritionist
"High protein diets cause an increase in blood flow through the kidney (glomerular filtration rate). The myth has been that if the dietary protein is restricted, this will make the kidney work less, and will ‘spare' the kidney from damage. Thus in the past, many have recommended low protein diets to ‘protect' a dog from developing kidney disease. This has been the focus of considerable research over the last 10 years. There has been no scientific evidence to support this theory. The feeding of low levels of dietary protein are NOT protective against the development of kidney disease.
"Reducing dietary protein in the older pet will not protect them from the development of renal disease. In fact, reducing the protein in the older dog's diet may have adverse effects. As pets age, their ability to utilize nutrients decreases. The older pet actually requires a higher level of protein to maintain its body stores of protein than does the younger adult dog. . . .
"Dietary protein restriction is appropriate in renal failure when the disease has become severe. Restriction of protein is based on the appearance of clinical signs. It has been recommended to start protein restriction when the dog's BUN (blood urea nitrogen) is greater than 80 mg/dL [28.6 mmol/L], and the serum creatinine is greater than 2.5 mg/dL [221 µmol/L]. Both BUN and serum creatinine are good indicators of kidney function. Protein is restricted in an attempt to keep the BUN below 60 mg/dL [21.4 mmol/L]. Dietary protein may need to be gradually decreased over time as renal failure progresses."
Effects of low phosphorus, medium protein diets in dogs with chronic renal failure
"In this study, 60 dogs with early CRF were fed either Medium Protein Diet, (CMP group) or a home-made diet (HMD group) which respectively contained 0.36% phosphorus, 27% protein, and 0.38% phosphorus, 21.5% protein on a dry matter basis, over a 28 week period. . . .
"From the results of this study, it can be concluded that many dogs with mild to moderate CRF can benefit from early diagnosis of the condition and dietary management using a diet with a low phosphorus and moderate protein content."
Effects of dietary phosphorus and protein in dogs with chronic renal failure.
Finco DR, Brown SA, Crowell WA, Duncan RJ, Barsanti JA, Bennett SE.
Am J Vet Res. 1992 Dec;53(12):2264-71.
"Four diets were formulated to contain: 16% protein and 0.4% phosphorus--diet 1; 16% protein and 1.4% phosphorus--diet 2; 32% protein and 0.4% phosphorus--diet 3; and 32% protein and 1.4% phosphorus--diet 4."
After 24 months, they concluded, "Dog survival was significantly enhanced by 0.4% P [phosphorus] diets (vs 1.4% P diets), but survival was not significantly influenced by amount of dietary protein. The 0.4% P diets (vs 1.4% P diets) significantly increased the period that GFR remained stable before it decreased, but dietary protein did not have significant effect."
Dietary Protein by Dr. Jeff Vidt, specialist in Chinese Shar-Pei and Renal Amyloidosis
Protein Restriction and Kidney Disease Extracts from Kirk's Current Veterinary Therapy XII, with links to a number of abstracts
"In perhaps the most noted clinical trial examining effects of high protein diet on progression of CRD, groups of dogs diagnosed with CRD were fed either high protein diets or low protein diets. No significant difference was observed in the rate of progression of CRD in the high-protein group compared to the low protein group. Therefore, excess protein in the diet did not appear to compromise renal function even in the presence of high endogenous levels of protein associated with the disease. In fact, on an individual basis some of the CRD dogs in the high protein diet group faired better. This finding was postulated to be associated with the fact that protein is required for cellular repair and function."
Kidney Failure in the Dog and Cat
"Contrary to popular myth, diets rich in protein ("high protein levels") do not cause kidney damage. Research done decades ago indicated that rodent kidneys were adversely affected by diets high in protein... and misguided researchers extrapolated that data to apply to the canine. There is no evidence that feeding dogs and cats diets rich in or "high" in protein actually causes kidney damage or disease. Some day this myth will be finally be put to rest. In fact, there is ample research and well documented studies that prove that dogs and cats thrive on diets with levels of protein consistent with a meat-eater's (carnivore) natural prey selection. Read more about protein in dog and cat diets here.
"Additionally,documented research on dogs indicates that reducing dietary protein levels in older dogs may be unwise; however, if kidney damage is already present to the extent that the BUN levels are 75mg/dl or above, some restriction of dietary protein may be beneficial for metabolic reasons... not renal reasons. '...restriction of protein intake does not alter the development of renal lesions nor does it preserve renal function. . . . Considering these (research) findings, the authors do not recommend reduction of dietary protein in dogs with renal disease or reduced renal function in order to achieve renoprotective effects.' (See Kirk's Veterinary Therapy XIII, Small Animal Practice, W. B. Saunders, page 861). Restricting dietary protein may be helpful to those patients whose BUN levels are rather high and that are already in advanced kidney failure."
Note that the above sites are from very traditional sources, including Purina and Iams. I think Hills is the only company still toeing the "low protein" line. The thinking now is that low protein can actually be harmful, and that a moderate amount of high quality protein is desirable for dogs with kidney disease. In addition, feeding reduced protein to dogs with normal kidneys does not help prevent kidney disease.
Why Feline Kidney Insufficiency is Still Tricky to Treat published in Veterinary Practice News, March 2015
(this article is about cats, but may apply to dogs as well)
A changing view of the best diet is a key development in the treatment of chronic kidney insufficiency, Norsworthy said, [Gary Norsworthy, DVM, Dipl. ABVP (Feline), owner of Alamo Feline Health Center in San Antonio]
In the past, the approach was a low-protein diet. Now the practice is to put such a cat on a high-protein diet, he said.
There are several reasons for this switch in thinking, particularly to keep the cat from losing too much weight and muscle. Muscle wasting is a common problem in senior cats, Norsworthy noted. Their spines and hips become very prominent because they have lost muscle mass. This occurs due to protein deprivation.
“The low-protein diet really hasn’t done the good that we thought it was doing,” Norsworthy said, adding that its benefits to kidney function “are minimal to nil” and that many older cats on a low-protein diet will begin to lose muscle and get thin.
Despite this change in thinking, many manufacturers continue to make low-protein diets that are officially labeled for the kidney disease, he said.
“I don’t even use those official kidney diets anymore,” Norsworthy said. Instead he recommends foods made for diabetes patients because they are low in carbohydrates, which is in excess amounts in many cat foods, and the food makers have replaced the carbs with mostly proteins.
Norsworthy noted that creatinine is artificially lowered when a cat loses a lot of weight, and muscle mass is needed to make creatinine.
“In a very thin cat, the creatinine will not be a true reflection of the kidney function,” he said.
That means the creatinine measurement is not reliable in a thin cat, and on the other hand the BUN measurement is falsely elevated in a cat that’s dehydrated. “Therefore, you have to consider body condition and hydration status when interpreting creatinine and BUN values,” he said.
High protein diets will elevate the BUN measurement, but that doesn’t mean the kidney function is down, he said. “It just means the cat’s on a high-protein diet.”
A picky appetite is one of the most common concerns Quimby hears about patients with chronic kidney disease. [Jessica Quimby, DVM, Ph.D., a faculty member in small animal internal medicine at Colorado State University Veterinary Teaching Hospital]
“Appetite should be actively addressed, as poor body condition is linked to a poor prognosis,” she said.
See http://lpi.orst.edu/infocenter/minerals/phosphorus/ for (human oriented) information on phosphorus and what excess levels in the blood do.
Below is a letter that I wrote to Hill's on June 20, 2005, in response to their complaints about an article I helped write for the Whole Dog Journal on the myth of low protein diets for dogs with kidney disease. Portions of both letters were printed in a later issue of the Whole Dog Journal. Hill's did not respond to my letter. Here is what I sent:
In response to the letter from Hill’s Pet Nutrition, dated June 6, 2005, regarding “Inaccurate information published in article: When to Say No to Low-Protein, Only Dogs Whose Kidney Failure is Advanced Need Very Low-Protein Foods by C.J. Puotinen.
I would like to respond to the allegations in this letter regarding supposed inaccuracies in the above article, and that some of the recommendations in the article “could cause significant harm to dogs suffering from CKD,” which I believe are wholly without foundation.
The letter says that “current recommendations made by veterinary nephrologists contradict those made by the author of this article,” then goes on to give the results of a study that compared the use of Prescription Diet® Canine k/d® with maintenance kibble.
I do not believe that dogs with CKD should be fed maintenance kibble, nor does the article we wrote advocate this. We made it clear that dogs with CKD need a reduced phosphorus diet, and advocated feeding a home made diet that reduces phosphorus, without overly restricting protein. Studies such as the one cited by Hill’s have provided some information to veterinarians on clinical responses to various commercially available foods, but they have not provided information to define the role of dietary protein intake on progressive renal injury.
“In addition, therapeutic foods such as Prescription Diet® Canine k/d® are indicated before kidney disease becomes severe; they should be fed when serum creatinine concentration is > or = 2 mg/dl.”
I agree that diet should be modified before kidney disease becomes severe, and the article we wrote specifically suggests reducing phosphorus “even with early stage CRF.” I do not agree that protein should be restricted at this stage, or that the level of phosphorus restriction in Prescription Diet® Canine k/d® is required for early stage renal disease. According to the Nutrition Support Service at Ohio State University Veterinary Hospital , “Results of recent research are changing our recommendations for nutrient modification in dogs with early signs of CRF. Restricting phosphorus intake to about 30 mg/pound/day [about half the amount found in normal diets] as soon as polyuria is recognized, and supplementing potassium intake (with alkalinizing salts if acidosis is a concern) to maintain serum potassium within the normal range may be all that is necessary until patients develop severe disease.” This compares to recommended levels of phosphorus intake as low as 10 mg/pound/day for late stage CRF.
“Prescription Diet® Canine k/d® contains a reduced amount of protein; however, it provides complete and balanced nutrition for maintenance of adult dogs as determined by animal feeding trails using Association of American Feed Control Official (AAFCO) procedures.”
Again according to the Nutrition Support Service at Ohio State University Veterinary Hospital , the minimum protein requirement for dogs with mild to severe CKD is 1 gram/pound/day (this refers to a nutritional analysis, not grams of meat). They recommend this amount be reduced only when the dog is uremic: “Protein restriction may be instituted when the BUN is greater than 80 mg/dl.” Prescription Diet® Canine k/d® (dry)  contains only 12-13.4% protein as fed, or 3.3 grams/100 kcals. Hill’s recommends feeding 4 ¼ to 5 ¾ cups to a 100 lb dog (420-570 grams). At 13% protein, 420-570 grams of food would have 55 to 74 grams of protein, which is below the minimum requirements for an adult dog of 1 gram per pound of bodyweight (100 grams of protein for a 100 lb dog). The Merck Veterinary Manual  and other sources  recommend 2.0 to 2.2 grams of protein per kg of bodyweight for dogs with CKD, which is equivalent to 1 gram/lb.
“As the quality and digestibility of the protein source increases, the amount of protein required in pet foods decreases. Prescription Diet® Canine k/d® contains proteins that are of high quality and digestibility, therefore, a lesser amount of protein can be used to meet maintenance requirements of dogs.”
There have been no studies done that I’m aware of showing that lower amounts of high quality protein do not cause problems due to protein deficiency in dogs with CKD. Suggestions I saw regarding the use of high quality proteins for dogs with kidney disease specifically suggested not reducing the quantity of protein below levels referenced above, but instead increasing the quality of protein fed .
I would also dispute Hill’s claim that it contains “proteins that are of high quality and digestibility.” The ingredients in Canine k/d® are “Brewers rice, pork fat (preserved with mixed tocopherols and citric acid), dried egg product, flaxseed, corn gluten meal, chicken liver flavor, soy fiber” plus supplements and artificial preservatives . Of these, only eggs would be considered high quality protein (and “dried egg product” is almost certain lower quality than fresh eggs). Corn gluten meal and brewers rice are poor quality and incomplete proteins. None of the other ingredients provide protein at all. As a side note, the ethoxyquin used in k/d® has been shown to cause kidney damage .
“To date, studies have not shown a conclusive effect on delaying progression of experimentally-induced CKD in dogs; however, there have been beneficial effects observed in dogs with naturally occurring kidney disease. It is generally accepted that reducing protein intake decreases blood urea nitrogen (BUN) concentrations and ameliorates clinical signs of uremia in patients with CKD.”
It is true that reducing protein decreases BUN, but this is only significant when BUN is high enough to cause signs of uremia (in general, over 80 mg/dl). Our article did advise reducing protein when a dog is uremic: “The only time dietary protein restriction is appropriate in renal failure is when the disease has become severe.” No studies have shown that reducing protein improves quality of life, extends life, or slows the progression of kidney disease when instituted before a dog is uremic. The studies cited by Hill’s that purportedly show otherwise apply only to dogs with canine X-linked hereditary nephritis (a rare, breed-specific form of juvenile renal disease), and the first study only indicates that a protein restricted diet reduces proteinuria, which is a symptom (comparable to BUN) that is related to the amount of protein in the diet and does not mean that there is any actual benefit from the reduction. The first study  also indicates that the dogs fed the low protein diet “did not maintain starting BW or plasma albumin concentration within the normal reference range,” dangerous side effects of low protein diets, and that “unintended differences in digestibility of protein and energy (P < or = .01) prevented assignment of the diet effect exclusively to protein.” I would argue that this study actually supports our contention that low protein diets can be harmful. The modified diet in the second study  also included reduced phosphorus, which has been shown to be beneficial to dogs with renal disease, so any positive effects cannot be attributed to reduced protein.
“In addition, restricting dietary protein may help ameliorate effects of metabolic acidosis, which is common in CKD. Although further evaluation is needed, dietary protein restriction is associated with reduced magnitude of urine protein loss (i.e., proteinuria) in dogs with glomerular diseases (i.e., kidney diseases characterized by increased permeability of glomerular blood vessels that allow protein leakage into the urine). Protein supplementation (e.g., meat, eggs, tripe) is contraindicated in these patients because it may worsen severity of proteinuria and subsequent hypoalbuminemia (decreased protein concentration in the blood).
I looked up the references cited in the quote from above. The first one cited is the study noted above , which indicates that low protein diets led to loss of body weight and hypoalbuminemia. Proteinuria may lead to hypoalbuminemia as well, but not because there is too much protein in the diet, but rather because protein is being lost faster than it is being restored. Proteinuria is a symptom, not a cause, of renal disease. In reality, higher protein diets may be required when dealing with proteinuria and hypoalbuminemia, due to the excessive loss of protein through the kidneys. A paper on Strategies for Protein Losing Nephropathy presented at the Atlantic Coast Veterinary Conference in 2001 specifically recommends adding protein in the form of egg whites to treat hypoalbuminemia .
The second citation is to information on Glomerular Diseases written by S.L. Vaden. I do not have access to the book cited, but did find articles by and about the same author on this topic on the web. One of them, "Glomerulonephritis remains an important cause of renal disease in dogs", has specific suggestions for controlling proteinuria in dogs with glomerulopathy. These suggestions include controlling inflammation, controlling systemic infections, and using angiotensin-converting enzyme (ACE) inhibitors. There is no mention of restricting protein in the diet. Another article by Dr. Vaden  concerning the treatment of SCWT with PLE/PLN advises moderate (not severe, as with Canine k/d®) protein restriction, coupled with ACE inhibitors and aspirin therapy to control proteinuria and hypoalbuminemia: “Dogs with PLN should be fed a diet that is moderately reduced in protein. Angiotensin converting enzyme inhibitors (usually enalapril) are frequently used to decrease the degree of proteinuria. Dogs that have serum albumin of < 2.0 g/dl should be given a low-dose of aspirin to help prevent thromboembolic disease. Systemic hypertension must be controlled.” An article by Gregory F. Grauer, DVM  specifically on proteinuria in dogs states that a direct link between proteinuria and renal damage has not been established in dogs, and again finds that the use of ACE inhibitors, and in this case omega-3 fatty acid supplementation, do more to reduce proteinuria than dietary changes. Protein malnutrition can be as harmful as protein excess and can even lead to death in dogs with kidney disease . Lastly, although moderate protein restriction may help with metabolic acidosis, this is not a common occurrence in dogs with kidney disease, particularly in the early and moderate stages, and is usually treated using alkalinizing agents such as sodium bicarbonate and potassium citrate, not by further protein reduction .
“The reader should also be aware that feeding a food with reduced protein is only part of the picture when talking about therapeutic foods such as Prescription Diet® Canine k/d®. Other beneficial components of these foods include moderate phosphate restriction, moderate sodium restriction, increased supplementation with polyunsaturated fatty acids (PUFA), increased B vitamins, and increased buffering capacity (to counteract the tendency toward decreased acid excretion by the kidneys).
Our article recommended phosphorus restriction and supplementation with B vitamins. Supplementation with omega-3 fatty acids was also highly recommended. Supplementation with omega-6 polyunsaturated fatty acids, which are found in flaxseed (an ingredient in Canine k/d®) along with omega-3 fatty acids, is not recommended and may be harmful to dogs with kidney disease. The Washington State University College of Veterinary Medicine , for example, says, “The omega 3 and omega 6 fatty acids are degraded to different eicosanoids. Preliminary studies suggest that diets high in omega 3 polyunsaturated fatty acids preserve renal function in dogs with induced renal failure whereas diets high in omega 6 fatty acids hasten decline.“ In addition, omega-3 fatty acids are quite fragile and oxidize quickly when exposed to light, heat or air. Adding them to a dry food is not likely to be helpful. Sodium restriction is only necessary when blood pressure levels are elevated; our article noted that a reduction in sodium might be necessary when this occurs. Regarding increased buffering capacity, calcium carbonate is one of the agents recommended to do this , and is the primary ingredient in the ground eggshells that our article recommended for use with home made diets for dogs with kidney disease. In summary, our diet recommendations had the same or better beneficial components as those in Prescription Diet® Canine k/d®.
“The author notes that feeding low amounts of phosphorus is a dietary goal for CKD patients, yet does not define what constitutes low phosphorus. A food that contains 0.15 to 0.3% phosphorus (dry matter basis) is recommended for most dogs with chronic kidney disease; Prescription Diet® Canine k/d® contains 0.22% phosphorus (dry matter basis). Because protein is a major source for phosphorus, it is usually necessary to limit dietary protein intake in order to limit dietary phosphorus intake. Following your recommendations to feed a food whose calories come 1/3 from fat, 1/3 from protein, and 1/3 from carbohydrates would result in excessive phosphorus intake, which could be detrimental to patients with CKD. In addition, your recommendation to supplement with fresh protein foods such as raw or cooked meat, eggs, or tripe will contribute further to consumption of excessive dietary phosphorus and worsening of kidney disease.”
Such extreme levels of phosphorus restriction are not indicated for early and moderate stage kidney disease, particularly when serum phosphorus levels are normal. The Virginia-Maryland Regional College of Veterinary Medicine  recommends 0.4% phosphorus for moderate stage kidney disease, dropping to 0.2% for advanced stage. The Nutrition Support Service at Ohio State University Veterinary Hospital  advises no phosphorus restriction for mild CKD, then restricting to about half the normal amount as the disease progresses to the moderate stage. Treating dogs with mild kidney problems the same way as dogs with severe kidney disease has not been shown to be beneficial and may in fact be harmful, due to excessive protein restriction. When attention is paid to the actual amount of phosphorus in various high protein foods, it is quite possible to restrict phosphorus substantially without having to reduce protein to dangerously low levels. For example, egg whites have a great deal of protein with very little phosphorus. Fatty meats can provide substantial amounts of protein with only moderate amounts of phosphorus. To say that one must restrict protein amounts to very low levels in order to achieve reductions in phosphorus is overly simplistic. By adding proper fresh food sources to Canine k/d®, as suggested in our article, protein levels can be raised to a healthier level with only nominal increases in the amount of phosphorus. For example, Canine k/d®  provides 57 mg phosphorus per 100 kcals, while ground lamb provides 56 mg phosphorus per 100 kcals, egg whites 29 mg, green tripe 84 mg, and 20% fat ground beef 62 mg . Feeding a diet whose calories come 1/3 from protein is considered a moderate level, and has been shown to be beneficial in dogs with renal disease : “Only beneficial responses, such as less proteinuria, less renal impairment and lower mortality, have been reported for diets containing low phosphorus [0.4% DM] and moderate protein (20-31% ME).” Note that with 3.3 grams of protein per 100 kcals, Canine k/d® derives only 13% of its calories from protein [2,3].
Also, although the article itself did not delve into the exact amounts that define “low phosphorus”, the additional resources to which the article referred do include that information. The specific home made diet suggestions found there, on which the article was based, produce a diet that is 0.37% phosphorus.
“In the past, owners of dogs with CKD often found that therapeutic kidney foods were not very palatable. Prescription Diet® Canine k/d® dry and wet formulas have undergone several improvements in palatability during the past several years and most dogs with CKD readily accept these foods.”
Hill’s admits that their diets have not been palatable in the past, but claims they are improved now. I would suggest that they would have made the same claims in the past, and will at some point in the future likely make further changes that they will then claim make their food more palatable than it is now. The fact is that low protein foods are inherently less palatable, and that commercial diets will never appeal as much to a dog as fresh food diets. Anecdotally, many people still report their dogs refusing to eat Prescription Diet® Canine k/d®. Even if some dogs are willing to eat this food, it is not unreasonable to suggest alternatives for those that will not, or additives that will make it more appealing, as well as healthier, for dogs with early to moderate stage CKD.
“The author incorrectly states that flax seed oil (a source of omega-3 polyunsaturated fatty acids) is not recommended for CKD patients. Dogs with CKD that consumed a food supplemented with omega-3 polyunsaturated fatty acids (PUFA) had lower mortality, better kidney function, fewer kidney lesions, and less proteinuria than dogs fed saturated fats or omega-6 PUFA. Based on these findings, supplementation with omega-3 PUFA are recommended for dogs with CKD.”
I couldn’t agree more that supplementation with omega-3 fatty acids is highly recommended for dogs with CKD, and our article made this recommendation several times. The critical difference is the source of the omega-3 fatty acids. Fish body oil supplies omega-3 fatty acids in the form of EPA and DHA, which are directly utilizable by the dog’s body, and fish oil does not contain any omega-6 fatty acids. Flax seed oil provides omega-3 fatty acids in the form of ALA (alpha-linolenic acid), which must be converted to EPA and DHA in the body. Even in humans, this conversion is inefficient, ranging from 0-15% [19,20], and is almost certainly lower in dogs . In addition, flax seed oil is only about 55% ALA, containing omega-6 as well as omega-3 fatty acids, and omega-6 fatty acids have been shown to be harmful to dogs with CKD .
Both studies cited by Hills used fish oil as the source of their omega-3 fatty acids, not flax seed oil: (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9605110&dopt=Abstract and http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10711867&dopt=Abstract). I find the statement above to be representative of how Hill’s manipulates information to distort the facts.
“There are no nutritional reasons that support providing excessive amounts of dietary protein; older dogs (>7 years) do not require more protein than young adults. In fact, subclinical kidney disease may exist in apparently healthy older or geriatric dogs and kidney function can be worsened by feeding excessive protein (and phosphorus) to these patients.”
There are many recent studies that indicate older dogs, even those with kidney disease, may need more protein than younger dogs.
“There are, however, potential problems with excessive protein restriction, which can result in protein malnutrition. The protein requirements of dogs and cats with CRF have not been established but it is likely that they may be quite similar or even higher than those of the healthy animal.”
“However it is highly questionable whether protein restriction is appropriate in the azotaemic, but non-uraemic patient. The main risk of protein restriction is protein deficiency. The protein and amino acid requirements of dogs and cats with chronic renal failure have not been established, but may well be increased. Uraemia is a catabolic state causing a range of abnormalities in protein metabolism. In addition, gastrointestinal blood loss and urinary loss of some amino acids may increase protein requirements.”
“A diet rich in protein is especially important for older dogs. Senior dogs appear less efficient at metabolizing protein, so they require additional protein in their diets to help compensate. In fact, research has shown that healthy older dogs may need as much as 50 percent more protein than normal young healthy adult dogs.”
“The importance of providing adequate dietary protein to senior dogs was brought out in research conducted at the Purina Pet Care Center. In this study, 26 English pointers, ranging from 7 to 9 years old, were fed diets that were either 15 percent or 45 percent protein over several years. Dogs fed the high-protein diet maintained a directionally higher percent of lean body mass and lower percent of body fat.”
“There is significant evidence, however, that the daily protein requirements actually increase slightly for dogs in chronic renal failure. Therefore, severely restricting the protein for such a dog is likely to result in protein malnutrition, in spite of the fact that the levels of blood urea nitrogen, or BUN (the primary by-product of protein metabolism) would be correspondingly lower.”
As to older dogs that “may” have subclinical kidney disease, low protein diets have not been shown to benefit dogs that do have kidney disease, nor have they been shown to reduce the risk of kidney disease or slow its progression. There are no studies indicating that feeding excessive protein to older dogs is harmful. “In contrast, research over the past 10 years or so has shown that protein does not harm the kidney of dogs. In studies conducted at the University of Georgia in the early 1990s, both in dogs with chronic kidney failure and in older dogs with only one kidney, protein levels as high as 34 percent caused no ill effects. . . . 'Our work was directed at learning whether a high-protein diet damages the kidney. After four years' studying geriatric dogs and two years' studying dogs with chronic renal failure, we found no indication that a high-protein diet was injurious to the kidney. However, the data did raise the question whether low-protein diets in geriatric dogs could be considered injurious.'” “Julie Churchill, D.V.M., assistant clinical specialist in companion animal nutrition at the University of Minnesota, was an investigator in studies to learn whether altering the amounts of dietary protein and fat could protect the kidney in aging dogs. "We found there is no benefit in restricting protein in geriatric dogs," Churchill says. "We observed no changes in morbidity or mortality. So, the question is, 'Why restrict protein if there is no benefit?'"”
In a study designed to test the hypothesis that restricting protein intake in older dogs may be renoprotective, experimental dogs seven to eight years of age were divided into two groups. Dogs in both groups had uninephrectomy performed to increase vulnerability of the remaining kidney to any protein effects. One group was fed a low protein diet (18% protein), and the other group received a high protein diet (34% protein) for the subsequent four years. Results of this study indicated that there were no adverse effects from the high protein diet, and mortality was actually higher in the low protein group .
A summary of eight studies done on dogs with reduced renal mass found that levels of protein up to 44% in the diet had no harmful effect on the kidneys .
“Dogs are omnivores and are capable of meeting their nutrient requirements by eating a variety of animal and vegetable substances including grains and starch (e.g., rice, corn starch). The carbohydrates in Prescription Diet® Canine k/d® are highly digestible (i.e., 97%).”
The question of whether dogs are carnivores are omnivores is beyond the scope of this letter. Suffice it to say that, while dogs are not obligate carnivores as cats are, their digestive systems, dentition, and classification as a subspecies of the gray wolf (canis lupus familiaris) would argue that dogs are, in fact, far more closely related to carnivores than to omnivores, such as humans. Although dogs may be capable of deriving nutrition from grains and starches, that does not mean these foods are an optimal diet for them, nor can dogs survive without any animal protein in their diet at all. Those of us interested in giving our dogs the best nutrition, and not just the cheapest, use primarily animal products, not low quality waste products from the human food industry such as Brewers Rice  and Corn Gluten Meal .
In summary, I stand by the recommendations in our article, and will continue to recommend that dogs with kidney problems receive adequate amounts of protein that Prescription Diet® Canine k/d® does not provide, along with fish oil (not the flaxseed found in Canine k/d®) and high quality fresh foods that they enjoy, and that moderate, rather than severe, restriction of phosphorus is all that is needed for early stage renal disease. I also stand by the assertion that older dogs, including those with CKD, do not benefit, and may be harmed, by excessive protein restriction.
I would also suggest that Hill’s, rather than defending their outdated formulas, might better direct that effort toward revising those formulas in light of recent research that even they admit “has radically changed and fine-tuned the dietary recommendations for canine CKD patients”, so that it would not be necessary to warn people about using them.
Researcher for the article in the Whole Dog Journal
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29. The Composition of Rice and Various Processed Types
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