June 20, 2005
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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 [1], “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 [2], the minimum protein requirement for dogs with mild to severe CKD is 1 gram/pound/day. 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.”[1] Prescription Diet® Canine k/d® (dry) [3] 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 [4] and other sources [5] 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 [4].

 
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 [3]. 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 [6].

 
“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 [7] 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 [8] 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.
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I looked up the references cited in the quote from above. The first one cited is the study noted above [7], 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 Nephopathy presented at the Atlantic Coast Veterinary Conference in 2001 specifically recommends adding protein in the form of egg whites to treat hypoalbuminemia [9].

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[10], 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 [11] 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 [12] 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 [13]. 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 [14].
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Our article recommended phosphorus restriction and supplemention 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 [15], 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 [13], 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®.
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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 [16] 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 [1] 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® [3] 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 [17]. 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 [18]: “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.
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“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.”
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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.
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“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.”
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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 [21]. 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 [15].

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.
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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.”[22]

“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.”[23]

“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.”[24]

“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.”[24]

“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.”[25]

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.'”[26] “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?'"”[27]

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 [28].

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 [14].
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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 grey 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 [29] and Corn Gluten Meal [30].

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.

Sincerely,

Mary Straus

Researcher for the article in the Whole Dog Journal

 
References

 
1. Nutrition and Chronic Renal Failure from the Nutrition support service at The Ohio State University Veterinary Hospital.

 
2. Veterinary Diet Manual Home Page, Nutrition support service at The Ohio State University Veterinary Hospital.

 
3. Prescription Diet® Canine k/d® http://www.hillspet.com/zSkin_2/products/product_details.jsp?PRODUCT%3C%3Eprd_id=845524441760642&FOLDER%3C%3Efolder_id=2534374302037389&bmUID=1119116913620

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4. Chronic Kidney Disease, Merck Veterinary Manual
http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/130603.htm

 
5. Allen et al Compendium of Continuing Education, July 1990

 
6. Manson MM, Green JA, Driver HE. Ethoxyquin alone induces preneoplastic changes in rat kidney whilst preventing induction of such lesions in liver by aflatoxin B1. Carcinogenesis. 1987 May;8(5):723-8.

 
7. Burkholder WJ, Lees GE, LeBlanc AK, et al. Diet modulates proteinuria in heterozygous female dogs with X-linked hereditary nephropathy. J Vet Intern Med. 2004 Mar-Apr;18(2):165-75.

 
8. Valli VE, Baumal R, Thorner P, et al. Dietary modification reduces splitting of glomerular basement membranes and delays death due to renal failure in canine X-linked hereditary nephritis. Lab Invest. 1991 Jul;65(1):67-73.

 
9. Melissa S. Wallace, DVM, Dipl. ACVIM Strategies for Protein Losing Nephropathy. Atlantic Coast Veterinary Conference 2001

 
10. Hoskins, J. Glomerulonephritis remains an important cause of renal disease in dogs. DVM Newsmagazine Mar 1, 2004.

 
11. Vaden, S. Description of PLE/PLN: Protein Losing Enteropathy/Nephropathy in Soft Coated Wheaten Terriers. NC State University College of Veterinary Medicine.

 
12. Gregory F. Grauer, DVM, MS, Diplomate, ACVIM (Internal Medicine) Proteinuria and the Progression of Chronic Renal Disease in the Dog. 27th WSAVA Conference

 
13. Andrea J. Fascetti and Sean J. Delaney  Nutritional Management of Chronic Renal Disease

 
14. Summary of Experiments on Dogs With Reduced Renal Mass That Examined Renal Effects of Diet.

 
15. Chronic Renal Disease and Failure (CRD, CRF). Washington State University College of Veterinary Medicine

 
16. Dietary Management of Renal and Cardiac Disease. Virginia-Maryland Regional College of Veterinary Medicine

 
17. USDA National Nutrient Database and GreenTripe.com

 
18. Kronfeld DS. Dietary management of renal senescence and failure in dogs. Aust Vet J. 1994 Oct;71(10):328-31.

 
19. Davis, B. C. and P. M. Kris-Etherton (2003). Achieving optimal essential fatty acid status in vegetarians: current knowledge and practical implications. Am J Clin Nutr 78(3 Suppl): 640S-646S.

 
20. Gerster H. Can adults adequately convert alpha-linolenic acid (18:3n-3) to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)? Int J Vitam Nutr Res. 1998;68(3):159-73.

 
21. “In dogs, a recent study published in the Journal of Nutrition showed that supplementation with ALA did not raise blood DHA levels.” Quoted by Susan G. Wynn, DVM

 
22. Dietary Management of Chronic Renal Failure.

 
23. Dietary Management of Chronic Polyuric Renal Failure.

 
24. D.P. Laflamme, DVM, PhD, DACVN, S.S. Hannah, PhD and R.D. Kealy, PhD, Geriatric Nutrition: Protein

http://www.purina.ca/images/articles/pdf/GeriatricNutritionProtein.pdf

25. Dr. Lucy L. Pinkston, D.V.M. Introduction to the Urinary System: Anatomy and Function.

 
26. Delmar R. Finco, D.V.M., Ph.D., professor of veterinary medicine at the University of Georgia. Proc the Waltham/OSU Symposium on Nephrology and Urology, Columbus, OH. Oct. 1992, p. 39.

http://www.purina.com/breeders/magazine.asp?article=256
http://www.freshpetfood.ca/tips.htm#DEMYSTIFYING_MYTHS_ABOUT_PROTEIN

27. Churchill J, Polzin D, Osborne C, et. al. Proceedings ACVM. 1997:675.

http://www.purina.ca/dogs/puppies.asp?article=471

28. Finco DR, Brown SA, Crowell WA, Brown CA, Barsanti JA, Carey DP, Hirakawa DA. Effects of aging and dietary protein intake on uninephrectomized geriatric dogs. Am J Vet Res. 1994 Sep;55(9):1282-90.

 
29. The Composition of Rice and Various Processed Types

 
30. Dr. Nick Christians, Corn Gluten Meal Research Site