Despite scientific advances that have increased the nutritional knowledge about the importance of protein in a dog's diet, there still are myths that feeding too much protein or the wrong kind of protein can be harmful.
Among breeders' key concerns about protein are: Does too much protein cause kidney failure in older dogs? Is it true that feeding large-breed puppies too much protein causes developmental bone problems? Can soy protein cause bloat, a potentially fatal illness, when fed to dogs?
The answers to these questions are no. In fact, research shows there is more reason to be concerned about feeding healthy dogs an inadequate amount of dietary protein.
Here, we present facts about protein to help you determine the best amount to include in your dog's diet for a healthy and long life.
The Geriatric Dog
Nutritional research has shown that healthy older dogs need more protein than young adult dogs - as much as 50 percent more. The additional protein is required to maintain a geriatric dog's protein reserves and support protein turnover, which are important in helping the dog's immune system function at its full capacity. Dogs that do not receive adequate protein are more susceptible to stress, such as injury or infection.
Dottie Laflamme, D.V.M., Ph.D., a Ralston Research Fellow, says, "It is important to provide older dogs with enough protein to help them fight the stress of aging, including injury and infection. Dogs fed inadequate amounts of protein may appear healthy, but may be less able to resist infection or fight off other diseases."
Despite this research, there has been a longstanding concern that excessive protein in diets for older dogs may cause kidney damage. The link is traced to clinical signs in dogs with kidney failure that relate to a buildup of byproducts from protein metabolism. However, research conducted during the past decade has shown that protein does not harm the kidneys.
Research first supporting the link between excessive protein and kidney disease was conducted in the 1920s, showing that male rodents exhibited progressive renal disease when fed a high-protein diet. 1 Another study on rodents, conducted in 1982 by B.M. Brenner, also showed that excess dietary protein caused kidney damage. 2 Though these studies were correct in rats, the results unfortunately were extrapolated to other species.
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. 3
Delmar R. Finco, D.V.M., Ph.D., professor of veterinary medicine at the University of Georgia, says, "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."
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. 4 The report confirmed that unless a dog has clinical evidence of kidney disease or other problems for which protein restriction may be beneficial, there is no reason to recommend a change to a low-protein diet.
Research on dogs at other universities and at the Purina Pet Care Center has generated similar results. 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. 5
"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?'"
Similarly, preliminary findings from the Purina Pet Care Center indicate that healthy geriatric dogs fed 45 percent dietary protein have maintained health and body condition, with no evidence of increased kidney damage due to protein intake. The evidence supports other recent research that protein at any level consistent with complete and balanced nutrition has no adverse effect on the kidneys of normal, healthy dogs.
It is known that as dogs age they become less efficient in metabolizing protein than young dogs so that older dogs require more protein than young adult dogs to fully replenish their protein reserves and maintain protein turnover. The specific amount of protein needed, as a percent of diet, depends on several factors.
"Geriatric dogs should be treated individually," Churchill says. "I think it's important to conduct a good health examination and blood biochemical profile by the time a dog is 7 years old in order to evaluate organ functioning and determine the best diet for that individual dog."
Similar to how the nutritional needs of dogs change with their age and lifestyle, their energy requirements tend to decrease with age. However, not all geriatric animals are less active or overweight. In fact, a greater proportion of older dogs are underweight than any other age group.
Many inactive older dogs need fewer calories, so it is important that they consume less food or a lower calorie food than they ate when they were younger. For this reason, many foods for older dogs are formulated to contain fewer calories, but it is important that they receive adequate intake of protein and other nutrients while reducing calorie intake.
The quality of the protein in dog food also is important. It must contain the right balance of amino acids and be digestible. This usually is accomplished by using two or more complementary proteins to achieve an optimum balance. For example, soybean meal and corn protein complement each other because the amino acids that are deficient in one are present in the other. Protein digestibility is achieved by the selection of ingredients and the processing. Processing that becomes too hot can destroy the amino acid content and lessens digestibility.
Dietary protein requirements are much higher for growing puppies than for fully grown dogs. In addition to supplying the protein needed to support protein turnover and normal cellular metabolism, protein is needed to build growing muscles and other tissues.
Research at the Purina Pet Care Center and at other facilities has shown that puppies fed inadequate protein do not grow as well and are more susceptible to health problems than those fed nutritionally complete diets. At the Pet Care Center, English setter puppies that were fed a low-protein diet showed stunted growth compared to puppies fed higher levels of protein. However, when the protein level was increased in the puppies at the Pet Care Center, the deficiency was corrected.
Concern about protein causing developmental bone problems in large-breed puppies has led some breeders to reduce the amount of protein they feed. However, in research published in 1993 based on studies of Great Dane puppies at Utrecht University in the Netherlands, it was shown that dietary protein does not contribute to these problems. 6
Herman A. Hazewinkel, D.V.M., Ph.D., professor of veterinary medicine at Utrecht University, led the research that found no detrimental effects from protein levels up to 32 percent of the diet. However, puppies fed a diet of only 15 percent protein showed evidence of inadequate protein intake.
"Too low protein decreases the growth rate of puppies and also their immunological response," Hazewinkel says. "This is true for large- and small-breed puppies. An adequate protein level should be higher than 15 percent."
This study, conducted in young Great Danes during their first half-year of life, concluded that dietary protein increased to 32 percent does not negatively affect skeletal or cartilage development in these dogs. The research also confirmed that dietary protein did not have detrimental effects on liver and kidney functioning.
Soy protein in dry dog food has been targeted as a potential cause of bloat because soy can sometimes cause flatulence, or gas, in dogs. Bloat, a potentially fatal disease in which swelling of the stomach compresses important body organs and blood vessels, most often occurs in large, deep-chested dogs.
Nutritionists at the College of Veterinary Medicine at Cornell University looked at whether ingredients, such as soy protein, could cause bloat. They reported no evidence that either soybean meal or any dry expanded dog food contributes to bloat. Rather, they found that dogs consuming meat-based diets are just as likely as dogs consuming soy-based diets to develop the condition that leads to bloat. Other research has shown that the gas associated with bloat actually is swallowed air rather than gas caused by fermentation of soy or other dietary components. 7
Dogs who develop bloat often are called "greedy eaters" because they scarf down their food. They also tend to "gulp" their water. Canine bloat is a complex disease. While many associated factors have been suggested, no causes have been proven. Further research is needed to identify the causative factors of bloat and to recommend proper preventive or curative measures. Despite the myths, soy-based diets are not a cause of bloat.
No single source of dietary protein is perfect. Neither meat nor soybean meal is an ideal protein; however, they both can be an excellent source if fed in combination with another source of amino acids, the substances that make up protein. As noted earlier, a combination of soybean meal and corn protein is effective because they complement each other, and the amino acids that are deficient in one are present in the other.
While the protein digestibility of pet food varies according to the quality of the protein source used and the processing, the protein in balanced dog foods made with soybean meal can be more digestible than those made with some meat or poultry meals.
Nutritional research also shows that soy contains a group of chemicals called isoflavones that are natural antioxidants and help protect cells from oxygen damage. Oxidation causes changes that are thought to lead to cancer and other diseases.
The key point is that soybeans provide an economic source of good quality protein and other nutrients. When properly processed, soy can be an excellent part of a complete dog diet.
1 Newburg LH, Curtis AC. Production of renal injury in the white rat by the protein of the diet. Arch Int Med. 1928; 42:801-21.
2 Brenner BM, Meyer TW, Hostetter TH. New England J. of Medicine. 1982; 307:652.
3 Finco DR. Proc the Waltham/OSU Symposium on Nephrology and Urology, Columbus, OH. Oct. 1992, p. 39.
4 Kronfeld DS. Aust. Vet. J. 1994; 71:328.
5 Churchill J, Polzin D, Osborne C, Tet. al. Proceedings ACVM. 1997:675.
6 Nap RC, Hazewinkel HAW, Vorhout G, Biewenga WJ, Koeman JP, Goedegebuure SA, van't Klooster A Th. The influence of the dietary protein content on growth in Giant breed dogs. Journal of Veterinary and Comparative Orthopaedics and Traumatology. 1993; 6:1-8.
7 Client Information Series: Gastric Dilatation Complex in the Dog. Canine Practice. 1994; 19:1.