Fresh Diets for Pets: Are they Healthier than Kibble or Canned?

Nutrition is one of the most frequent subjects my clients want to discuss, and often one of the most controversial. Raw diets,1 vegetarian or vegan pet foods,2 ketogenic diets for pets,3 the significance of “artificial” versus “natural” pet food ingredients,4 and the fundamental question of whether veterinarians are trusted counselors when it comes to nutrition are just some of the controversies in pet nutrition I have covered in Veterinary Practice News. 

These are all tough issues, both because of the passionate opinions they engender and because of the inevitably limited and imperfect scientific evidence available to adjudicate them. A key tenet of evidence-based medicine, however, is that we have to make judgments based on the evidence we have, not the evidence we wish we had. Another core principle is that our confidence in any judgment we make should only be as strong as the evidence allows. 

Today’s topic is one where claims and passions far exceed the available evidence—fresh pet food. Various terms are used to describe such diets, including fresh, lightly-cooked, whole-food, etc., and there is no standardized terminology for these diets. I will mostly use “fresh food” as a shorthand for the myriad diets marketed in this way.

In addition to homemade fresh diets prepared by individual dog owners, a number of companies are now selling cooked commercial diets that are designed and packaged like fresh, homemade foods rather than extruded kibble or traditional canned pet food. These companies market such diets with implicit, or often explicit, claims that they are healthier than traditional commercial foods. 

In an extreme example, the founders of Just Food for Dogs (JFFD) have written a polemical book titled “Big Kibble: The Hidden Dangers of the Pet Food Industry” to promote their alternative to traditional commercial diets. They have not been restrained or respectful in their response to criticism of their claims and marketing methods. The company explicitly claims their product is healthier that traditional kibble, and the leaders are not impressed by calls for evidence to prove this: “The mainstream veterinarian needs research and proof that real food is healthier, and that just boggles my mind,” Dr. Chavez adds. “We’re the last healthcare profession that is recommending an ultra-processed daily sustenance. It’s just crazy.”

So, is it crazy to wonder if fresh foods really are healthier than canned or kibble? Can we assume that dogs eating traditional commercial diets will have shorter lives and more health problems than dogs eating fresh diets? Regular readers of this column will already know my answer—nope! Equally “obvious” claims about the complicated relationship between environmental factors and health outcomes have been stunningly wrong many times in the history of human and veterinary medicine, and we should place very little confidence such beliefs without scientific evidence.

The ideal evidence for these claims, of course, would be long-term comparative feeding studies showing dogs eating fresh diets live longer and experience less disease than those eating kibble or canned foods. Such studies would be extremely complex and expensive to run, and I don’t see much chance companies on either side of the debate will step up to support them. This means that, as usual, we need to rely on less robust evidence (and proportion our confidence accordingly).

There is certainly epidemiologic evidence that consumption of whole foods, particularly fruits and vegetables, is associated with improved health outcomes in people compared with packaged and convenience foods. But it needs to be emphasized that commercial dog food is not the nutritional equivalent of potato chips just because both come in bags. Human snack and convenience foods are deliberately designed to be appealing, not nutritious. Pet foods are formulated with much more emphasis on nutritional value, and have been used and evaluated extensively for decades for their impact on health. They may well not be the optimal food we should be feeding, but they are hardly the egregious poison their detractors claim. And as the analogy breaks down, so does the relevance of the epidemiologic evidence in humans to pet feeding practices.

There is little direct research on the potential health impact of fresh diets compared with other cooked pet foods. There is research showing that homemade diets are often nutritionally unbalanced and incomplete, but little evidence pertaining to commercial cooked fresh diets.5–10

A small study was reported as a poster at the 2014 American Academy of Veterinary Nutrition Research Symposium, and this is often cited by fresh food advocates as positive evidence for their claims. Twenty-one dogs of various breeds were transitioned from kibble to a frozen cooked fresh-food diet and basic bloodwork and exams were conducted at the beginning of feeding the diet and again at 6 months and twelve months later. This was a pilot study, so there was no control group, no blinding, no pre-specified outcomes or hypotheses, no reported accounting for repeated measures or multiple comparisons in the statistical analysis, no discussion of any other aspects of the dogs’ health or environment, and overall no significant control for bias or random error. This limitation is especially relevant given that the lead author is Chief Medical Officer for JFFD and also an author of the book I mentioned earlier.

A few differences were found in some clinical laboratory measures before and after the transition to the JFFD diet. Increases were seen in red blood cell count and globulins, for example, though all values remained within reference intervals for all dogs. This sort of data might suggest hypotheses for future testing, but it doesn’t support any specific conclusions about the relative merits or health effects of different types of diet. It certainly does not support the claims in a JFFD press release that their foods “could benefit immune health” and that if the purported trends in the blood values continue for a lifetime “we may see a decrease in chronic diseases such as cancer, renal failure, kidney disease, inflammatory bowel disease, dental disease, etc.” 

There is some laboratory research showing fresh diets have higher digestibility than extruded pet foods, and there may be effects on gut flora and other physiologic parameters.11–14 However, these are, once again, only useful bits of data that suggest testable hypotheses, not conclusive evidence for real-world health effects.  

Personally, I am sympathetic to the hypothesis that pet diet which contain less processed whole ingredients may be superior to conventional canned or extruded dry diets in terms of health outcomes. The epidemiologic evidence in humans, and pre-clinical research in laboratory animal models is suggestive, though by no means conclusive. There are, of course, other issues besides health impacts that must be considered in comparing the merits of different types of pet food. The affordability and accessibility of different diets, storage and stability, safety, environmental sustainability, and many other factors are relevant as well to the recommendations of veterinarians and the feeding choices of dog owners.

The bottom line, as always, is that we should make decisions based on the best possible evidence, and we should limit our claims and confidence to what the evidence can support. Currently, the most optimistic assessment of diets identified by marketing materials as fresh, lightly-cooked, whole-food, human-grade, etc. is that it is plausible they may have health benefits if properly formulated by veterinary nutritionists and properly handled and fed by owners. Biologic plausibility and pre-clinical evidence are necessary starting points, but evidence from the real world on meaningful health outcomes will be needed before we can have any confidence in claims about the benefits of such diets.

References

1.        McKenzie BA. Debating Raw Diets. Vet Pract News. January 2019:30-31.

2.        McKenzie B. Are Vegan of Vegetarian Diets Good for Pets? Vet Pract News. July 2019:26-27.

3.        McKenzie BA. Is Keto Kind to Pets? Vet Pract News. January 2020:30-31.

4.        McKenzie B. Is banning “artificial” ingredients based on fear or science? Vet Pract News. March 2019:36-37.

5.        Lauten S, Smith T, Kirk C. Computer analysis of nutrient sufficiency of published home-cooked diets for dogs and cats [abstract]. J Vet Intern Med. 2005;19(3):476-477.

6.        Heinze CR, Gomez FC, Freeman LM. Assessment of commercial diets and recipes for home-prepared diets recommended for dogs with cancer. J Am Vet Med Assoc. 2012;241(11):1453-1460. doi:10.2460/javma.241.11.1453

7.        Larsen JA, Parks EM, Heinze CR, Fascetti AJ. Evaluation of recipes for home-prepared diets for dogs and cats with chronic kidney disease. J Am Vet Med Assoc. 2012;240(5):532-538. doi:10.2460/javma.240.5.532

8.        Taylor MB, Geiger DA, Saker KE, Larson MM. Diffuse osteopenia and myelopathy in a puppy fed a diet composed of an organic premix and raw ground beef. J Am Vet Med Assoc. 2009;234(8):1041-1048. doi:10.2460/javma.234.8.1041

9.        ROUDEBUSH P, COWELL CS. Results of a Hypoallergenic Diet Survey of Veterinarians in North America with a Nutritional Evaluation of Homemade Diet Prescriptions. Vet Dermatol. 1992;3(1):23-28. doi:10.1111/j.1365-3164.1992.tb00139.x

10.      Pedrinelli V, Gomes M de OS, Carciofi AC. Analysis of recipes of home-prepared diets for dogs and cats published in Portuguese. J Nutr Sci. 2017;6:e33. doi:10.1017/jns.2017.31

11.      Oba PM, Utterback PL, Parsons CM, Swanson KS. True nutrient and amino acid digestibility of dog foods made with human-grade ingredients using the precision-fed cecectomized rooster assay1. Transl Anim Sci. 2020;4(1):442-451. doi:10.1093/tas/txz175

12.      Do S, Phungviwatnikul T, de Godoy MRC, Swanson KS. Nutrient digestibility and fecal characteristics, microbiota, and metabolites in dogs fed human-grade foods. J Anim Sci. 2021;99(2). doi:10.1093/jas/skab028

13.      Tanprasertsuk J, Perry LM, Tate DE, Honaker RW, Shmalberg J. Apparent total tract nutrient digestibility and metabolizable energy estimation in commercial fresh and extruded dry kibble dog foods. Transl Anim Sci. 2021;5(3). doi:10.1093/tas/txab071

14.      Buff PR, Carter RA, Bauer JE, Kersey JH. Natural pet food: A review of natural diets and their impact on canine and feline physiology. J Anim Sci. 2014;92(9):3781-3791. doi:10.2527/jas.2014-7789

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7 Responses to Fresh Diets for Pets: Are they Healthier than Kibble or Canned?

  1. Sofia says:

    It absolutely makes sense to base things on evidence but like you mentioned, we will probably NEVER have the type of studies we need to really know if fresh food is better than kibble.

    But we do have studies on other things like the fact that dogs ingest over a hundred times the amount of maillard reaction products than humans, which can have toxic and carcinogenic effects. And the fact that dogs on kibble and canned diets have extremely high levels of glyphosate in their urine (compared to none in dogs fed raw diets). We also know facts about the digestibility differences and microbiome effects of fresh vs kibble.

    While we don’t have the absolute *best* evidence at this time, should we not be at least extrapolating the data that we do have? Or do we ignore these findings and say “eh, it’s fine”?

    If we never question anything, we will never progress. And if we wait around for the solid evidence and refuse to extrapolate, well then we’re going to stay right where we are.

  2. skeptvet says:

    You are correct that we have to make decisions on the basis of the evidence we have, even if it is imperfect. However, we should also proportion our confidence in our conclusions to the strength of the evidence. When the evidence is weak, then we cannot make strong, confident claims, and we have to be ready to change our conclusions as the evidence changes.

    Most of the concerns about conventional diets and the proposed benefits for fresh or raw diets are based on indirect evidence- rodent studies, pre-clinical pilot studies, nothing that directly demonstrates health effects, positive or negative. This level of evidence is reasonable for driving further research, but it is a shaky foundation for radical changes in feeding practices.

    As far as some of the specifics you mention:
    a. Maillard products or AGEs- These exist in all cooked food, with levels varying depending on the details of each diet and its processes. The evidence for toxic effects comes from rodent studies, and there are no direct data showing negative health effects in dogs from exposure in cooked foods. This doesn’t mean we shouldn’t investigate or be concerned, but it does mean we shouldn’t recommend a radical change from diets with decades of experience and research evidence behind them to diets with mostly anecdote and very sparse, preliminary research evidence to support them.
    b. glyphosphate- most evidence suggests very low toxicity potential in mammals, and no evidence at all for health risks to dogs from consumption of commercial diets. This is popular bogeyman right now, but the research is more reassuring than worrying.
    c. digestibility- there are a few studies showing differences between cooked and uncooked diets, and cooking increases digestibility of some ingredients and reduces it for others. This only matters if the differences in digestibility lead to differences in calorie or nutrient adequacy, and this has not been shown to be the case. The fact that conventional diets almost certainly meet nutritional and caloric needs argues that this is not a difference with meaningful implications for health.
    d. microbiome differences- again, a few studies show differences in the types of flora in dogs eating different diets. We know almost nothing about what this means or what health implications this has, if any.

    I’m all for questioning things, but people who are concerned about conventional diets rarely turn the same level of critical scrutiny on claims for alternatives. When you do, you find that the evidence for these is much less robust, so there is a long way to go before we should be confidently saying that such diets have health benefits

  3. Bryan says:

    Thanks for covering this. I’m now more curious to understand what it would take to see a at least one strong, comparative feeding study take place. Is it mostly cost that has prevented this from happening? Is anything else preventing it?

    And maybe a better question… do you have a favorite feeding study or one that comes to mind when you think about quality feeding studies? It could be any, not necessarily involving fresh diets, because as you’ve said there aren’t many. I think it would be interesting to look at the history of high quality feeding studies and how they got there.

  4. skeptvet says:

    Great question! I suspect cost is a big factor. However, the reality is that it is easy to sell diets with marketing not based on scientific evidence because consumers don’t demand this, so there is no incentive to do such studies when it won’t increase sales. Grain-free diets, for example, have no proven benefits and possibly some risks, and yet they went from uncommon to 40% or more of the diets on the market entirely without any meaningful scientific evidence.

    Nutrition is also complex. If you were, for example, to compare a fresh-cooked diet and a kibble, there would be dozens, if not hundreds of differences between them besides the degree of cooking/processing. How would you decide what factors were the critical ones responsible for any health differences? Even simple things like comparing the carbohydrate content of two diets can be really tricky since a low-carb diet is, by definition, either a high-protein or a high-fat diet, and which factor or what interaction between them gets the credit/blame for any differences seen in a study? So another barrier is that you can design a good study and spend a lot of time and money running it and still not have a definitive conclusion that will be persuasive to most people.

    There have been some great feeding trials. Purina did a caloric restriction study in a group of labs that lasted for 14 years and generated incredibly valuable evidence about possible methods to reduce age-related disease and extend lifespan. I’m not sure what factors motivated them to do this, but it didn’t have any obvious or direct impact on revenue.

    My guess is that the improvement in our knowledge about nutrition will come mostly from associational, epidemiological studies than from controlld clinical trials, even though these cannot proven causation. The Golden Retriever Lifetime Study, for example, will certainly collect information about diets, and some associations may be seen that can give some insight into dietary factors and health, though there will be lots of caveats and limitations to any conclusions.

    In my fantasy world, all pet food companies over a certain size (TBD) would contribute to a communal research fund to be administered by an independent committee and taking grant applications fro nutritional research projects from academics not affiliated with specific companies. That would be great way to stimulate more and better studies. But it doesn’t seem likely to happen.

  5. JCM says:

    Dr. Skeptvet you have the patience of a saint.

    Any idea where this sudden influx of hysteria about Maillard reaction came from? It’s being brought up on the regular by raw food acolytes. (Meaning, what holistic magazine/doggy “wellness” web site started this rumor that Maillard reaction = instant cancer? I have a guess, but wondering if you know for sure.)

  6. skeptvet says:

    It’s a great question, and I don’t know the answer. The reaction is ubiquitous in cooking, so why it is such a prominent villain when discussing commercial pet food is unclear.

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