Raw, Cooked, and Dry Cat Diets–A New Study Examined

The debate about raw diets for pets is an ongoing “hot button” topic in veterinary circles. There is also a less heated but still vigorous debate about cat nutrition in particular and what form and composition constitutes the optimal diet for our pet cats. I have previously written about raw diets extensively, and I still hold the opinion that there is little reason to believe they are superior to cooked diets, including commercial pet foods (1, 2, 3, 4, 5, 6, 7).

The theoretical arguments advanced for using these diets are weak and often simply a form of the appeal to nature fallacy. “Natural” is not synonymous with “optimal,” so even for cats, which are obligate carnivores and often still actively predatory, actual experimental evidence is needed to justify the claim that the natural diet of raw, live small prey, or other raw diets promoted as equivalent, is better than the alternatives.  This is even more true for dogs, who have been so deeply altered by breeding and association with humans from their wild ancestors that it is a stretch even to claim there is a “natural” diet that domestic dogs should be eating.

While the benefits of raw diets remain unproven, there are legitimate concerns about risks, including infectious disease, parasites, and potential dangerous from specific components of some diets, such as bones. These risks do not yet appear dramatic, but they need to be weighed against benefits which can be substantiated by real research, which does not yet exist.

One recent study has compared a commercial kibble with both cooked and raw alternatives in a few cats, and some raw diet advocates are suggesting this is evidence in favor of the benefits of raw diets. Having looked at the study, I think it is an interesting beginning in terms of comparing different feeding options, but I am not convinced it provides significant evidence to support most of the claims made for raw pet diets.

K.R. Kerr, B.M. Vester Boler, C.L. Morris, K.J. Liu, and K.S. Swanson. 2012. Apparent total tract energy and macronutrient digestibility and fecal fermentative end-product concentrations of domestic cats fed extruded, raw beef-based, and cooked beef-based diets. Journal of Animal Science: 90: 515-522.

The study was nicely designed, though this also meant labor intensive, which likely contributed to the small sample size. Nine cats were each fed three different diets in rotation; a beef-based raw diet, the same diet cooked in a microwave to the standards for food safety recommended by the USDA, and a high-protein commercial dry diet. The cats ate each diet for 21 days. The last 4 days of each period, all feces and urine was collected for analysis, and on the last day a blood sample was taken.

The nutritional composition of the three diets was complete and roughly equivalent, though the ingredients of the commercial kibble were significantly different from the ingredients of the other two diets.  And of course the moisture content of the dry diet was significantly less than that of the others.

The only difference among the diets in terms of the bloodwork was a higher creatinine (a measure of kidney function, hydration, and protein metabolism) and triglyceride (a fat) in cats eating the cooked and raw fresh diets than in those eating the commercial dry diet. Though these values differed between the diets, they were within the normal range for all cats. 

Interestingly, there were no differences in the urinalysis values between any of the diets, and all the cats had very high urine specific gravity measurements (>1.064). One of the arguments for feeding fresh or canned diets is that the increased fluid content should improve hydration and reduce urine specific gravity, which is hypothesized to be useful in preventing and managing kidney disease. However, despite having water available at all times, the urine specific gravity was extremely high in all of these cats, and there was no difference between the kibble and the fresh diets.

Any interpretation of these numbers is speculative given the small size of the study, but one would expect the fresh diets to lead to lower renal bloodwork values and lower urine specific gravity than the kibble, particularly if they are to have any protective benefits against kidney disease. This data seems to undermine this hypothesis.

The amount the cats were fed was adjusted regularly to maintain an ideal body condition, so no differences in the effect of diet on body condition could be evaluated. Cats on the dry diet did eat more, in terms of volume and calories, than cats on the other two diets (which were not different from each other). This agrees with the finding that the dry diet appeared to be less digestible than the other two diets (which again did not differ from each other). 

The various measures of digestibility showed the dry diet to be between 4.2% and 11.7% less digestible than the cooked and raw fresh diets. However, as the authors state, “All diets tested in this experiment were highly digestible,” so the real-world significance of this isn’t clear. Presumably, a cat would have to eat a greater quantity of a less digestible diet and would produce more stool, but it is unclear how great a difference this might be or whether it would have any health implications.

Stool quality is often a marketing point for raw diet advocates. In this study, stool quality was evaluated on a 5-point scale:

1 = hard, dry pellets
2 = dry, well-formed stools
3 = soft, moist, formed stool
4 = soft, unformed stool
5 = watery liquid that can be poured

Cats on the dry diet had a higher score (average =3.3) compared to the other diets (which did not differ significantly at averages of 2.8 and 2.9). The ideal score is 3.0, so all of the diets generated a normal fecal consistency. Oddly, the dry diet appeared to lead to moister stools than the higher moisture content diets, which is the opposite of what one might expect.

A number of fecal compounds were measured, but the only conclusion drawn from the differences seemed to be again that the dry diet was less digestible and so led to greater colonic bacterial metabolism than the other two diets. However, the measurements were consistent with normal values seen in healthy cats. The health implications of this are unclear.

So what conclusions did the authors draw from these data?

Although the raw and cooked beef-based diets were more digestible than [the dry diet], all diets were highly digestible in this experiment.

Few differences in serum metabolites were detected

Urine variables did not differ among diets.

All scores of fecal consistency were within a desirable range, but cats fed [the dry diet] had greater scores

Carbohydrate fermentation was similar for all diets.

Fecal putrefactive compounds…were increased in cats fed [the dry diet] but were similar to values reported in the literature for healthy cats.

Because cooking may minimize risk of microbial contamination, and the results from the cooked beef-based diet tested herein were not different than the raw diet, cooking may be an appropriate modification to this feeding strategy

And, finally, the authors argued that more research is needed, which is certainly the case.

Overall, this study is interesting but doesn’t seem to offer much support to the claims made for raw, or even fresh high-moisture diets. No difference of any kind was reported between the raw and the cooked diet, and the vast majority of the measurements made did not differ between these and the dry kibble. And even the high moisture diets led to a high urine specific gravity, which would suggest less than optimal fluid intake.

The dry was slightly less digestible than the fresh diets, but all were easily and thoroughly digested. But as the authors state, “Because the ingredient composition of [the dry diet] was different than that of [the other diets], we acknowledge that the influence of the dietary composition and extrusion cannot be separated.” So it is not even clear if the difference found has to do with the form of the diet or simply the particular ingredients used. Even less clear is whether these differences have any implications for health or disease.

I certainly support further research into different pet feeding methods. My personal guess is that raw will not prove to have any benefits sufficient to outweigh the risks, though I do think feeding moist diets may turn out to be better for cats than feeding dry diets. Homemade diets may have some advantages, but these are probably outweighed by the persistent problems with nutritional adequacy and quality control unless a nutritionist is involved in formulating and supervising them.

I look forward to studies which either confirm these guesses or prove me completely wrong. However, until there is adequate research, these are simply guesses, not facts. The biggest concern I have about raw diet advocates is that they tend to make grand health claims far beyond anything supported by evidence. I wouldn’t be surprised if this paper is held up as evidence for those claims, but a close reading of it does not support that interpretation.

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18 Responses to Raw, Cooked, and Dry Cat Diets–A New Study Examined

  1. Rita says:

    Interesting results, if only because, like so much literature and studies on food for humans, all ethical issues are left aside. Keeping obligate carnivores as pets, sourcing their food (which must, if humans are doing the feeding, be derived from defenceless victim populations of domesticised nonhumans), environmental effects of animal agriculture and so on do not enter into the calculations. Of course, the study was not aimed at answering these questions, but isn’t it about time that such studies started factoring in ethics?

  2. skeptvet says:

    Well, I agree that the ethical issues are worh considering. I’m a vegetarian largely on ethical grounds. Still, I don’t think scientific studies of nutrition are the way to approach those kinds of issues. The question of what is the optimal diet for pet cats from a biological point of view is a separate question from what we might decide to feed them based on ethical considerations involving other animals used as food for cats and the environmental issues. When people try to use scientific data to support ethical claims, I think they are running off the rails and into trouble (think about the eugenics movement, social Darwinism, and so on). Values ought to be influenced by facts (though, sadly, they often aren’t), but values aren’t facts themselves and are ultimately determined by feelings as much as beliefs, so the most science can contribute to debates about values is as accurate an understanding of the facts about the context as possible.

    How one brings those issues into discussion s a question I don’t have a ready answer for. Given the current cultural climate, I can’t imagine most cat owners would consider it a serious question, so I wouldn’t expect a very receptive audience. Of course, I feel the same way about rationalism and skepticism in medicine, yet I keep plugging away here anyway. 🙂

  3. Rita says:

    Yes, I take your point, using science to “prove” ethics is just dressing up the naturalistic fallacy in a white lab coat – I’m really asking just ” how one brings those issues into discussion” – after all, it’s science that tells us about climate change etc: perhaps a government environmental warning on tins, like on cigarette packets? 😉

  4. Aleja says:

    catinfo.org ’nuff said.

  5. Kenneth says:

    It’s interesting that you note the nutritional composition of the food was roughly equivalent between the three (albeit small) groups. This seems to support what I’ve heard time and again: nutrition is the all-important factor. It’s a common pitfall of the raw diets that are becomingly more popular among us homo sapiens.

    Obviously we want to feed our pets food that is not going to result in obesity (which I hear dry food has a slightly higher risk of obesity in pets, likely in part because it is more likely to be free-fed instead of schedule-fed) but will be nutritionally-complete for good health and longevity. We also don’t want to feed our pets something that is known to be a risk to their internal organs, such as bones posing a risk of perforations. Nutritionally-complete and safe to eat, basically.

    So my take-away from this article is that so long as the diet you are feeding your pet is nutritionally-adequate and safe on the digestive system, then there’s not really much difference between the ones tested, meaning that nutrition is and remains the primary concern. Is that about right, or am I completely off the mark?

    Also my wife and i have been considering switching our two cats over to a wet or mixed wet/dry diet — in part to get the huskier one on a schedule that I hope will result in weight loss — and what you’ve said here I think will influence my decision, though I believe a wet diet isn’t as friendly on the pocketbook.

  6. skeptvet says:

    You’re absoultely correct that nutrition is, in general, more important than specific ingredients. People tend to look at the ingredient list and make judgements about the nutritional value of the food, but that is not a useful way of evaluating diets. So I think you’re also correct that diets which meet the basic standards of nutritional adequacy and have minimal risk are roughly equivalent. Of course, we don’t know what the optimal level of many nutrients is (as opposed to the minium necessary to avoid deficiency), so there may still be meanigful differences between diets even if they all meet the minimum standards. But claims that one diet is superior to another need to be based on real testing, not just claims like “Nutrient X is good for you and our foood has more sdo it is better,” and such.

    As far as canned food, there is some plausible reason to think that canned food may be better for cats in terms of fluid content (higher)) and carbohydrate content (lower) copared to dry. However, I was surprised that in this study the high moisture foods didn’t lead to any lower a urine specific gravity than the kibble, so that questions isn’t totally solved yet.

    As for weight managemen, the absolute most important factor above any other is calorie intake. Cats are designed to do little and eat little, and they utilize calories very efficiently. They become overweight due to excessive calories, not problems wit the composition of diets. The most recent consensus statement on feline diabetes and obeisity, for example, had these conclusions:
    http://skeptvet.com/Blog/2011/06/dietary-carbohydrates-and-cats-its-the-calories/

    1. Are dietary carbohydrates an essential or required nutrient for cats?
    Answer- No. Based on a good quality and quantity of evidence, most cats do not require dietary carbohydrates. There are some simple sugars in feline milk, so it is possible that nursing kittens may require these but no clear deficiency has been demonstrated.

    2. Can cats effectively utilize dietary carbohydrates for energy and nutrition?
    Answer- Yes. Based on a good quality and quantity of evidence, cats can effectively digest, absorb, and utilize dietary carbohydrates.

    3. Do dietary carbohydrates in the diet cause obesity?
    Answer- No. The cause of obesity in almost all cats is excessive calorie intake irrespective of whether the calories come from protein, fat, or carbohydrate. In fact, low carbohydrate foods may be more likely to lead to obesity if they are higher in fat than regular diets.

    4. Do dietary carbohydrates contribute to the development of diabetes?
    Answer- The consensus was that they do not, however the research evidence is very limited and not always consistent. The consensus was that even if carbohydrates do play a role as a risk factor for diabetes, this is dwarfed by the much more important factor of obesity.

    5. Are low-carbohydrate diets useful in the management of feline diabetes?
    Answer-Maybe. The evidence is limited and conflicting, and the committee did not achieve a consensus.

    The lead presenter summarized the central finding of the panel with three words:

    “It’s the calories, Stupid!”

  7. Art says:

    I was surprised that in this study the high moisture foods didn’t lead to any lower a urine specific gravity than the kibble, so that questions isn’t totally solved yet.>>>>

    That’s why when we make a diet adjustment we need to measure an effect. I bet if they used old formula dry cat food we had in the 60s we could measure a difference. I remember purina sending vets to speak at CE telling us purina cat chow was all these blocked cats needed
    Art

  8. Kelly says:

    I wonder if this study also measured the amount of water ingested by the cats on the various diets – it does not appear to; or did I miss that?. If the cats on dry, commercial food, also drank more free water, than one might expect their stool to be moister – puzzle solved?

    btw: just found and really appreciate your website!! Have spent well more than an hour reading many blogs, including “HuffPo Offers Vet Woo Too, Courtesy of Dr. Palmquist” . I took my dog to Dr. Palmquist a couple years back. I will admit – gladly – that he did more for my dog, and prevented an unnecessary surgery, than 2 other ‘conventional’ vets and am very grateful I found him. Although I was a bit skeptical about some of the ‘treatments’ he convinced me to use. And I consider myself at least a reasonably skeptical scientist (biologist)! It’s easy to believe an argument/treatment when you want to find things to help your companion animal.

  9. skeptvet says:

    If the water intake for the cats was quantified, I can’t find that in the paper. Obviously, since the fresh diets have a much higher water content, cats on a dry diet would need to drink more to maintain the same level of hydration. One argument often made against dry diets is that cat are very insensitive to mild dehydration and tend not to increase their water intake to compensate, which supposedly leads to hard stools and other, more serious, consequences of chronic underhydration.

    However, urine specific gravity was not different among the groups and was universally extremely high (>1.060), which argues against increased water intake and hydration as an explanation for the difference in fecal scores between the groups. All groups appeared equally hydrated (or underhydrates, based on the USGs measured).

    Glad you enjoy the web site. Rick Palmquist and I have become virtual acquaintences, and I appreciate his committment to his patients and clients. I also still disagree with him on many of his recommendations and conclusions. He is a bright and educated doctor, but he is also much more comfortable than I am making recommendations or strong conclusions about safety and efficacy based on anecdote, intuition, personal experience, and unsupported theories. Respectful, substantive disagreement is both right and necessary to advance the profession, and hopefully we both continue to agree on at least that. 🙂

  10. Renee says:

    I’ll just say I had to try raw after my Maine Coon developed IBS/IBD 18 months ago, no kibble could ‘cure’ his diahrrea. . .once on a raw diet, the diahrrea only occurred if he got into something he shouldn’t. . . .so IMHO there are good reasons to feed raw. . .with my boy gone, I’ve given up on the raw as it is a royal PITA to deal with, but I’d do it all over again to have him back.

  11. v.t. says:

    Renee, while a raw diet may have seemed to work for your kitty, chances are, it was no better than anything else commonly used in a diet for IBD. There is not exactly a one-size-fits-all approach with IBD cats, and treatment varies with every cat. Treating inflammation, vomiting, diarrhea and/or constipation, and even GI tract infections are mainstay in treatment, often involving medications to alleviate the symptoms. You might have gotten similar results from a home-prepared diet formulated by a board-certified veterinary nutritionist. When there is potential for bacterial infection in the GI tract (combined with inflammation), raw diets carry a great deal of risk for an IBD cat.

  12. David says:

    For one of my cats, all it took was a switch to a low ingredient dry diet. My guess is that for him, these ingredients were easier to digest.

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  16. Martijn Lievaart says:

    Anecdotal evidence, but a sample size greater than 1. Having had cats all my life, feline lower urinary tract disease was just a fact of life. In a dozen cats, about half had feline lower urinary tract disease. Since switching to barf and another half a dozen cats later, no feline lower urinary tract disease. None.

    Is there any scientific data on barf and feline lower urinary tract disease?

  17. skeptvet says:

    No controlled data that I know of. A lot of us suspect high moisture diets, which would include BARF, lower the risk, but we don’t have any evidence to demonstrate this. If this turns out to be true, you might get the same benefit with a cooked homemade or a canned diet without the downsides of BARF, but we’ll just have to wait on the research to know.

  18. Tiffany says:

    I’m curios as to why in the raw diet, was it an all beef diet? I have never seen a raw all beef diet recommended for cats. it’s usually majority chicken with a small percent of beef, organs and bone.

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