Are Vegan Diets Healthier for Dogs & Cats?

Introduction
Given the media frenzy the article has kicked off, it is unlikely very many people are unaware of a recent study reported in the journal PLOS One that claims to show vegan diets are healthier for dogs and cats than diets containing meat. 

Knight A, Huang E, Rai N, Brown H. Vegan versus meat-based dog food: Guardian-reported indicators of health. PLoS One. 2022 Apr 13;17(4):e0265662.

Dramatic headlines have appeared in many major news sources proclaiming that vegan diets are “linked to better health” or “are healthier and safer” than conventional diets. Getting beyond the headlines, one may even read that this study demonstrates raw diets to be healthier than conventional foods, though that is less emphasized in both the original article and the media coverage.

I’ve written about vegan diets for dogs before, and indeed this article cites my column on the subject, though only to dismiss my claim that vegan diets “should not be recommended” as “without evidence.” This is not entirely accurate, since my article did cite the limited evidence available, and I did not actually recommend against vegan diets for dogs, though I did conclude they were a bad choice for cats. My actual claims were:

There is no evidence vegetarian diets have health benefits for dogs and cats, and no real reason to believe they should, based on the physiology and nutritional requirements of these species.

Dogs are omnivores that are able to eat both plant and animal foods, and in theory, they should be able to thrive on vegetarian or vegan diets. However, these diets must be carefully formulated, and many commercial vegetarian dog foods do not appear to be nutritionally adequate. There is also little reliable research evidence showing dogs can remain healthy when fed only a vegan diet. Given the unexpected health problems seen with theoretically adequate grain-free diets, we should be cautious about the potential risks of vegetarian formulations for dogs until there is better and more evidence showing their long-term health effects.

Cats are clearly obligate carnivores with nutritional requirements that are unlikely to be effectively met by vegan diets. Such diets offer only risks and no benefits for cats and should be avoided.

In any case, I am sympathetic to many of the potential environmental and health benefits of plant-based diets, and I have been a vegetarian, though not a vegan, for almost 20 years  (technically, an ovo/lacto/pescatarian, though realistically I don’t like fish and so eat very little of it). However, the evidence for benefits of plant-based diets for dogs and cats is far weaker than that for humans, which is itself often nuanced and not always conclusive. And I believe in following the evidence wherever it leads, even when it doesn’t support my personal beliefs or practices.

So, does this paper change the game in terms of showing real health benefits to raw and/or vegan diets? Spoiler, but not really! Let’s look at the actual findings a bit closer.

The Study- Results
The study reported the following statistically significant differences between “conventional,” raw, and vegan diets (leaving aside the fuzzy definitions of these, the ubiquitous feeding of unidentified treats to all pets, and the lack of clarity about how strictly feeding practices corresponded to each category).

  • Dogs fed raw and vegan diets were less likely to have had multiple veterinary visits in the year observed.
  • Dogs fed raw and vegan diets were less likely to have been given medications
  • Dogs fed raw and vegan diets were less likely to have been transitioned onto a therapeutic diet
  • Dogs fed raw and vegan diets were less likely to have an owner-reported veterinary assessment of poor health status
  • Dogs fed raw and vegan diets were less likely to have an owner reported assessment of poor health
  • Dogs fed raw and vegan diets were less likely to be reported to be “unwell,” and when unwell dogs on raw diets had fewer reported disorders than dogs on conventional diets.
  • There were differences in the occurrence of specific health conditions between diet groups as illustrated in this figure and table:
  • Dogs fed raw and vegan diets were less likely to have had multiple veterinary visits in the year observed.
  • Dogs fed raw and vegan diets were less likely to have been given medications
  • Dogs fed raw and vegan diets were less likely to have been transitioned onto a therapeutic diet
  • Dogs fed raw and vegan diets were less likely to have an owner-reported veterinary assessment of poor health status
  • Dogs fed raw and vegan diets were less likely to have an owner reported assessment of poor health
  • Dogs fed raw and vegan diets were less likely to be reported to be “unwell,” and when unwell dogs on raw diets had fewer reported disorders than dogs on conventional diets.
  • There were differences in the occurrence of specific health conditions between diet groups as illustrated in this figure and table:

All-in-all, these results would seem to be pretty bad news for conventional diets (whatever that means) and pretty good news for feeders of raw and vegan diets. Anyone reading this, however, can probably sense the “but” coming, so let’s get into the caveats.

The Study-Limitations
The biggest limitation (and boy is it a big one!) is that all the data were collected from online surveys of pet owners. Everything from the diet fed to the health status and even the reported veterinary assessment of health all relied entirely on the responses of pet owners who found the survey online and took the trouble to complete it. None of the facts, not the feeding practices nor the health of the pets, were verified objectively in any way. Right from the start, then, that makes this a study of what some small subset of pet owners believe about their pets’ diet and health, not about the actual feeding and health of these animals.

It is not difficult to find reasons to be concerned that these responses may not reflect reality, or even the opinions of other groups of pet owners. The respondents were overwhelmingly female (92%) and European (86%), which is a pretty narrow population to survey. They were, of course, also in a socioeconomic class inclined to participate in online pet health surveys, which is also not representative of many other pet owners.

More concerning, 22% of the population of owners were themselves vegan. This is a group likely to have strong beliefs and biases about plant-based vs meat-based nutrition and health, so it would be, frankly, shocking if they did not report that plant-based diets were healthier than other diets. 13% of these owners fed their pets vegan diets and 33% fed raw meat, indicating nearly half fed unconventional diets not usually recommended by veterinarians or veterinary nutrition specialists. Such a group is clearly a population biased in favor of the kind of outcomes reported in this study, and given the study only shows us the opinions of owners, not actual objective data about health and diet, the outcomes are simply a fancy way of reporting what people in this group think. 

Other studies in other populations have reported lower rates of veganism (5.8%) and of feeding vegan diets to pets (1-3%), which emphasizes that this study surveys a very specific, narrow group of owners. A review of previous owner surveys also shows more nuanced and variable feeding practices reported in these studies, again suggesting this paper may not be applicable outside of the specific population included in the survey.

In terms of potential sources of bias beyond the survey population, it is worth pointing out that the lead author is a consistent advocate for plant-based diets, for humans and pets, on environmental and ethical grounds. The study was also funded by a plant-based diet advocacy group. As I have discussed in detail in previous articles on conflict of interest, these facts do not indicate the research is fraudulent or inaccurate, nor are they a justification for ignoring the claims, arguments, and evidence provided in this paper. They are, however, a reason to consider carefully the potential for uncontrolled unconscious bias in the design, conduct, analysis, and reporting of the study. Given that the study itself was essentially a survey of subjective opinions, methods to control such bias are minimal, so the results have to be viewed in that context.

The authors, to their credit, do acknowledge some of these limitations. They call out the fact that, for example, fewer reported veterinary visits in dogs fed raw diets may be due to the fact that feeders of raw diets are often skeptical of conventional veterinary medicine and less likely to seek care rather than to any actual difference in health status. The same logic, of course, applies to the frequency of vet visits for dogs fed vegan diets, and to the reported use of medicine and therapeutic diets by owners who have a clear preference for unconventional health practices to begin with.

Conclusions
So what does this study mean? Overall, it means that the particular population of pet owners surveyed believes that feeding raw and plant-based diets are associated with better health in their pets. They also believe that their veterinarians think their pets are healthier (though whether these vets actually believe this is unknown). And these owners report less use of veterinary medical services, though whether this means their pets are healthier or simply that they try harder to avoid taking their pets to the vet is also unknown.

Like previous studies relying on owner surveys and both conducted and funded by folks with strong a priori opinions about diet and health, this is a useful insight into such beliefs. It is not compelling or probative evidence for actual health effects of different feeding strategies. Sadly, the media coverage of the paper rarely recognizes or emphasizes this.

As the authors themselves suggest, though with little evident enthusiasm, controlled studies with objective measures of outcome and more defined and verified feeding practices are required to draw any meaningful, actionable conclusions about the healthiest feeding strategy for our pets. I am neither for nor against vegan diets for dogs, and I am even open to reversing my objection to feeding vegan to cats or raw diets to cats or dogs if strong evidence is generated that these are safe or beneficial practices. However, regardless of the difficulties in funding and conducting the necessary research, we are not justified in making confident claims about the health impact of raw, vegan, or conventional diets without it.

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8 Responses to Are Vegan Diets Healthier for Dogs & Cats?

  1. Kate says:

    It’s interesting that some off the questions had to do with frequency of vet visits and use of medications and therapeutic diets. It’s entirely possible that people who fed raw or vegan diets had less trust in veterinarians and that that explained fewer visits and less use of conventional veterinary medicine. Lots of confounders in this study for sure.

  2. Kevin says:

    Thanks for the review. I heard this story on BBC and was rather skeptical of the claims made. A study based on no objective evidence is not very convincing.

  3. Emma says:

    2 problems i have with this discussion:
    Some studies have found nutritional deficiencies in commercial vegan diets therefore even properly formulated products are having issues (I’m guessing nutritional interactions such as plant based anti-nutrents are not being in-depthly researched).
    The second is the premise that dogs are omnivores to begin with as most ecological sources discuss them as mainly preditory scavengers (borderline hyper carnivores). As a subspecies of grey wolf that can interbreed with grey wolves with completely fertile young, at what percentage wolf is a wolf-dog suddenly not an omnivore anymore? There are a couple of gene’s that help them digest certain carbs that’s true, but how does this suddenly mean they are capable of having only plant based foods? (Most candids can’t do this, even the most vegetarian dog like species (maned wolf) eats 50% vegetation 50% animal based foods) And if the only difference between cats and dogs is this one type of gene and the need for taurine then why is vegan cat food suddenly dismissed when they will have the same nutritional considerations within this type of diet but with additional taurine?

  4. Jazzlet says:

    I’d very much like to know what was in the vegan diets fed to dogs, and particularly to cats – I’m not expecting an answer here obviously. Especially in the case of cats it is difficult to see how a vegan diet could be healthy and at all ‘natural’. I can’t help wondering how much the cats supplement what they are fed by their owners with hunted food.

  5. skeptvet says:

    The current consensus is that dogs have co-evolved with humans, and this has involved significant adaptations to a more omnivorous diet. The adaptations are a bit more extensive than a couple of genes, and dogs have been thriving on essentially human diets for a long time. Whether or not there are health benefits or risks to properly formulated vegan diets isn’t truly known, and can’t be without proper feeding trials, but the imperfect evidence we have makes it at least plausible that they should b able to do well on such diets.

  6. Molly Hodgdon says:

    One thing I’ve noticed in discussions of vegan cat food is that vegans will often point to the nutritional label, saying “Look, it has everything cats need” without acknowledgement that even if it’s printed on the paper it might not be as bioavailable to cats as the nutrients in meat sources. As you noted, there is a strong ideological bias coloring their understanding of pet food, their observations, and their reporting. And I say this as a completely committed vegan myself. Veganism is an expression of my values, but that’s the thing: they’re MY values – I’m not going to impose those values on my cats unless our vets assure us there is a vegan alternative with more than just anecdotes and shaky owner reporting surveys to back it up.

  7. lorac says:

    “ubiquitous feeding of unidentified treats to all POETS” (emphasis mine). I do have a book of poems by dogs, so maybe identification of dogs as poets is justified. 🙂 🙂

  8. skeptvet says:

    I’m all for seeing poets well fed! 😉 (typo fixed)

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