Updated: Sep 20
Back in April 2022, a study made the headlines, after a peer-reviewed analysis of around 2,500 pets found that vegan dogs visit the vet less often and require fewer medications. This week, we have another, with the Guardian stating ‘Cats may get health benefits from vegan diet, study suggests’ and ‘Owners who fed their pet a plant-based diet reported fewer visits to the vet and less medication use’
It certainly sounds amazing - a diet that is healthier for cats and dogs, while also saving the environment, and resulting in fewer vet visits and less medication. However, journalism like this must be taken with extreme caution, as it is so difficult for owners to be able to break down wordy and complex literature to understand the bias and limitations behind them. These studies make it look as if vegan cat and dogs’ foods are the way forward, when in actual fact they should only be undertaken with some serious considerations.
At the heart of every owner’s decision – especially with the surprisingly emotive topic of diet – is the want to provide the very best for our pets. Many of these decisions can also be based on or swayed by our own bias and beliefs too. This is even highlighted in the study:
‘Our results affirmed the importance of pet health to guardians. Among 2,596 respondents, health and nutrition was the factor considered most important in purchasing decisions.’
Incredibly, when looking at the new cat data, the article written in praise of vegan cat food states:
“The study, published in the journal Plos One, surveyed 1,369 cat owners, about 9% of whom reported feeding their cat a vegan diet. When asked about 22 specific health disorders, 42% of owners whose cat ate a meat-based diet reported at least one disorder, compared with 37% of owners of cats on vegan diets.”
While this may look positive, although overall the vegan cats scored better on all health indicators – which is expected when you do survey-based client-reported outcomes – it is crucial to note that these differences were not statistically significant.
We are increasingly aware of the global climate crisis and environmental concerns, and as such interest in veganism has grown, with the belief that it may be better for our health, better for animal health and welfare and better for the environment. Naturally, these ideas will also filter into our decision making for our pets, and thus we have also seen a growth in interest in vegan diets for dogs. This is also acknowledged in the study:
‘Vegan diets are among a range of alternative diets being formulated to address increasing concerns of consumers about traditional pet foods, such as their ecological ‘pawprint’, perceived lack of ‘naturalness’, health concerns, or impacts on ‘food’ animals used to formulate such diets’
Dogs are omnivores, and as such can technically live on a vegan diet – and there are many that do. However, we are in the infancy of knowledge about these diets and their effect on health and longevity in dogs – and as such, this kind of bold claims in the media could have far reaching and potentially quite damaging effects if the evidence isn't presented in an open and transparent way.
Cats, on the other hand, as ‘obligate carnivores’, and therefore need specific amino acids to survive. Cats lack the ability to synthesise (make) a number of important nutrients, due to a complete deletion or severe limitation of the enzyme or pathway that makes each nutrient. Important examples include taurine, arginine, and Vitamin A, but there are others.
Conflict of interest
First thing to note is the conflicts of interest – the researcher that devised and led both peer-reviewed studies, Andrew Knight, follows a vegan diet himself, and the study that was funded by the charity ProVeg – a charity whose aim is to raise awareness of the importance of plant-based food for people, animals, and the planet through their campaigns. The study notes ‘This research and its publication open access was funded by food awareness organisation ProVeg International’.
This transparency and the investigation is worthy of praise, as we all have a responsibly to ensure the health of people, animals and our planet – but starting from this ideological position does inevitably introduce a risk of bias towards a plant-based diet in results, and we should bear that risk in mind when we are assessing the results.
Downfalls of a survey
Although the sample size for the studies were good, with 2,639 dogs and 1,369 cats and their owners (termed ‘guardians’ in the studies) included, this study is a self-reported survey, rather than an observational study or randomised controlled trial. Surveys have the potential for issues that may result in the inability to draw robust conclusions from data received and analyzed, and this study was no different, with a number of potential downfalls.
Of the 2,610 human respondents who provided their sex in the dog study, 92% (2,412) identified as female. Similarly, in the cat study of the 1,399 human respondents who provided their sex, 91% (1,269) identified as female, and only 9% (124) as males. This is heavily skewed, and may indicate that respondents aren't wholly representative of the general pet owning population.
Many of the respondents also had an existing interest in vegetarianism or veganism, with vegan (22%, 586), reducetarian (omnivore reducing animal product consumption) (21%, 567), and vegetarian (10%, 266) making up 53 % of respondents in the dogs study. In the cat study the diet choice of the humans respondents were as follows; omnivorous (35%, 500), vegan (26%, 372), reducetarian (omnivore reducing animal product consumption) (22%, 318), vegetarian (10%, 146) and pescatarian (consuming fish but no other meats) (5%, 72). This is significantly higher than the estimated 7% of the UK population who are currently vegan, and could have affected results due to potential bias.
In the study, there was a much larger sample size for conventional diet compared to vegan, which could introduce some uncertainty in the results. Of 2,536 dogs in the first study, dog guardians reported their dog’s main diet, were jointly maintained on one of three main diets: conventional meat (1,370–54%), raw meat (830–33%), and a vegan Diet (336–13%). In the cat study, 91% reported a meat-based diet, with 9% reporting a vegan diet.
When looking for robust data to support a conclusion, ideally there should be no significant difference between the groups being compared – so a larger sample size of vegan-diet animals would be less prone to outlier results, and will hence provide more accurate results.
In the cat group, it is noteworthy that at 6.24 years, the average age of cats in their sample fed vegan diets was nearly two years less than that of those fed meat-based diets, at 8.14 years, and that difference was significant. This is important, because younger cats may have decreased risks of certain health disorders, meaning the cats in the vegan group may have naturally reported less age-related health issues, simply because they are younger, not because they are vegan.
The dogs and cats weren't on an exclusive diet
Another limitation to the study was that these diets were usually not fed exclusively - as might occur within a controlled study in a research institute. Of the 2,536 dogs in the three main diet groups, 76% received a variety of treats at least once daily, and 37% were also regularly offered dietary supplements. In the cat study the author noted ‘The researchers could not rule out the cat's obtaining meat through other means’.
Outdoor Vs indoor
When assessing the impact of diet, it is also important to control for other significant variables in the compared populations, that might otherwise impact on the results. Yet, as the Guardian noted:
The researchers could not rule out the cats obtaining meat through other means, but said that this was unlikely to influence the findings. “Most of the cats on vegan diets were indoor cats,” said Knight. “They weren’t going outdoors and hunting. It could’ve been for the other ones that there was some supplemental hunting going on.”
This is certainly relevant, because while the researchers feel they could rule out vegan cats sneaking unseen meat into their diet, the meat-based cats were more likely to be outdoor cats – which means they might be more likely to need veterinary visits from the injuries and trauma that’s often associated with their outdoor lifestyle. Indoor cats are far less likely to get into fights, or to encounter traffic, or to pick up parasites and some diseases, which could account for why they’re less likely to need that visit to the vet.
Certain ideologies mean you are less likely to visit a vet – at all!
We know that certain ideologies around human health can make people less likely to visit a doctor or less likely to access conventional medicine and experience poorer patient outcomes – and this can be seen in their animals too.
This theory is noted by the researchers who discuss it in regard to health checks. The study suggested that routine health checks are normally conducted annually, whereas multiple veterinary visits within a single year may sometimes indicate a health problem. The study was interested in those dogs who saw veterinarians more than once in the previous year.
In their survey, they found that a larger number of dogs fed on a raw meat diet did not see a veterinarian at all in the last year, compared to the other two dietary groups, and there were more raw-fed dogs who had not seen a vet at all, than in the other groups. But not visiting a vet does not always equate to the animals being healthier – it might just mean they were not getting any professional care. Guardians feeding raw meat diets also had lower neutering rates – it might just be that such guardians are less likely to seek, or to comply with, veterinary advice and routine preventative healthcare advice.
The researchers even acknowledge this:
‘there is reason to believe that guardians of dogs fed raw meat are less likely to visit veterinarians, for reasons not directly related to the health of their animals. It is known that those who feed a raw meat diet are less likely to seek advice from their veterinarian, and more inclined to gather information from other sources, such as online resources which vary greatly in their reliability. The perceived opposition of most veterinarians to the feeding philosophy and choices of guardians feeding raw meat diets, may make these people less trusting of veterinary advice, and less likely to visit veterinarians, in general. This is likely to have altered this apparent general health indicator, for reasons unrelated to the health of these dogs.’
The researchers fail to note that this phenomenon may also be seen in the vegan group, who also had a higher percentage than conventional meat group for not seeing a vet at all, and could also be down to similar ideologies seen in the raw feeding group.
For example, in the vegan group there was an increased risks of internal parasites in dogs - the vegan lifestyle adhered to by such guardians commonly involves a commitment to minimising harm to living creatures, and it is possible some vegan guardians consider internal parasites to be living creatures deserving of consideration, reducing their use of anthelmintics (de-wormers). Or it may just be that the owners following a vegan diet might also have an aversion to pharmaceutical products, leaving their pets at risk of parasites. The ideological choices of owners can filter down and have serious implications for the health of their pets.
Personal Bias of Respondents
Personal bias can also sway results when obtaining survey results. We already know that when surveys are distributed, we can often an experience a bias where people interested in that particular topic are more likely to respond, as they are passionate about the subject. This means that personal bias of respondents can lead to their over-representation within results, in what is known as ‘selection bias’.
Pet diet is actually a very emotive topic and can be a source of much angst – as such those with a particular passion or interest in their pets's nutrition may have been more likely to respond to this survey.
To have personal biases is to be human - we all hold our own subjective world views, and are influenced and shaped by our experiences, beliefs, values, education, family, friends, peers and others - but if we want to make robust scientific conclusions, we need to find ways to eliminate as much of the effects of that bias on the results as possible.
Another source of potential bias when relying on respondent's answers, is unconscious bias. This could occur if a guardian using a conventional or unconventional pet diet expected a better health outcome as a result, and if this expectation exerted an unconscious effect on their answers about pet health indicators.
The caregiver placebo effect is also an unconscious issue that is seen in owners - essentially, as the owners have faith in a treatment, they are self-motivated to perceive a notable benefit in their pets, even if the animal is not actually clinically any better. This effect can be strong - it can be as prevalent as 39.7% in owners – and it works in combination with a number of other factors.
When guardians were asked for their own assessments of their dogs’ health status there was a shift of roughly 5% in all groups toward considering dogs to be healthier than veterinarians were expected to rate them – this suggests that owners are not objective at assessing their dog's health status. It is conceivable that vegans, or even the respondents following other dietary groups, such as omnivores, might have had greater subconscious expectations of good health, when animals were fed diets similar to their own, and inadvertently adjusted their assessments and their answers accordingly.
Diet change to ‘therapeutic diets’
In the survey, if a dog had to move onto a ‘therapeutic’ diet, then the dog guardians were asked to respond about the diet they were previously on. Again, raw-fed dogs were less likely to do this, perhaps for some of the reasons outlined above. However, there were significant differences in likelihood of subsequent progression onto a therapeutic diet - dogs initially fed vegan diets had more than three times the risk of this outcome (needing to be changed to a therapeutic diet), compared to those initially fed raw meat.
In the feline study, cats fed a vegan diet had, on average, 56.5% lower odds of progressing onto a therapeutic diet. This was not a statistically significant finding, but it also may have been affected by guardians feeding vegan diets being reluctant to progress their cats onto therapeutic diets, if those therapeutic diets were not also vegan.
Encouraging but more data is needed
The results of this study are encouraging for the potential of a new and complete dietary choice for owners that also allows them to maintain their belief system while having the joy of pet ownership.
However, despite a lot of ongoing research in the field of vegan dog and cat diets – and these papers add to the body of evidence supporting its benefits – there is currently a lack of robust data mapping the long-term health consequences of feeding a vegan diet to a large number of dogs over many years. As such, evidence such as the Dilated Cardiomyopathy in diets using grain-free recipes and pea and legume proteins could shed more light into the potential inadequacies of such protein source. In cats, nutritional deficiencies in vegan foods could result in secondary nutritional hyperparathyroidism, thrombocytopenia, impairment of platelet aggregation, DCM development and more.
Previous data found that laboratory analyses of the four main commercial vegan cat and dog foods showed that none of them met all of the nutritional recommendations of FEDIAF and AAFCO (2019) for the species for which they were intended, prompting the BVA to respond:
“While on paper a vegan diet for cats may include supplements or alternatives to animal-based protein, for example, there is no guarantee that these would be bioavailable to the cat or that they wouldn’t interfere with the action of other nutrients. That is why robust, peer-reviewed research is needed to ensure that non-animal protein sources can meet the pet’s dietary requirements.”
Despite what you might read in the media and on social media, you should never rush into a vegan diet for your cat or dog, and if it is something you are considering, be sure to seek the input of a veterinary nutritionist, who can help guide you appropriately to ensure the health and welfare of your pet.