Evidence Update: A Systematic Review of Studies Evaluating Vegan Diets for Dogs and Cats

Over the years, I have reviewed the general evidence and some specific studies concerning vegetarian and vegan diets for dogs and cats. Despite the aggressive claims of some advocates for such diets (including some egregiously unscrupulous individuals), the actual evidence has not been extensive or definitive. My conclusions in previous posts have been that there is no clear evidence vegetarian or vegan diets have benefits for dogs and cats, and there is some real potential for harm, especially in cats:

Vegetarian Diets for Dogs & Cats, 2019

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

Dogs are omnivores shaped by domestication to be 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 that dogs can remain healthy fed only a vegan diet.

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.

Plant-based vs Meat-based Diets for Cats: Which is Healthier?, 2021

This study didn’t actually evaluate the effect of plant-based vs. meat-based diets on health or longevity in cats. What the study evaluated was the perceptions of owners about their cats’ diet and health. The difference is crucial. 

All we can say is that owners who choose to feed a plant-based diet believe their cats are a healthier weight than owners who feed meat-based diets. Since plant-based diets are fed to a small minority of cats, the people who feed these diets must choose to do so based on pre-existing beliefs about their health value. Such individuals already believe these diets are healthier, and they are likely to see and report what is consistent with these beliefs, whether or not it is the reality of their cats’ condition.

Prospective, blinded, randomized feeding studies would be needed to allow any strong conclusions about whether or not plant-based diets are safe and healthy for cats.

Are Vegan Diets Healthier for Dogs & Cats?, 2022

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)…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. 

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. 

I recently came across a systematic review of the literature that summarizes and assesses the available evidence concerning vegan diets for dogs and cats.

Domínguez-Oliva A, Mota-Rojas D, Semendric I, Whittaker AL. The Impact of Vegan Diets on Indicators of Health in Dogs and Cats: A Systematic Review. Veterinary Sciences. 2023; 10(1):52. 

The data from review largely support my concerns about the lack of evidence, though the authors draw a somewhat more optimistic conclusion than I would:

In this review, we conducted a formal assessment of the evidence in the form of a systematic review. We found that there has been limited scientific study on the impact of vegan diets on cat and dog health. In addition, the studies that have been conducted tended to employ small sample sizes, with study designs which are considered less reliable in evidence-based practice. Whilst there have been several survey studies with larger sample sizes, these types of studies can be subject to selection bias based on the disposition of the respondents towards alternative diets, or since answers may relate to subjective concepts such as body condition. However, there is little evidence of adverse effects arising in dogs and cats on vegan diets. In addition, some of the evidence on adverse health impacts is contradicted in other studies. Additionally, there is some evidence of benefits, particularly arising from guardians’ perceptions of the diets. Given the lack of large population-based studies, a cautious approach is recommended.

The short version of this is that there aren’t many studies, and most have serious flaws or limitations, so strong conclusions either way aren’t justified.

No obviously horrible risks have shown up, but given that the studies which looked at actual health outcomes were few, short-term, and involved small numbers of animals on several very different diets, it would be a mistake to draw the general conclusion that vegan diets are safe.

Similarly, a few studies claim to show health benefits, but these are all based on owner surveys with groups of owners self-selected to include people already convinced that vegan diets are healthier. This kind of evidence, also frequently cited to support health claims for raw diets, is highly biased and tells us more about what believers in such diets expect to see than about the actual health effects on their pets.

Finally, some nutritionists have pointed out a significant problem with this kind of review– the idea that broad categories of diets (e.g. canned, kibble, raw, fresh, vegan, etc.) can be identified as beneficial or harmful doesn’t really make sense. The health impact of a diet depends on the specific composition, the ratio and availability of nutrients in the diet, and the nutritional needs of each individual. Vague generalizations, such as “vegan diets are healthy” or “kibble is unhealthy” are so broad as to be pretty meaningless, and they don’t help us decide what the best diet is for our specific pets. While some level of generalization is, of course, necessary and useful, these characterizations go too far to be accurate or helpful.

Bottom Line
There is very little research examining the health effects of vegetarian or vegan diets in dogs and cats. The existing evidence has significant limitations, which makes any firm conclusions impossible. 

General theoretical arguments for why such diets should be healthy are not especially plausible nor convincing. It is likely that a properly formulated vegan diet could be adequate for many dogs, and possible for cats as well, but whether there are any benefits to feeding such diets, and whether these might outweigh potential risks, is not yet known. 

Given this uncertainty, and the much better evidence for the appropriateness of cooked, meat-based diets, feeding dogs and cats a specific vegan diet is essentially a haphazard experiment, with as much or more potential for harm as for benefit.  

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