To most pet guardians, dogs are viewed as members of the family. This has resulted in pet nutrition trends to closely following popular trends in human nutrition (Buff et al., 2014; Knight and Leitsberger, 2016; Dodd et al., 2018). Consumption of an entirely plant-based (vegan) diet that avoids consuming any ingredients that come from an animal has increased in popularity in human nutrition because of the potential positive impacts on the environment and overall health (Berschneider, 2002; Michel, 2006; Dodd et al., 2018; Eisen and Brown, 2022; Feigin et al., 2023; Knight, 2023). As more pet guardians switch to a vegan diet, the potential for moral conflicts has developed when it comes to feeding their pets because most commercial diets are formulated using animal-based ingredients (Rothgerber, 2013; Dodd et al., 2018). A survey conducted in 2019, indicated that three quarters of vegan pet guardians would reportedly feed a plant-based diet to their pets if one was available to meet their needs (Dodd et al., 2019). A third of pet guardians who did not already feed a plant-based diet expressed interest in doing so, and a fifth of those pet guardians who expressed interest stated that further knowledge about plant-based diets is needed before they will feed such diet to their pets (Dodd et al., 2019). The gap in required research to determine the feasibility of vegan diets for dogs pertains to the long-term nutritional suitability with regards to the ability of these diets to meet all nutritional requirements (Berschneider, 2002; Michel, 2006; Remillard, 2008; Dodd et al., 2018, 2019; Knight et al., 2022).

The limited research regarding nutritional adequacy of vegan diets, poses questions for both veterinarians and pet guardians on their long-term sustainability. Commercial vegan diets for dogs have been voiced by veterinary nutritionists as potentially being insufficient in meeting the maintenance requirements stated by the National Research Council, The Association of American Feed Control Officials (2019), and the European Pet Food Industry Federation (McMillan et al., 2006; Beloshapka et al., 2016; Dodd et al., 2021). These concerns are based on lower concentrations of nutrients and poor bioavailability of nutrients in plant-based ingredients used in vegan diet formulations (Dodd et al., 2021). Digestibility studies however, on individual plant ingredients, mildly cooked human-grade, and whole plant-based diets have demonstrated that plant-based ingredients if processed correctly have the potential to demonstrate similar nutrient digestibility when compared to conventional animal-based ingredients (Kendall and Holme, 1982; Bednar et al., 2000; Liversidge et al., 2023). This is likely due to coevolution of dogs and humans which has led to dogs becoming closer related biologically to humans rather than to their wolf ancestors (Buff et al., 2014). Unlike their wolf ancestors, the canine gut microbiota shows to have greater ability to adapt to either a high-protein or high-carbohydrate diet and contains a diverse number of bacteria used for fermentation of these nutrients (Alessandri et al., 2019). Specifically, the gut microbiota of dogs has adapted to show an increase in gut microbiota used for fermentation of plant material and reduction of gut microbiota used in the fermentation of protein and fats from animal ingredients (Coelho et al., 2018; Alessandri et al., 2019; Liu et al., 2021). Thus, if a vegan diet that was formulated with highly digestible plant-ingredients was consumed, the canine gut microbiota has the ability to shift making it possible for dogs to meet daily nutrient requirements with consumption of a plant-based diet.

In humans, diet has a large impact on the richness and diversity of microorganisms present within the gastrointestinal (GI) tract (Bosch et al., 2009; Kamboj and Nanda, 2018; Alvarenga et al., 2019). Individuals following a vegan diet compared to an omnivorous diet showed differences in the diversity and abundance of several taxa of the GI microbiota (Honda and Littman, 2012; Singh et al., 2017). Specifically, the gut microbiota has an increased abundance of bacterial communities that aid in fermentation and absorption of plant-materials and a decreased abundance of communities that breakdown animal-derived protein and fat sources (Winglee and Fodor, 2015; Zhenjiang and Knight, 2015; Singh et al., 2017). Vegetarian – devote of meat products but including dairy and eggs – or vegan diet consumption has been seen to shift gut microbiota in a state of dysbiosis to aid in reducing inflammation and treating conditions such as inflammatory bowel disease, obesity, and type-2 diabetes in humans (Li et al., 2020; Santhiravel et al., 2022; Sahoo et al., 2023). This shift in the GI microbiota appears to be helpful to properly digest and utilize plant-based ingredients and is essential for plant-based diets to provide beneficial effects. Some researchers have investigated fecal microbial effects of single plant-based ingredients in dogs (Carciofi et al., 2010; Bazolli et al., 2015; Hankel et al., 2020); however, currently, there is no research conducted on the effects of vegan diets on the gut microbiota in dogs.

Following thousands of years of domestication, the GI microbiota of dogs and humans have developed structural and functional similarities (Coelho et al., 2018). Based on these resemblances, we hypothesized that similar alterations in microbial taxonomy seen in humans consuming a vegan diet would occur in dogs fed a diet consisting of only plant-based ingredients, in combination with mineral or synthetic additives, but devote of meat-based ingredients. Therefore, the objective of this study was to examine the canine fecal microbial composition and structure in response to an experimental vegan diet compared to a commercially available animal-based diet.

All experimental procedures for this study were approved by the University of Guelph Animal Care Committee (AUP#4192) and the Research Ethics Board (Research Ethics Approval Number 19-02-036), and were in accordance with institutional, provincial, and national guidelines for the care and use of animals and humans participating in research.

Of the total 61 client owned dogs completing the study, 47 were included for fecal microbiota analysis (27 consuming PLANT and 20 consuming MEAT). Sample size was reduced due to a lack of fecal samples submitted by the pet owners on the day of sample collection (n = 5), poor DNA yield after extraction (n = 7), or a repeated inability to obtain adequate sequence numbers (n = 2).

As mentioned previously, the COVID-19 pandemic added variation in trial duration for 10 dogs participating in the study. Examination of labeled scatter plots revealed these dogs were not identified as outliers and results were not significantly different from the dogs consuming the experimental diet for the intended 12 weeks.

The dogs were aged 3 years or older (mean age being 4.734 ± 0.347), both neutered male (n = 22) and spayed female (n = 25) of various purebred or crossbreeds. All dogs enrolled in this study had BCS between 4 and 7 on a 9-point-scale with BW ranging between 5 and 50 kg (baseline BW: 24.12 ± 1.48; exit BW: 24.13 ± 1.53). All dogs in the study tolerated their diets well, with none showing signs of malnutrition or illness during the baseline or trial periods.

The present study revealed that feeding an entirely plant-based or vegan diet for 3 months had minor effects on the gut microbiota in dogs when compared to a conventional animal-based diet. A recent study conducted a gene catalog comparison between dog, human, pig, and mouse to determine which animal the dog’s GI microbiota was most closely related to (Coelho et al., 2018). The distribution of genes at the phylum level of this study showed that the canine GI microbiota has a higher taxonomic and functional overlap with the human GI microbiota (Coelho et al., 2018). These structural and functional similarities imply that human research findings could predict results in dogs and vice versa (Coelho et al., 2018). Based on this similarity to the human gut microbiota, it was expected that similar fecal microbiota changes consisting of increased abundance of microbiota that aid in carbohydrate fermentation and decreased abundance of microbiota related to fermentation of protein and fat would occur in dogs fed an entirely plant-based diet as those seen in humans consuming a vegan diet compared to an omnivorous diet. However, in the current canine study the bacterial fecal microbiota was minimally affected. Despite the lack of change in beta-diversity comparisons, there were a few taxonomic changes on the phylum level between the plant-based and animal-based diet groups, but no significant differences were found at the family or genus level.

In humans, a healthy adult intestinal microbiota is characterized by the dominance of Bacteroidetes and Firmicutes phyla (Simpson and Campbell, 2015; Bamberger et al., 2018). Variability between these two bacteria is shown to be heavily affected by diet, specifically between omnivorous, vegetarian, or vegan diets. In one study comparing Indian participants, whose diets mainly contained plant-based foods, with Chinese participants, whose diets contain mainly animal-based fats and proteins, researchers found the population of Bacteroidetes of Indian participants was nearly four times greater than in the Chinese individuals (Jain et al., 2018). An additional study involving European individuals identified a higher relative abundance of Bacteroidetes in vegans and vegetarians than in omnivores (Losasso et al., 2018). These findings agree with previous research showing that the relative abundance of Bacteroidetes in Italian children, typically consuming an omnivorous diet high in animal protein and fat, was less than half of that seen in Burkina Faso children who consume a diet rich in plant-based starch, fiber, and plant protein (De Filippo et al., 2010). These studies indicate that in humans a lower consumption of animal products is associated with a greater abundance of Bacteroidetes. In contrast, Lin et al. (2013) reported that children from the United States following a Western diet had a higher abundance of Bacteroidetes than children from Bangladesh following a Mediterranean diet. Like humans, the dominance of Bacteroidetes and Firmicutes was confirmed in healthy dogs (Deng and Swanson, 2015). However, the abundance of Bacteroidetes behaved differently in the present study in response to a plant-based diet. Dogs fed on the control animal-based diet had a higher abundance of Bacteroidetes than dogs on the experimental plant-based diet at the exit time-point. These distinct results in humans and dogs may be due to the many exogenous factors affecting the microbiota within the GI tract (Wu et al., 2011; Deng and Swanson, 2015; Li et al., 2017). For instance, results reported by Lin et al. (2013) indicated an abundance of Bacteroidetes between children consuming a Western diet compared to a Mediterranean diet; the researchers found both age and geographical differences as the potential explanation for their unexpected results (Lin et al., 2013). In dogs, similar significant associations between canine alpha-diversity indexes and geographical regions have been observed (Jha et al., 2020). Moreover, the same study observed differences in alpha-diversity indexes between rural and suburban dogs (Jha et al., 2020). In the current study, these exogenous factors may have played a role in affecting the GI microbiota, as client-owned dogs of various ages came from different household environments.

The balance between Bacteroidetes and Firmicutes is affected by each other where a decrease in Firmicutes levels usually occurs in favor of increasing Bacteroidetes levels (Tomova et al., 2019). The ratio between Bacteroidetes and Firmicutes in humans is related to a predisposition to disease development (Ley et al., 2006; Verdam et al., 2013). For example, increases in Firmicutes abundance are observed in obesity and GI diseases such as inflammatory bowel disease (Ley et al., 2006; Verdam et al., 2013). Humans consuming a vegan diet have a lower abundance of Firmicutes compared to omnivores; this finding is linked to positive health benefits, including weight loss, decreased inflammation, and improvements to the immune system that lower the risk of developing chronic diseases (Ley et al., 2006; Kim et al., 2013; Kumar et al., 2014; Singh et al., 2017). In the current study, however, dogs fed the experimental plant-based diet had a higher abundance of Firmicutes than the control animal-based diet. In human literature, these changes are attributed to endogenous factors that can affect the gut microbiota within the GI tract (Wu et al., 2011; Deng and Swanson, 2015; Li et al., 2017). In humans, the nutrient profiles between an omnivore diet and a plant-based diet are typically very different, with a vegan diet in humans expressing lower total energy and protein intake but increased fat and fiber intake when compared to an omnivorous diet (Clarys et al., 2014). In the current study, the experimental plant-based diet was formulated to have a similar nutrient profile to the control animal-based diet, despite different ingredients. Potentially, the changes in human fecal microbiota when comparing individuals consuming a vegan diet to individuals consuming an omnivorous diet may be in response to nutrient differences in the diets, not in response to ingredient changes. Thus, due to the nutrient profiles being as close as possible between the PLANT and MEAT diet this may be the reason that the canine gut microbiota in the current study was shown to be opposite of what is reported in human literature with dogs in the PLANT group expressing higher abundance of Firmicutes and lower abundance Bacteroidetes.

In humans, differences in Fusobacteria abundance have been documented when comparing individuals consuming a vegan diet to individuals consuming an omnivorous diet, with a greater abundance of Fusobacteria associated with an omnivorous diet (Losno et al., 2021). Similarly, the abundance of Fusobacterium (Fusobacteria phylum) was increased in dogs fed entirely animal-based raw food compared to dogs consuming omnivorous extruded diets (Pilla and Suchodolski, 2020). Fusobacteria is more abundant in other carnivorous species such as cats and wolves (Zhang and Chen, 2010; Bermingham et al., 2013). In the current study, relative abundance of Fusobacteria decreased from the baseline to exit time-point in dogs fed the PLANT diet. Furthermore, dogs fed the ANIMAL diet showed a higher abundance than those fed PLANT at the exit time-point. These results complement previous research found in humans (Losno et al., 2021) and other predator species (seals, tigers, polar bears, etc.) supporting that high abundance of Fusobacteria is associated with digestion of meat (Jiang et al., 2020).

The current study had several limitations including a smaller sample size, the use of client-owned animals, sample collection times, and the ability to extrapolate the study results to other vegan diets for dogs. Due to the relatively small sample size, one should be careful extrapolating of the results to a greater population. Nonetheless, the current study had a larger sample size then previously published gut microbiota research in client-owned dogs (Beloshapka et al., 2016; Herstad et al., 2017; Sanches et al., 2020; Onozawa et al., 2022). With use of client-owned animals we are reliant completely on their dedication to the trial protocol. Pet guardians were provided with a small Styrofoam cooler and waste collection bags and asked to collect feces immediately after defecation and freeze using the provided resources. With the use of client-owned animals, despite instructions to freeze fecal samples immediately after defecation, there was no supervision by the research team to ensure the fecal collection and storage was performed properly by pet guardians prior to delivery of the samples. However, the authors do not expect this to influence the results based on previous research in companion animals showing limited changes in the microbiota from short term (2 weeks) exposure to ambient temperature and time (Weese and Jalali, 2014; Tal et al., 2020; Chui et al., 2023). A further limitation factor with respect to data collection and the research variables assessed in the study was the frequency of sampling. Fecal collections from the dogs were only done at baseline (after the adaption period where all dogs were fed the MEAT diet) and exit (after feeding either the PLANT or MEAT diet exclusively for 3 months) timepoints. Based on recent research, the gut microbiota in dogs is very resilient meaning microbiota shifts and stabilization following a dietary intervention can be seen in just 2 weeks (Lin et al., 2022). In the current study changes may have occurred soon after introducing the MEAT or PLANT diet however, the pattern of change was indeterminable due to the data collection timepoints. This reduces the ability to interpret the significance of some of the changes. More frequent measurements to document the temporal trends and patterns in variables would have been beneficial. Furthermore, extrapolation of findings from this trial to all vegan dog foods should be made with caution. The PLANT diet used in the current trial was formulated to exceed minimum AAFCO nutrient recommendations (The Association of American Feed Control Officials, 2019) and also postproduction nutrient analysis occurred to ensure nutrient content was on target. Other commercially available vegan diets may not provide the same nutrient profile which could affect the gut microbiota composition. Lastly, COVID-19 lockdowns introduced some variability into the duration of the diet trial. Dogs were in various stages of the study (adaptation or experimental) when data collection was forcibly paused during the global pandemic. This limitation was unaccepted however, was taken into account during statistical analysis and the authors feel that the variation in trial duration did not influence the results.

The current study showed that feeding an entirely plant-based or vegan diet for 12 weeks to healthy dogs has some potential to change the composition of the canine fecal microbiota, but these changes were not as dramatic or as distinct as those reported in humans. Future microbiota research investigating vegan dog food should consider the effects of different nutrient profiles as well as length of feeding. Moreover, further research is warranted to assess the metabolic function of the microbiota through fecal metabolomics and metagenomics in response to vegan dog foods, in addition to the fecal microbial composition and structural effects examined in the present study.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: Borealis (the Universities Repository), https://doi.org/10.5683/SP3/VOELA6.

The animal studies were approved by the University of Guelph Animal Care Committee (AUP#4192) and the Research Ethics Board (Research Ethics Approval Number 19-02-036), and were in accordance with institutional, provincial, and national guidelines for the care and use of animals and humans participating in research. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent was obtained from the owners for the participation of their animals in this study.

BL: Writing – original draft. DG: Writing – review & editing. SD: Writing – review & editing. JM: Writing – review & editing. JA: Writing – review & editing. SB: Writing – review & editing. AV: Writing – review & editing.