An academic review encompassing 31 studies determined that vegan and vegetarian pet foods are typically highly digestible, showing overall similarity to traditional meat-based formulas. Nonetheless, investigations in controlled laboratory settings involved far more dogs than cats through various trials, and the authors warned that digestibility by itself cannot confirm a diet’s nutritional completeness.
Comparable Digestibility Across Diet Types
A study featured in the journal Animals analyzed research on vegan and vegetarian diets for pets, including both complete diets and individual plant-based proteins. The author, Andrew Knight, is affiliated with Murdoch University, Griffith University, and the University of Winchester.
Seven studies evaluated the digestibility of complete vegan or vegetarian pet diets, six of which included comparisons with non-vegan or non-vegetarian diets. Five of those studies measured apparent total tract digestibility. Across these studies, digestibility of vegan and vegetarian diets ranged from 80.6% to 84.7% for dry matter, 85% to 86.3% for organic matter, 88.6% to 89% for nitrogen-free extract, and 86.5% to 87.5% for energy. Crude protein digestibility ranged from 79.9% to 89.4%, while fat digestibility ranged from 88.8% to 97.1%.
For animal-based diets evaluated in those same studies, apparent total tract digestibility ranged from 80.5% to 83.4% for dry matter, 83.5% to 87% for organic matter, 76.4% to 85.6% for crude protein, 86.5% to 97.3% for fat, and 87.5% to 89.5% for nitrogen-free extract. Andrew Knight wrote that the digestibility of vegan pet diets was broadly comparable to that of animal-based diets, with all seven studies concluding that vegan diets were well-digested.
Findings varied by nutrient and study. One comparison of mildly cooked vegan dog foods with a chicken-based diet showed slightly higher fat digestibility in vegan options but lower organic matter digestibility. No significant differences were found for dry matter, crude protein, or energy. Another study involving 61 dogs over 12 weeks found total tract digestibility above 80% for dry matter, 85% for crude protein, and 97% for fat in both vegan and chicken-based diets, with no significant differences.
Protein Ingredients and Processing Factors
The review also examined individual protein ingredients, including soy, peas, lentils, chickpeas, wheat gluten, corn gluten meal, potato protein, dried yeast, and proteins produced through microbial fermentation. Soy was the most extensively studied vegan protein source, with comparisons to poultry proteins producing mixed results. Depending on the study and digestibility measurement, some differences favored soy, others favored poultry, and some were not statistically significant.
Even where poultry ingredients showed higher digestibility for particular measures, the review found soy-based diets generally remained highly digestible. Across the included studies, apparent total tract crude protein digestibility of soy diets ranged from 80.9% to 89.3%, while fat digestibility ranged from 88.4% to 95.5%. Processing also emerged as an important consideration. The studies indicated that appropriate processing can reduce the effects of some anti-nutritional factors. At the same time, heat processing can decrease the digestibility of certain amino acids through Maillard reactions.
Andrew Knight wrote that pet food manufacturers should be aware of these nuances, and should optimize processing conditions for the specific ingredients used.
Limited Evidence for Cats
Limited research was available for cats, with only two of 31 studies focusing on them, compared to 22 for dogs. Seven studies applied to both species. However, research showed cats could digest plant-based proteins like soy, rice protein concentrate, corn gluten meal, and chickpeas. Knight stressed the need for more feline-specific research.
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Research gaps extend beyond species. The studies generally involved adult animals consuming maintenance diets and did not establish the suitability of vegan or vegetarian diets for puppies, kittens, senior animals, or animals with compromised digestive function. Most studies were also conducted under controlled laboratory conditions.
High digestibility of proteins or macronutrients does not guarantee a diet’s nutritional completeness. Andrew Knight cautioned that protein digestibility alone does not fully indicate nutritional completeness. The review primarily considered dry matter, organic matter, fat, nitrogen-free extract, crude protein, amino acids, and energy. It did not evaluate micronutrient digestibility, and the author identified micronutrients as an area requiring additional research.
Andrew Knight wrote that overall, the available evidence does not indicate an inherent digestibility disadvantage for nutritionally sound vegan and vegetarian pet foods. Nevertheless, further research involving cats, and in real-world, in-home settings would be beneficial.
Although the paper synthesized findings from numerous studies, it was a review rather than a meta-analysis. Andrew Knight noted that it was not conducted as a systematic review. The paper identified relevant research, established inclusion criteria, and compared findings across the selected studies, but did not statistically combine results into a pooled effect estimate as would occur in a meta-analysis. Instead, its conclusions were based on a qualitative assessment of findings across studies with different designs, populations, ingredients, processing methods, and measures of digestibility.
Andrew Knight acknowledged that a systematic review could provide more thorough coverage of the available literature. The review included 22 studies specific to dogs, two specific to cats, and seven applicable to both species.
Processing Effects on Nutrient Availability
Beyond protein source selection, thermal processing methods significantly influenced digestibility outcomes across multiple studies. Extrusion, baking, and mild cooking each altered the bioavailability of both macronutrients and specific amino acids. Andrew Knight noted that while appropriate processing reduced anti-nutritional factors such as phytic acid and enzyme inhibitors, excessive heat exposure triggered Maillard reactions that diminished lysine availability.
Several studies reported that fermentation-derived proteins maintained higher amino acid integrity when subjected to lower-temperature processing protocols.
Limitations in Study Design and Scope
The majority of included studies relied on laboratory-based feeding trials with controlled environments, limiting generalizability to typical home-feeding scenarios. Only one study explicitly conducted in-home digestibility trials involved 61 healthy adult companion dogs over a 12-week period, comparing a vegan extruded kibble against a commercial chicken-based diet with no significant differences in dry matter, crude protein, or fat digestibility.
Additionally, the review did not evaluate micronutrient digestibility, focusing instead on macronutrients like crude protein, fat, and energy. Andrew Knight identified this gap as requiring further investigation, particularly regarding bioavailable forms of iron, zinc, and taurine in plant-based formulations. The absence of long-term feeding studies also left questions unanswered about sustained nutritional adequacy across life stages.
