A. Verbrugghe; S. Dodd
Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada
Human diets minimizing, or completely eschewing, animal products have been increasing in prevalence in developed countries. Many meat-avoiding pet owners have a moral dilemma, as they live with animals that rely on animal products for their nutritional sustenance. This conflict can result in feelings of guilt and internal conflict. For some, this may stress the human-animal bond to the point where they do not feel comfortable sharing their home with a carnivorous pet, and abstain from this, despite their desire to do so. Considering a prevalence of vegan pet owners in Canada of around 10%, a large number of owners and pets may be affected.
The main concerns that vegan pet owners have regarding the use of animals for production of pet food are challenging to alleviate. They consistently reported concern with the animals’ rights to not be farmed and processed. While claims of improved animal welfare, health effects, and/or sustainability are becoming very popular on pet food labels, these will not help to sway the decisions of this particular group. In Canada, 25% of vegan pet owners reported feeding their dog a plant-based diet, while of the 75% who did not do so, 75% indicated an interest in doing so.
Plant-based diets have been introduced to the pet food market. This practice is certainly a solution for meat-avoiding pet owners. However, considering the novelty of these diets and the unique challenges posed when attempting to formulate and design a plant-based diet for opportunistic carnivorous dogs and obligate carnivorous cats, there is concern regarding the nutritional adequacy these diets. Lack of evidence of adequacy is the number one reason for hesitance among pet owners who do not feed a plant-based diet, but are interested in doing so. Nutritional analyses have yielded conflicting results. In North America, there is poor labeling compliance, and concerns have been expressed regarding the amino acid content. Though diets may be formulated to meet industry recommendations, manufactures rarely perform nutritional analyses and feeding trials. Moreover, very few commercial plant-based diets exist, which may leave some owners choosing to feed a homemade plant-based diet instead, with increased risk of nutritional imbalances and potential predisposition for nutritionally associated diseases.
Nutritional insufficiencies and imbalances may occur with any nutrients; however, those of greatest concern are the essential nutrients which are found mostly or exclusively in animal tissues. These include the amino acids methionine, cysteine and taurine; the omega-3 (n-3) fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA); the omega-6 (n-6) fatty acid arachidonic acid; calcium, phosphorus, and vitamins A, B12 and D.
Protein and Amino Acids
Protein is readily found in many plant ingredients. Though both the total quantity of dietary protein and the biological value, a function of digestibility and constituent amino acid content, must be considered. When compared to animal proteins, plant proteins may have lower digestibility and may require more processing to mitigate anti-nutritional factors. Moreover, the amino acid profile of plant proteins may be incomplete, particularly with methionine and lysine being limiting amino acids. For dogs, methionine is the only strictly essential sulfur amino acid, as cysteine and taurine can be synthesized if enough methionine is provided. Cats have even higher dietary requirements for sulfur amino acids; furthermore, they cannot synthesize taurine. Also, the bioavailability of sulfur amino acids can vary depending on ingredient source, and high levels of dietary fibre appear to increase biliary taurine losses and/or support bacterial degradation of dietary taurine. Taurine deficiency causes ocular changes, dilated cardiomyopathy and heart failure, while signs of sulfur amino acid deficiency also include weight loss, lethargy, and skin lesion. Recently, grain-free diets, especially diets high in legumes, including plant-based diets, were suggested as a risk factor for development of taurine-deficiency dilated cardiomyopathy in dogs. Protein content differs among plant sources, although some, for example soy meal, sunflower meal, and Brewer’s yeast, contain total protein and sulfur amino acids comparable to animal protein ingredients commonly used in pet food. Still, the inclusion of synthetic DL-methionine is recommended, and synthetic taurine also especially if methionine concentrations are insufficient or close to minimum.
N-3 and n-6 fatty acids are required for cellular structure and physiological functions. For adult, non-reproductive cats and dogs, ½-linolenic is the only essential n-3 fatty acid. During growth, puppies and kittens require provision of dietary EPA and DHA, as these essential fatty acids selectively accumulate within the developing nervous tissues. For dogs, linoleic acid is the only essential n-6 fatty acid, and while it is also essential for cats, they additionally require arachidonic acid, which is found mostly in animal fats. Marine sources are the only ingredients which contain EPA and DHA, and in pet food, fish oil has been the traditional source. Species of algae are known to contain EPA, DHA and arachidonic acid, though their use for pet food is still rare.
Some minerals such as calcium, phosphorus, potassium, and zinc, are found in low concentrations in most plant-based ingredients and can be deficient in diets that are not formulated or supplemented appropriately. Calcium and phosphorus in pet diets are typically provided by animal-derived dietary ingredients, such meat and bone meal, or bone meal, as well as in mineral supplements. Phosphorus is also abundant in non-animal ingredients, though both calcium and phosphorus in plant-derived ingredients may be poorly available due to complexing as phytate. Calcium and phosphorus are critical for healthy skeletal development, and imbalances in these minerals can result in severe skeletal abnormalities, particularly in growing animals. Not only provision of adequate amounts of each macro-mineral is a concern, also the appropriate ratio of calcium to phosphorus should be maintained. Due to the relative ubiquity of phosphorus in non-animal ingredients, risk of an inverse calcium to phosphorus ratio is great if appropriate supplementation of calcium is not included.
Most diets, when appropriately formulated, are supplemented with vitamins. Although non-animal sources exist, vitamins A, B12 and D are traditionally obtained from animal sources. Vitamin A is essential only for cats. Dogs can synthesize sufficient vitamin A from dietary precursors. Vitamin A is essential for embryological development, immune function and vision. Vegetables rich in ½-carotene can be included in pet foods in order to provide adequate precursors for vitamin A metabolism. Synthetic vitamin A analogs, retinyl esters, can also be added. Of the B vitamins, B12, or cobalamin, is the least prevalent in plant-based diets. In nature, cobalamin is produced by microorganisms within the gastrointestinal tract of herbivorous animals and is found within their tissues. Dogs and cats do not possess the microorganisms, and thus require a dietary source. Yeasts and yeast extracts are a source of cobalamin, and also synthetic cobalamin is available and frequently used within the pet food industry. Neither dogs nor cats can synthesize adequate vitamin D in their skin, making it an essential vitamin. Vitamin D exists in two forms in nature: As D3, or cholecalciferol, found in animal tissues, or D2, or ergocalciferol, found in fungi. While it is thus relatively simple to include vitamin D as ergocalciferol, its biological effectiveness in cats and dogs is not well known. Indeed, it would appear that cats do not utilise D2 with near the same efficacy as D3. Still, the amounts required are currently unknown.
Dogs and cats have dietary requirements for energy and essential nutrients, but they do not have requirements for specific ingredients, no matter if these ingredients are animal-derived, plant-derived or synthetic. However, special care must be taken when formulating plant-based diets to ensure that all nutrient requirements are met, accounting for nutrient interactions, and palatability and digestibility should be verified with feeding trials. Dogs and cats fed homemade plant-based diets are at similar, or even greater, risk of nutrient imbalances and deficiencies as pets fed other homemade diets due to the unique challenges of nutrient provision when sourcing nutrients exclusively from animal-free ingredients. It is recommended that pet owners use recipes formulated by a qualified veterinary nutritionist and that these pets are routinely examined by a veterinarian as they are considered high-risk animals.
1. Bauer JE. Metabolic basis for the essential nature of fatty acids and the unique dietary fatty acid requirement of cats. J Am Vet Med Assoc. 2006;229:1729–1732.
2. Bauer JE. The essential nature of dietary omega-3 fatty acids in dogs. J Am Vet Med Assoc. 2016;249:1267–1272.
3. Dodd SAS, Adolphe JL, Verbrugghe A. Plant-based diets for dogs. J Am Vet Med Assoc. 2018;253:1425–1432.
4. Dodd SAS, Cave NJ, Adolphe JL, Shoveller AK, Verbrugghe A. Plant-based (vegan) diets for pets: A survey of pet owner attitudes and feeding practices. PLoS One. 2019;14(1):e0210806.
5. Kanakubo K, Fascetti AJ, Larsen JA. Assessment of protein and amino acid concentrations and labeling adequacy of commercial vegetarian diets formulated for dogs and cats. J Am Vet Med Assoc. 2015;247:385–392.
6. Morris JG. Cats discriminate between cholecalciferol and ergocalciferol. J Anim Physiol Anim Nutr. 2002;86,229–238.