Both cats and dogs are formally classed as carnivores, in the biological order Carnivora, although the metabolism and nutritional needs of the dog approach those of the omnivore classification (Fascetti and Delaney, 2012; Böswald et al, 2018). This is because dogs are able to synthesise taurine, arachidonic acid and vitamin A from the metabolic precursors cysteine, linoleic acid and beta-carotene respectively. Whether dogs should be classed as carnivores or omnivores is outside the scope of this article, but there are as many references that would argue either way.
However, cats are obligate carnivores and require these specific nutrients in a pre-formed state, that can only be found in meat-based diets, in order to survive. This is what veterinary professionals are taught at college, but has this changed? There has been a lot of discussion in the veterinary profession, with the British Veterinary Association releasing a statement stating that cats should not be fed a vegan diet (Shotton, 2022). This has lead to discussions about ingredients in diets and awareness that those nutrients that were classed as only able to be obtained from animal sources can now be obtained from non-animal sources.
Cats also require arginine, niacin and vitamin D directly from their diets, but these can be obtained from plant sources (Fascetti and Delaney, 2012). The amino acids methionine and cystine are required in higher amounts by cats than most other species. Nutritional deficiencies are possible, especially in cats fed home-prepared or vegetable-based foods (Kirk et al, 2000).
Vegan feline diets
Veterinary professionals are commonly taught that taurine, arachidonic acid and vitamin A cannot be derived from plant material and will need to be chemically synthesised (as with taurine) or be derived from animals. The processes behind some of these manufacturing methods will not be stated on the packaging of pet food but these could still be synthesised from an animal-based ingredient.
Taurine
Taurine can be easily synthesised and is available as a supplement. Thousands of tonnes of taurine are made annually — mainly in China — and approximately 50% of this is for the pet food industry (Tully, 2000). Taurine can be chemically made by two different routes (direct or indirect) and is completely synthetic. The European Food Safety Authority (2012) claimed that the use of synthetic taurine in pet foods for cats, dogs and carnivorous fish was efficacious. The clinical signs of taurine deficiency occur only after prolonged periods of depletion (i.e. 5 months to 2 years) (Kirk et al, 2000). Clinical signs of deficiency include feline central retinal degeneration, dilated cardiomyopathy or both, with only approximately 40% of taurine-deficient cats exhibiting clinical signs (Kirk et al, 2000).
Arachidonic acid
There is a considerable amount of work looking at finding sustainable and reliable sources of polyunsaturated fatty acids, to meet market requirements. The main polyunsaturated fatty acids of interest are docosahexaenoic acid, eicosapentaenoic acid and arachidonic acid (Shanab et al, 2018). Arachidonic acid and docosahexaenoic acid are the major constituents of the brain phospholipid membrane, can act as an immune suppressant and induce inflammatory responses, blood clotting and cell signalling (Kinsella et al, 1990; Hansen et al, 1997; Neitzel, 2010). Docosahexaenoic acid has also been shown to protect against neurogenerative diseases such as Alzheimer's disease and Parkinson's disease as well as multiple sclerosis (Valenzuela et al, 2012).
Definition of vegan
Veganism is a way of living which seeks to exclude, as far as is possible and practicable, all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose (Vegan Society, 2020).
Eicosapentaenoic acid and docosahexaenoic acid (n-3) play an important role in the cardiovascular system and treating psychiatric disorders (Hallahan and Garland, 2005). There has been a small amount of research looking at medium chain triglycerides in dogs and their neuroprotective role, but this has not been replicated in cats (Pan et al, 2010). Certain microbes, algae and microalgae produce the polyunsaturated fatty acids that are required by cats. The non-pathogenic fungi, Mortierella spp., produces approximately 70% arachidonic acid from the total amount of all the lipids it produces (Dediukhina et al, 2011). As there are so many positive implications for these polyunsaturated fatty acids in the human market, research has been conducted on how these can be sourced and engineered in larger quantities. This means that there is now a potential source of non-animal produced arachidonic acid for the pet food market.
Vitamin A
Supplemental vitamin A is derived primarily from fish liver oils (in which the vitamin occurs largely in esterified form) and from industrial chemical synthesis. Before the era of chemical production of vitamin A, the principal source of vitamin A concentrates was the liver and/or body oils of marine fish. Since industrial synthesis was developed in 1949, the synthetic form has become the major source of the vitamin to meet the requirements of domestic animals and humans. The synthetic vitamin is usually produced as the all-trans retinyl palmitate or acetate. Pre-formed vitamin A (retinyl esters that can be used by the cat) can also be manufactured by chemical synthesis from either a plant or animal base (Google Patents, 2008).
Urinary pH
Currently the Vegan Society's web page on feeding vegan diets to cats recommends regularly testing the pH of the cat's urine and adding in supplements if necessary to make the urine more alkaline (Vegan Society, 2015). This is because diets that are high in meat protein cause a more acidic urine; when the diet changes to plant based the urine becomes more alkaline which will encourage formation of struvite crystals in cats. However, if the urine is over-acidified, oxalate crystals could form. It is a very fine balancing act, feeding an appropriate diet that benefits health and welfare without the need for corrective supplements. Supplements that are used to change urinary pH are normally potassium citrate for urinary alkalinisation: 75 mg/kg or 2 mmol/kg by mouth every 12 hours, this is a licensed product (Cystopurin, GSL-AVM) (British Small Animal Veterinary Association, 2020). Vitamin C is used to acidify urine, but there is no licensed supplement and no suggested dose rates. The British Small Animal Veterinary Association formulary warns that prolonged use of vitamin C can increase the risk of urate, oxalate and cystine crystalluria and stone formation, because of its acidifying nature (British Small Animal Veterinary Association, 2020). There are many factors that influence crystal formation, including body weight, behavioural and/or stress aspects, feeding behaviours, food composition, age, sex, neuter status. What the animal is fed and how that food is offered is an aspect of these factors.
Are vegan diets ethically sound?
There is an ethical responsibility to address concerns regarding food sustainability and the effects of farming on the planet, but they should not compromise the pet's health. It is vital that the pet's nutritional needs are met, but whether or not this is done irrespective of the owner's personal views or beliefs on animal-based diets can be very difficult to balance. As there are non-animal-based sources of taurine, arachidonic acid and vitamin A, complete and balanced vegan pet foods are theoretically viable. Knowing the full list of ingredients for the diet being fed to the cat is very important to know if the diet is truly vegan. However, knowledge of the nutrient aspect rather than the specific ingredients is required to ensure the correct type of nutrients are contained within the diet. Being able to interpret the food labels and make an informed judgement on the diet might not be possible during the nurse consultation. Do not be afraid to tell the owner that asks about a specific diet that you will need to research the diet and then get back to them.
In the UK pet food manufacturers can join the Pet Food Manufacturer's Association. In order to join this, the diet needs to be proved to be a balanced and complete diet in lines with FEDIAF (European Pet Food Industry Federation) guidelines. At present no manufacturers of vegan diets foods are members of this association.
Are vegan diets nutritionally balanced?
Zafalon et al (2020) analysed four vegan diets (cat and dog) for macro- and micronutrients. All of the diets were lacking in at least one nutrient as recommended by FEDIAF and the Association of American Feed Control Officials. One study looking at protein sources in diets found that 50% of all the vegan and vegetarian diets tested contained mammalian DNA (Kanakubo et al, 2017). Other studies have found non-vegan commercial dog and cat foods containing meat proteins that were those not stated on the ingredient labels, this also included diets labelled as defined novel protein diets (Ricci et al, 2018). Inadequate labelling and contamination of pet foods is a common occurrence (Ricci et al, 2018). Meat protein does have to be declared on the label as an ingredient (Figure 1). This demonstrates the importance of independent batch testing of foods, and RVNs/veterinary surgeons should only recommend diets that have been batch tested by the manufacturers.
Animal Welfare Act 2006
Do vegan diets adhere to the Animal Welfare Act 2006? This states that all animals need to be supplied with a suitable diet. The Department of Environment and Rural Affairs (2017) produced the Code of Practice for the Welfare of Cats that states that cats need to be supplied with a meat-based diet in order to meet their nutritional requirements.
The welfare of pets is vital. Nutrition is just one element of this, and behaviour is another. It is important to consider what pets are fed and also how they are fed. Nutrition and behaviour go hand in hand with felines and it is an aspect that should be discussed with owners. Cats need to be allowed to exhibit hunting (play) behaviour.
It does raise two very important questions: just because we can do something (make a nutritionally balanced vegan diet for pets), should we; and are the current manufacturers of vegan diets researching their diets, performing long-term studies, and batch testing their foods for nutritional adequacy? Just because we can do it, are they doing it?
Conclusions
Whether you believe vegan pet foods to be just another new fad in the industry or a much needed move towards resourcing foods differently, they are very much in their infancy. Vast amounts of research is needed in the pet food industry, not just into vegan foods. There is so little research around diets as a whole, focusing on individual nutrients. Long-term studies are needed on the feeding of vegan diets to cats, as well as dogs.
KEY POINTS
- Theoretically vegan foods for cats are possible, they are just not being made yet.
- Food manufacturers need to be undertaking research regarding short- and long-term benefits and risks of feeding their diets.
- Sustainability of ingredients, and knowing where ingredients are sourced, is an important element of all pet foods.
- Clinically signs of deficiencies can take years to manifest, long-term studies are vital because of this. Taurine deficiency can take up to two years before clinical signs can be seen.