The death and suffering inflicted upon approximately seventy billion chickens, pigs, sheep, cows, and other land animals, both intensively and extensively farmed, who are slaughtered annually, and upon trillions of intensively farmed or wild-caught fish, in order to fulfil the desire of some people for meat, has been thoroughly documented; as have the deleterious environmental impacts of both intensive and extensive animal farming.

It is because of the ethical concerns of a growing population of vegetarian animal guardians, and because of medical conditions such as allergies caused by beef, lamb and other animal-derived dietary ingredients, that vegetarian pet food brands were first developed. However, vegetarian companion animal diets remain the subject of controversy, which is exacerbated by ignorance of the health and nutritional issues involved, even among veterinarians and other experienced animal carers. The following summarises some of these issues.

Health hazards of meat-based diets

The health hazards to dogs and cats (and, of course, so-called ‘food’ animals) inherent to meat-based companion animal diets, are extensive, and difficult to avoid. These diets provide a vast industrial dumping ground for slaughterhouse waste products, ‘4-D’ meat (from animals that are dead, dying, diseased or disabled, on arrival at the slaughterhouse), old or spoiled supermarket meat, large numbers of rendered dogs and cats euthanised in animal shelters, which may contain detectable levels of euthanising solution, old restaurant grease complete with high concentrations of dangerous free radicals and trans fatty acids, and damaged or spoiled fish, complete with potentially hazardous levels of mercury, PCBs and other toxins. The combined results are rendered so delicious to cats and dogs by the addition of ‘digest’ – a soup of partially dissolved entrails, usually from chickens – that more than 95% of US companion animals subsist primarily on commercial meat-based diets (Perry 1996), generating in excess of $11 billion annually for the US pet food industry (API 2002).

Also hazardous to the health of our companion animals are the pathogenic bacteria, protozoa, fungi, viruses and prions, their associated endotoxins and mycotoxins, and the hormone and antibiotic residues and dangerous preservatives, that are found within meat-based diets.

Diseases described in the scientific literature following long-term maintenance of cats and dogs on commercial meat-based diets include kidney, liver, heart, neurologic, visual, neuromuscular and skin diseases, bleeding disorders, birth defects, immunocompromisation and infectious diseases (Dow et al. 1989, DiBartola et al. 1993, Strieker et al. 1996 and Freytag et al. 2003).

After examining and treating many thousands of animals over more than a decade as a practicing veterinarian, I’ve become convinced that rates of diseases such as cancer, kidney and liver disease, are far higher than would occur naturally. I believe that many cases are probably exacerbated or directly caused by long-term exposure to the numerous hazardous ingredients of meat-based diets. Kidney disease, for example, is one of the top three killers of companion animals, and may be exacerbated by the extra load placed on the kidneys by the high protein content (Di Bartola et al. 1993) and poor quality ingredients of many meat-based commercial diets. Left untreated, kidney disease may result in the systemic buildup of toxins, leading to uraemic poisoning, appetite loss, vomiting, neurological disorders, and death.

Vegetarian companion animal diets

On the other hand, reported cases suggest that vegetarian diets may increase overall health and vitality, decrease incidences of cancer, infections, hypothyroidism (a hormonal disease), ectoparasites (fleas, ticks, lice and mites), improve coat condition, control allergies and body weight, decrease arthritis, and even result in the regression of diabetes and the resolution of cataracts (PETA 1994, Peden 1999, Gillen 2003).

Dogs may be biologically classified as omnivores, due to their ability to subsist on a mixed diet of animal and plant-based material in their natural environments. In contrast, cats are classified as obligate carnivores, because their evolutionary anatomical, physiological and biochemical adaptations to a carnivorous lifestyle prevent them from deriving substantial benefit from the plant-based material available in their natural environments. Both wild cats and dogs do consume plant material, primarily sourced from the gastrointestinal tracts of their consumed prey.

However, these factors are of little relevance to domesticated cats and dogs maintained on artificial diets. In fact, there is no scientific reason why a diet comprised only of plant, mineral and synthetically-based ingredients cannot be formulated to meet all of the palatability, nutritional and bioavailability needs of the species for which they are intended. In fact, several commercially-available vegan (no animal product) companion animal diets aim to do so, and have jointly supported a healthy population of thousands of cats, dogs and ferrets (who are also naturally carnivorous) for many years (Weisman 2004). However, sound quality control procedures, including periodic laboratory nutritional analysis, should be implemented to ensure formulated diets consistently meet these requirements.

Safeguarding health

If a diet is nutritionally inadequate – whether vegetarian, or meat-based – disease is likely to result sooner or later. Hence use of a nutritionally complete and reasonably balanced commercial diet, or of a nutritional supplement with a home-made diet, is essential, to prevent disease and safeguard health.

Regular urine pH monitoring is also important to detect urinary alkalinisation, with its consequent potential for urinary stones, blockages and infections, that may result from a vegetarian diet in a small minority of cases. Urinary alkalinisation may be corrected via a range of dietary additives. Further information can be found under vegetarian canine or feline diets.

Changing to a vegetarian diet

The fixation of some cats or dogs to meat-based commercial brands to which they have become accustomed is primarily due to the addition of ‘digest,’ the industry euphemism for partially dissolved entrails, usually from chickens. According to Small Animal Clinical Nutrition, “Digest is probably the most important factor discovered in recent years for enhancing the palatability of dry food for cats and, to a lesser degree, dogs.” (Lewis et al. 1987).

Consequently, considerable patience and persistence may be required when altering diets, particularly of fussy cats. In difficult cases it may first be necessary to withhold all food (not water!), for one day. This will stimulate the appetite without harming a healthy adult. It may be necessary to change the diet gradually, e.g., by using 90% old and 10% new diet for a few days, then switching to 80% and 20% for a few more days, thereby transitioning to the new diet over several weeks, or even longer if necessary. A gradual change is more acceptable behaviourally in difficult cases, and also allows an appropriate transition of digestive enzymes (to the extent possible) and intestinal flora (bacteria), thereby minimising the chance of gastrointestinal reactions such as diarrhoea.

Guardians should demonstrate by their behaviour that they consider the new diet just as edible as the old (without possibly warning or alarming their pet by making a fuss). They should not be concerned if their pet eats around the vegetarian food at first; just having it in close proximity to the usual food will help the pet make the necessary mental association. Mixing the food thoroughly may help, as may the addition of odiferous (the sense of smell is very important) and tasty additives, such as nutritional yeast, vegetable oil, nori flakes and spirulina. Gently warming the food may also help. Guardians should remove uneaten food, and offer only fresh food.

The most important factors for transitioning difficult pets onto a vegetarian diet are gradual change and persistence. Using these principles, the most stubborn of cats and dogs have been successfully weaned onto healthy vegetarian diets. Vegetarian pet food recipes may be obtained from Peden (1999) and Gillen (2003) and from some suppliers.


  • Animal Protection Institute(API). What is REALLY in your pet's food?: you may not want to know. Revised 29 Jan. 2002. WJRT-TV/DT., accessed 17 May 2004.
  • DiBartola SP, Buffington CA, Chew DJ, McLoughlin MA, Sparks RA. Development of chronic renal disease in cats fed a commercial diet. J Am Vet Med Assoc 1993; 202(5): 744-751.
  • Dow SW, Fettman MJ, Curtis CR, LeCouteur RA. Hypokalemia in cats: 186 cases (1984-1987). J Am Vet Med Assoc 1989; 194(11): 1604-1608.
  • Freytag TL, Liu SM, Rogers QR, Morris JG. Teratogenic effects of chronic ingestion of high levels of vitamin A in cats. J Anim Physiol Anim Nutr (Berl) 2003; 87(1-2): 42-51.
  • Gillen J. Obligate Carnivore. Seattle, WA: Stein Hoist Books. 2003.
  • Lewis L, Morris M, Hand M. Small Animal Clinical Nutrition. 3rd Edn. Topeka, KS: Mark Morris Associates. 1987: 2-3.
  • Peden J. Vegetarian Cats & Dogs. 3rd Edn. Troy, MT: Harbingers of a New Age. 1999.
  • People for the Ethical Treatment of Animals (PETA). Dog health survey. Unpublished. 1994., accessed 12 Aug. 2004.
  • Perry T. What's really for dinner?: the truth about commercial pet food. The Animals' Agenda. 1996; Nov. - Dec., accessed 02 Apr. 2012.
  • Smith C.A. Research roundup: changes and challenges in feline nutrition. J Am Vet Med Assoc 1993; 203: 1395-1400.
  • Strieker MJ, Morris JG, Feldman BF, Rogers QR. Vitamin K deficiency in cats fed commercial fish-based diets. J Small Anim Pract 1996; 37(7): 322-326.
  • Weisman E. Personal communication to Andrew Knight re: Evolution Diet vegan pet food. 24 Feb. 2004.