Many people, especially animal lovers, choose to follow a vegetarian or vegan diet.

There is evidence that diets with no or low levels of animal products may be better for the environment and have human health benefits.

A recent study found a growing number of pet owners are feeding their dogs and cats vegetarian or vegan diets, or would like to do so.

However, meat-free diets can potentially kill or sicken pets.

The gut instinct

The shape of an animal's teeth and length of its gut are excellent indicators of the kind of diet needed for good health.

Plant eating animals like cows and horses have long faces to accommodate abundant flat grinding teeth. Their large bellies contain long, complicated guts which break down the hard-to-digest plants that dominate their diet.

Dogs and cats are naturally carnivorous. They have sharp pointy teeth for shearing meat, and short, simple guts. Meat-based diets are high in nutrients and very digestible meaning they need less time in the digestive tract.

Human digestive systems fit in between the two. We are omnivores and thrive eating a varied diet that contains both animal and plant products.

The act of chewing offers psychological, bone and dental benefits to dogs. ( Pixabay: SNGPhotography )

We have sharp biting incisors and flatter molars, and our intestines are reasonably long but not complicated.

Because humans are naturally omnivorous it is possible to live healthily on diets that are free of animal products, if we choose to do so.

To stay healthy, vegans and vegetarians need to eat a balanced diet that pays attention to incorporating vitamins like B12 and D, as well as minerals like calcium, iodine and zinc or these diets can mean an increased risk of bone breakage in humans.

Can your cat be vegetarian?

But what happens if we turn our cats and dogs into vegos, or vegans?

As anyone who owns a cat knows they are special, and this extends to their diet

Cats are obligate carnivores — a true carnivore which must eat meat to survive. Meat provides cats with an essential amino acid called taurine, as well as vitamins A and B, arginine and arachidonic acid.

Puppies and kittens are at more risk if they're fed a meat-free diet.

Without these nutrients cats develop a series of devastating health problems including eye and liver problems, birth defects and heart failure.

In the UK, cat owners who feed their pets a vegan diet could be prosecuted under the Animal Welfare Act.

The risk of death is very high if cats are fed a meat-free diet.

Since the 1980s supplementing cat food with taurine has largely eliminated these health problems. But supplementing food with required nutrients is not always the answer.

What about dogs?

The co-evolution of the domestic dog alongside their human companions means that although dogs have carnivore anatomy, they have evolved some nifty adaptions for living alongside people and scavenging our scraps.

As a result, dogs are happy on omnivorous diets including meat and plants.

Dogs have evolved nifty adaptions to living alongside people and scavenging scraps. ( ABC News: Norman Hermant )

However, dogs still need an easily digestible diet that is rich in protein so that nutrients can be rapidly absorbed into their bodies.

For this reason, vegan dogs will struggle to digest the high fibre of plant-based diets and are at risk of vitamin D and B deficiencies.

In one study of vigorously exercising dogs, those eating a plant-based diet showed damage to their blood. The blood remained normal in meat-eating dogs.

However, other studies have showed the health and fitness of dogs could be maintained on vegan diets. And meat-free diets even benefit some breeds — reducing the incidence of bladder stones in Dalmatians, for example.

But dogs enjoy the taste of meat and the physical act of chewing is entertaining and offers psychological, bone and dental health benefits

But what about vegan pet food?

Although plants and meat may contain similar nutrients, plant sources are not always easily digestible or have nutrients in the correct amounts.

The short guts of dogs and cats mean plants are less digestible than meat because pets have less time to extract the nutrients they need as the food makes its way through their bodies. As a result sources of plant protein, like soybean, are often not suitable for animals.

It's now easy to buy commercially produced vegetarian or vegan food for your pet. Whether this food is nutritionally complete is less clear.

Independent analysis shows 25 per cent of meat-free pet diets in the US have nutrient deficiencies despite claiming to be nutritionally complete.

But it should be acknowledged that this problem is not unique to meat-free pet diets as nutritional deficiencies and excesses have also been reported in Australia in commercial cat foods.

Can you cook your own vegetarian meals for pets?

It is difficult to successfully formulate a nutritionally sound vegetarian diet for pets. Many dogs are lactose intolerant and can't handle milk and eating too many raw eggs can result in deficiencies in essential nutrients like biotin.

Puppies and kittens fed meat-free diets would be at higher risk of problems because their high protein needs (up to five times an adult dog) and requirements for a complex balance of nutrients is important and could lead to bone disease or stunted growth.

Proponents of meat-free diets in cats, suggest supplementation of nutrients like vitamin A. Too much vitamin A in adult cats can cause bones to fuse, stopping the cat from moving naturally and causing pain and distress.

Vegan appeal is clear

Interest in the vegan and vegetarian pet food market is clear.

Many advocates of vegetarian pet diets cite justifiable concerns about climate change, animal welfare and food sustainability.

Many meat free pet food companies also claim ingredients are organic or human grade.

Most meat in pet food comes as a bi-product of meat production for human consumption. It is arguable that this allows a majority of the sacrificed animal to be used, reducing food waste.

It is absolutely justifiable to explore the best way possible for both animals and people to live sustainably, and for animal welfare to be maximised. But it is also important for pet owners to be aware of the risks of death and disease associated with feeding vegetarian or vegan diets to their pets.

Nutrition is complex. Discussion with your veterinarian is highly recommended before you change your pet to a different diet.

However, forcing naturally carnivorous animals to eat diets which may jeopardise their health would be counter productive.

Rachel Allavena is an associate professor in the School of Veterinary Science at the University of Queensland.