OVER the top animal lovers who want their pets to share a "cruelty free" vegan lifestyle are actually risking their own cats' health, vets have warned.

Unlike other mammals, including humans and dogs, cats are "obligate" carnivores, meaning they require meat in their diet.

But that hasn't stopped a rise in vegan pet food products being sold in Sydney's supermarkets.

Bondi Vet Hospital's Dr Kate Adams said pet owners needed to be "very careful" about vegan cat food advertised as a "complete and balanced" diet.

"I am a supporter of a meat-based diet in cats, on top of that they need to be fortified with vitamins and minerals," she said.

"I've had a couple of clients who fed only vegan - all of those cats came to not very good consequences."

Dr Adams said she was concerned cat owners might be led to believe all vegan food was okay for their furry friends.

Managing vet at RSPCA NSW Sydney Veterinary Hospital Dr Chris Greenwell said cats needed to eat specific molecules in meat, including taurine, arginine, vitamin A, vitamin D, niacin and arachidonic acid.

"When these are depleted over time there are many negative health impacts," he said. "If you're denying proper care to a cat you're effectively breaching your own theoretical values with dietary choices."

Emma Williams from All About Cats - an online nutrition guide - said the cat food industry was "changing dramatically" and warned owners to be wary and always speak to a vet before making any diet changes.

"It's best to check with your vet, get them to write up a meal plan if you want to reduce meat but you still couldn't cut it out completely," she said, adding that excluding meat from a cat's diet was "naive".

Cameron the cat and Snow Puppy the dog agreed with the vets when The Daily Telegraph presented the animals with two meals - one vegan and one not - both devouring the meat option.

Dr Jonine Penrose-Wall from the World League for Protection of Animals said the charity would only adopt animals out to owners who provide a healthy and balanced diet.

"When we re-home animals to vegans we certainly advise them about a comprehensive complete diet for the animals, including some raw meat," she said.