Gluten-free diets for our four-legged companions are on the rise, but the health benefits of this booming pet food market is controversial.

As an ever-growing percentage of the human population opt out of eating grains due to genuine, or perceived, gluten intolerance, pet owners are starting to impose the same diets on their pets.

Kiwis spent $1.8 billion in 2016 on their companion animals, and food was the largest part of that.

One advertising ploy at the forefront of pet food around the world is the notion of "grain-free" - with one UK study identifying it accounted for 15 per cent of the market.

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And New Zealand is not exempt. A perusal of the pet food isle of one inner city Auckland supermarket, found 13 separate brands declaring themselves "grain free" - about 10 per cent of the products featured on the aisle.

But while some pet owners spoken to by the Herald on Sunday attested to the health benefits of such dietary change, a veterinary science expert says Kiwi pet owners are unjustifiably forcing their own dietary preferences onto their cats and dogs.

Massey University Associate Professor in small animal medicine Nick Cave said the market of grain-free pet food was "grossly disproportionate by several orders of magnitude" to the frequency of pets with allergies to gluten.

Cave stressed the only animals who benefit from gluten free diets are those with a genuine allergy to gluten.

The equivalent allergy in humans is Celiac disease, which is an immune reaction to gluten that accounts for broadly 1 per cent of the global population.

"Unfortunately, we as consumers tend to fall foul of the concept that if anything is advertised as being free of something, that something must therefore be bad," Cave says.

"So as there was an increase in the availability of gluten-free foods, so there grew this gross misconception that there was actually a problem with gluten.

"That chaos has spilled over into veterinary nutrition, and if it is deemed unhealthy for me, it is assumed it is unhealthy for dogs and cats."

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Such views are backed up by a 2018 study commissioned by the Ministry of Business, Innovation and Enterprise on pet food industry, which revealed an "increasing number of pet-owners are treating their pets as part of the family" as trends in pet food mirror trends in human food choices.

Veterinarian at Grey Lynn's Vet Care clinic Dr Shalsee Vigeant said there was not yet enough information on allergies among companion animals to advise owners against any one diet.

"I tell my clients, it doesn't matter what they feed their pets as long as it's balanced and healthy for that pet," Vigeant said.

"I do believe as vets we should tell owners there's a lot of controversy about raw diets and grain free diets, and basically, like humans, every dog or cat is different and you have to adjust their diets to them."

But two owners spoken to by the Herald on Sunday swear by the diets.

Aucklander Gracey Whiting has also been feeding her 10-year-old snoodle, Enzo, a grain free diet since he was two.

"He was getting really dry, flakey skin and the vet suggested we try grain-free," Whiting said.

"His skin cleared up quickly, he lost a couple of kilos, and we don't go to the vet very much any more."

Central Otago resident Isla Irvine said her 8-year-old red-nose pitbull had an eczema like skin condition that was dramatically improved by a transition to a grain-free diet - to the point where she was taken off steroidal medication.

"Isis' eyes were all closed, she couldn't really see anything, her paws were red raw, it was actually horrible. And now she's a completely different dog."

Despite success stories among some owners, another serious concern around grain-free dog food is a link to fatal heart disease.

The US Food and Drug Administration (FDA) has undertaken an investigation into a "highly unusual" number of cases of canine dilated cardiomyopathy.

The consensus within the veterinary community is that the link to heart disease is likely caused by the absence of a amino acid called Taurine in many grain-free pet foods.