Matthew Grant tried diet after diet to help lessen the food allergies of Cooper, his old English bulldog. "Raw diet, novel protein, kangaroo, venison, we tried just about everything," he says. Nothing worked - until he went vegan.

Now his three dogs are on plant-based diets, and he says their coats are shinier and softer. Cooper's allergies are completely gone. So remarkable was the change that Grant says he's now thinking about going vegan himself.

As more vegetarian kibble shows up in pet stores, switching Fido and Fluffy to plant-based nutrition may seem like a good idea. According to a study published this year in PLOS One, over a third of pet owners in English-speaking countries, the United States included, have considered putting their pets on plant-based diets.

Yet experts warn to be cautious. The science is sparse. While going plant-based - or at least forgoing meat - may be healthy for humans, it's not necessarily so for our furry friends - and for cats in particular.

Lisa Freeman, veterinary nutritionist at Tufts University, believes the answer to whether cats can be healthy on a vegetarian diet is an "unequivocal no."

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Unlike dogs, cats are obligate or "true" carnivores, meaning they rely entirely on animal flesh for some important nutrients. Take vitamin A: dogs and humans can convert beta carotene from plant foods into active vitamin A. Cats cannot - they need to ingest the vitamin directly with meat.

"There are also some fatty acids, like arachidonic acid, which is essential for cats and only found in animal fats," says Jennifer Adolphe, animal nutritionist at the University of Saskatchewan. Other challenging nutrients are vitamin D, vitamin B12 and taurine - an amino acid found in mammalian tissues (humans and dogs can synthesize it from plant sources). Taurine deficiency in cats can cause heart disease and vision problems.

Yes, it's possible to whip up a cocktail of supplements and sprinkle it over your cat's porridge. But, Adolphe warns, "you wouldn't want to just simply add those ingredients and hope for the best."

In real foods, nutrients come combined with thousands of others, interacting, complementing each other, changing the way they work. In supplements, such nutrients are taken out of context and don't necessarily work in the same way. Studies on humans show, for instance, that popping beta carotene or vitamin E supplements can actually increase mortality. And although supplements can be essential to health in case of deficiencies, they cannot replace proper nutrition, experts say.

Besides, almost no research exists about the health of vegetarian cats. The few studies were done on very small numbers of animals. In one, published in the Journal of the American Veterinary Medical Association, blood samples were obtained from 17 vegetarian cats. All were fine in terms of vitamin B12; as for taurine, three cats were found deficient. But the concentrations of vitamin A and arachidonic acid weren't even measured.

Such a lack of research worries vets.

"I don't think there are a lot of people with advanced training in pet nutrition that think it's a good idea to feed cats vegan," says Cailin Heinze, animal nutritionist at Tufts University.

If you want to have a vegetarian pet, dogs are more promising - they've certainly come a long way from their wolf ancestors. Whole-genome sequencing study of dogs and wolves revealed that, over the domestication process, 10 genes related to starch digestion and fat metabolism have significantly changed.

"It's therefore likely that dogs digest starch more efficiently than wolves," says Erik Axelsson, geneticist at Uppsala University in Sweden, who led the study.

But this does not mean you can put Fido on vegan chow without a second thought.

"There are some amino acids that are essential for dogs, that are not so for people," says Heinze, adding that plants tend to be poor in these particular amino acids.

According to a 2018 review of plant-based diets for dogs, our Fidos also cannot easily produce vitamin D3 from sunlight, the way humans do, and need to get it from animal products. Yet, overall, switching a dog to a vegetarian diet is "doable," Heinze says. For some breeds, such as Dalmatians, going veg could be even beneficial to health - a low-protein diet can lower the risk of bladder stones, which these dogs get frequently.

Another issue is the quality of commercial vegetarian pet foods (meat-based products have problems, too). A 2015 University of California at Davis study of 24 vegetarian diets for dogs and cats showed that a quarter did not meet all amino acid requirements.

For this reason, Adolphe advises pet owners to do "some homework to find out who is behind the company, if it employs a full-time qualified nutritionist, what kind of quality control measures do they use." The safest bet? Veterinary therapeutic diets, Heinze says.

One way or the other, consulting a certified pet nutritionist is always advisable.

The good news is that some new protein sources offer hope for owners who want to safely switch their pets to less meaty diets.

In Europe, dog food made from insects is already on sale. In California, a start-up called Wild Earth is working on lab-grown mice meat for cats (although the progress recently stalled because of a lack of a regulatory approval process). This can be good news for those who are worried about their cats or dogs health on vegan chow but are also concerned about potential effect of meaty pets' diets on the environment.

Looking at data on climate impacts of pets' diets can certainly send a chill down your spine. Of all greenhouse gases released by humans, 14.5 percent are from livestock - about the same as emissions from all transportation methods combined: passenger cars, ships, trucks, airplanes, you name it. In 2017, Gregory Okin, a geography professor at University of California at Los Angeles, calculated that if American dogs and cats formed their own country, it would rank fifth in the world in meat consumption. More recent Japanese data paints a similar picture: Nutrition of cats and dogs there is responsible for as much greenhouse gas emissions as Latvia or Cambodia.

Add to this other meat-production related issues - water pollution, deforestation, animal suffering - and it's clear why some may be tempted to eliminate livestock products from their pets' diets.

Heinze suggests a possible halfway solution for pet owners: reduced meat for dogs and, yes, for cats, too, although more cautiously, and in consultation with your veterinarian.

"There seems to be this dichotomy right now: your cat either eats a meat-only diet or your cat eats a vegan diet," she says. "And I think probably the healthiest option is somewhere in the middle."

This article was written by Marta Zaraska, for The Washington Post.