Why do cats eat grass and then throw up?

We love our cats, but why does everything they do seem to result in a little puddle of vomit? And why, if they know that grass makes them sick, do they so contentedly graze on a patch of lawn like hungry sheep?

Theories abound about why cats eat something they can’t seem to tolerate, and some of these ideas have merit. Let’s talk about all the theories first, and then the science that explains the real reason why cats eat grass. But first…

Can cats digest grass?

Cats are obligate carnivores.[1] This means that cats must eat meat. “Obligate” means “by necessity” and carnivore means “meat eater.” Just because there are vegan cat foods on the market does not mean they meet a cat’s nutritional needs. Meat is a biological necessity for cats. There is some thought that cats have no need for carbohydrates at all.[2]

(For more information about cats and vegan/vegetarian diets, read this post.) 

Humans and dogs, by contrast, are omnivores, meaning we are able to remain healthy by eating both animals and plants. (“Omni” means “all.”)

But even humans and dogs who eat “everything” don’t eat grass. Grass is non-toxic and is edible, and plenty of animals thrive on it. Why don’t we?

The reason we don’t eat grass is because of the cellulose. The cell walls of most plants contain cellulose, a complex carbohydrate, and humans, cats, and dogs, cannot digest cellulose. The plants we do eat contain other nutrients that we can digest and so we take what we can from them. But grass is mostly cellulose and there isn’t much nutrition in it for us.[3]

Animals that can eat grass have very specialized digestive systems that allow them to break down all that cellulose in grass. Cows, for example, have a special chamber in their stomachs called a rumen. Microbes in the rumen produce an enzyme called cellulase that breaks down the cellulose into more digestible bits. There’s a lot more to the whole process than that, but the bottom line is that cats (humans and dogs) don’t have a rumen. They can’t digest grass.

Do cats eat grass because they like to throw up?

There are all kinds of theories about why cats eat grass if they can’t digest it. One of them is that cats like to throw up.[4] Well, not that cats actually enjoy vomiting, but that vomiting makes them feel better. The idea behind this theory is that wild cats who don’t eat supper from a can would eat their prey in its entirety: bones, fur, and feathers. Vomiting allows a cat to eliminate all the uncomfortable indigestible stuff from their bellies.

Another related theory is that cats eat grass because they don’t feel well for some other reason. Maybe a grass-eating cat has an inflammatory bowel disease or food allergy that causes gastrointestinal distress.[5] Maybe, in the absence of a medicine cabinet full of Tums and Gas-X your cat reaches for a few blades of grass to alleviate discomfort.

This theory has some holes, however. Cats who do not suffer from gastrointestinal problems or disease of some other sort eat grass too. And it does not explain why cats who do not have any fur or bones in their belly have an urge to regurgitate anyway.

Do cats eat grass for the folic acid?

Another theory is that cats chew on grass to extract the vitamin folic acid from its juices.[6] Folic acid (vitamin B9) helps the cat’s body produce hemoglobin, a protein that transports oxygen in the blood. It’s true that cat mothers produce folic acid in their milk because it’s so important that their kittens get enough. Adult cats who are low in folic acid can become anemic.

The idea that cats eat grass for the folic acid assumes that there is an epidemic of cats deficient in folic acid, but the science doesn’t support that contention. DSM, one of the world’s leaders in animal nutrition, contends that “folic acid produced by intestinal microflora is usually adequate to meet requirements.”[7] In other words, the bacteria in cats’ own bellies makes all the folic acid a cat needs.

A true folic acid deficiency was noted specifically in cats on special diets who were also taking antibiotics. Folic acid deficiency is probably not a concern for most cats.

Do cats eat grass for the fiber?

Another theory holds that because grass has so much indigestible cellulose (aka fiber), that cats eat it to help keep their digestive tracts in good working order[8] – the way a human suffering from constipation might down a glass of Citrucel.

It is true that constipation is a common cat problem.[9] It is possible that cats are self-medicating with grass to alleviate an overfull colon. Notice if your cat hasn’t pooped in more than a day or two, or if he is crying or straining in the litter box and consult your veterinarian posthaste.

If your cat is not suffering from constipation, this is probably not the reason he is eating grass.

Do cats “stress eat” grass?

It is true that cats can suffer from some of the same emotional ailments that haunt many humans, including anxiety. Cats who experience anxiety may perform repetitive behaviors to self-soothe, including over-grooming and excessive vocalization.[10] One theory holds that the reason cats eat grass or household plants is that it gives an anxious cat something to chew on – as a kind of distraction from the emotional discomfort she is feeling.

This theory may address the question of grass-eating for a small population of cats suffering from anxiety disorder. But it does not explain why cats who enjoy good mental health also eat grass.

Do cats eat grass because we don’t give them the “whole mouse”?

Another theory to explain grass eating in cats is that wild cats eat rodents, which in turn eat grasses and grains. When a cat eats a mouse, he’s not just eating mouse protein, he’s also eating that mouse’s last meal, what Dr. Hazel Carney described to Westvet.net as “a salad with it’s main dish.”[11]

Perhaps a grass-eating cat is just trying to make up for the lack of greens his wild ancestors (or feral friends) would have inadvertently consumed.

While this theory is interesting and most certainly true in part, it assumes that cats need vegetation in their diets. Current science does not support the idea that cats need vegetables or fruits to complete their diet.[12]

Science says grass-eating is an instinctual behavior to keep parasites under control

Scientists think they have an answer that explains grass-eating in cats. Last year, at the annual convention of the International Society for Applied Ethology in Norway, researchers shared results of a study[13] involving more than 1,000 cat owners who admitted that they spent at least three hours a day watching and spending time with their pet. (Who doesn’t?)

77% of the cat owners surveyed said that they personally witnessed their cats eating grass at least six times. Only 11% said they never saw their cats eating grass at all.

For one thing, most grass-eaters in the study didn’t vomit at all. Only a quarter of the grass-eaters threw up afterwards, and most of these (91%) didn’t appear ill before vomiting, according to their owners.

This observation tends to refute the theory that cats eat grass because they have an upset stomach and are trying to vomit to relieve their discomfort.

According to researchers, eating plants is just instinctual. These scientists observed grass-eating behaviors in other wild carnivores and noted that eating non-digestible plants is a common tactic used by meat-eaters to keep the number of parasitic worms in their intestinal tract to a “tolerable load.”

So, grass-eating cats are probably just doing what their ancestors have always done to help themselves live to see another day, even though most well-loved household cats already enjoy a life free of parasites.

Don’t let your cat eat any old grass

While eating grass is, at best, a pleasurable activity for your cat, and, at worst, an indulgence in a non-toxic (albeit indigestible) snack, not all grasses are created equal.

First of all, a cat that is looking for something green to eat might turn to other plant matter in the absence of real grass. Many household and landscape plants are poisonous to cats. Know whether your household and yard plants are dangerous for cats and seriously consider rehoming them if they are.

Also, read the labels of the fertilizers, herbicides, and pesticides that you may apply to your indoor and outdoor gardens. Many of these chemicals are toxic to cats and may be inadvertently ingested by a cat seeking a little greenery to nibble.

For cats who simply must have grass to munch on, consider buying her a special kitty garden. These come with untreated seeds (or already established plants) of safe grasses, such as oats, rye, wheat, or barley.

Grass is safe for cats, except in the unlikely event of…

Keep an eye out for the unlikely occurrence of a grass foreign body in your cat’s sinuses. Rarely, a blade of grass can make its way up into a cat’s sinuses instead of out of the mouth when she vomits. Excessive sneezing or nasal discharge from one nostril are possible signs of this unusual scenario, which absolutely requires a trip to the veterinarian.[14]

Similarly, an undigested piece of grass can wend its merry way all the way through the digestive tract and become entangled in feces. Rarely, a long strand will become so snarled that it becomes difficult for the cat to pass.[15] A loving cat owner may have to glove up and gently pull on the strand to help it along.   

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Dawn LaFontaine is a lifelong animal lover who always seems to have a little pet hair in her keyboard. Her blog, Kitty Contemplations, helps cat guardians better understand and care for the special beings they share their lives and homes with. Her cat-products business, Cat in the Box, sells beautiful, well-made, and award-winning products that she designed to meet the biological needs of cats.

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FOOTNOTES

[1] Gates, Margaret. “Https://Feline-Nutrition.org/Answers/Answers-What-Exactly-Is-an-Obligate-Carnivore.” StackPath, Feline-Nutrition.org, 9 Feb. 2013, 14:33, feline-nutrition.org/answers/answers-what-exactly-is-an-obligate-carnivore.

[2] Danks, Lee. “The Cat as a Carnivore: Proteins, Carbohydrates and beyond...” Veterinary Practice, 1 June 2020, veterinary-practice.com/article/the-cat-as-a-carnivore-proteins-carbohydrates-and-beyond.

[3] (Razeez), Sparkonit. “Why Can't Humans Digest Grass If Cows, Goats And Sheep Can?” Sparkonit, 14 Sept. 2019, sparkonit.com/2016/10/10/cant-humans-digest-grass-cows-goats-sheep-can/. 

[4] Negron, Vladimir. “Why Do Cats Eat Grass?” PetMD, 20 Apr. 2009, www.petmd.com/cat/wellness/evr_ct_eating_grass.

[5] Dr. Wailani Sung MS, PhD. “Why Does My Cat... Eat Grass or Houseplants?” Vetstreet, 22 Sept. 2016, www.vetstreet.com/our-pet-experts/why-does-my-cat-eat-grass-or-houseplants.

[6] “Why Do Cats Eat Grass?” Animal Planet, 15 May 2012, www.animalplanet.com/pets/why-do-cats-eat-grass/.

[7] “Folic Acid.” Www.dsm.com, www.dsm.com/markets/anh/en_US/Compendium/companion_animals/folic_acid.html.

[8] “Cat Behavior and Expert Answers: Why Do Cats Eat Grass?” Pet Central by Chewy, 23 July 2019, petcentral.chewy.com/cats-eat-grass/.

[9] “Constipation.” Cornell University College of Veterinary Medicine, 13 Jan. 2020, www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/constipation.

[10] O'Brien, Christine. “Why Do Cats Eat Grass?: Hill's Pet.” Hill's Pet Nutrition, 4 May 2018, www.hillspet.com/cat-care/behavior-appearance/why-do-cats-eat-grass.

[11] Hoskin, Christina. “Why Cats Eat Grass-and When to Worry About It.” WestVet Veterinarian Hospital Services, 13 Mar. 2017, www.westvet.net/blog/post/dr-hazel-carney-on-why-cats-eat-grass-and-when-to-worry-about-it/.

[12] Meliker , Shayna. “Can Cats Eat Broccoli, Carrots and Other Fruits and Vegetables?” Vetstreet, 17 Feb. 2015, www.vetstreet.com/our-pet-experts/best-fruits-and-vegetables-for-cats.

[13] Hart, Benjamin L., et al. “Characterization of Plant Eating in Cats.” International Society for Applied Ethology 2019 - Book of Abstracts, International Society for Applied Ethology, 2019, cdn.eventsforce.net/files/ef-r47ypsy562lc/website/9/e-book_international_society_for_applied_ethology_2019_-_book_of_abstracts.pdf.

[14] “Cat Behavior and Expert Answers: Why Do Cats Eat Grass?” Pet Central by Chewy, 23 July 2019, petcentral.chewy.com/cats-eat-grass/.

[15] Doctor, Pete Wedderburn - Animal. “Dogs and Cats Eat Grass - but Why Do They Do It?” Independent, Independent.ie, 27 Apr. 2016, www.independent.ie/regionals/newrossstandard/lifestyle/dogs-and-cats-eat-grass-but-why-do-they-do-it-34658732.html.