A tiny piece of plain, unseasoned nori now and then is unlikely to hurt most healthy cats, but it adds nothing a complete cat food doesn't already provide (Cornell Feline Health Center). Skip it if your cat eats a vet-prescribed diet, such as an iodine-restricted thyroid diet or a kidney or heart diet, unless your vet says otherwise (WSAVA).
The bigger problem is the seaweed cats actually get offered: snack sheets brushed with oil and salt, flavors with garlic or soy sauce, sushi rolls, kelp supplements and whatever washed up on the beach. Those are a no.
Cats are obligate carnivores, which means they rely on nutrients found only in animal products. If your cat begs for a nibble of plain nori, it's a treat. If you skip it, your cat misses nothing.
Seaweed for Cats at a Glance
Seaweed | OK for cats? | Why |
|---|---|---|
Plain nori (sushi nori, unseasoned) | A tiny piece, occasionally | Lowest in iodine of the common kitchen seaweeds tested; no salt or flavorings added |
Roasted seaweed snacks (oil and salt) | No | Added salt; often oil and flavorings too |
Seasoned seaweed (garlic, onion, soy, wasabi, chili) | No | Salt, and garlic or onion powder are toxic to cats |
Sushi rolls | No | Rice, soy sauce, seasonings and often raw fish |
Dried kelp, kombu, kelp flakes or powder | No, unless your vet suggests it | Very high and variable iodine |
Hijiki | No | Can contain high levels of inorganic arsenic |
Wild seaweed from the beach | No | Unknown species, salt and sand |
Kelp listed in a complete cat food | Yes, as part of that food | The food is formulated and balanced as a whole |
What Is Seaweed?
Seaweed is a group of marine algae that grow in the ocean and along coastlines. Instead of roots, stems and leaves, it has a holdfast that grips rocks and flat, leaf-like blades (fronds) that take in nutrients straight from the water.
The kinds you'll find in a kitchen are mostly nori (the dark sheets around sushi), kombu and other kelps (thick, leathery strips used for broth), wakame (in miso soup), dulse and hijiki. They differ a lot in what they contain, and that matters for cats.
Do Cats Like Seaweed?
Most cats don't seek it out. Seaweed isn't part of a cat's natural diet, and cat tastes run toward meat. Some cats are curious about the crunch or the ocean smell, and a few will pester you for a piece of your snack. The snack kind is usually the salted, oiled one, which is exactly the part they shouldn't have.
Can Cats Eat Nori or Seaweed Sheets?
Plain nori is the best of the bunch. It is much lower in iodine than kelp: one analysis of edible seaweeds found dried nori at about 16 µg of iodine per gram, wakame at about 42, and kelp flakes at more than 8,000 µg per gram (Zava and Zava, Thyroid Research). By that count, kelp flakes carry more than 500 times the iodine of nori.
If you want to share:
- Check the label. The only ingredient should be seaweed. Sushi nori sold for rolling is often plain; snack packs usually aren't.
- Tear off a tiny piece. The World Small Animal Veterinary Association (WSAVA) advises picking treats of a size and shape that lowers the risk of choking, and a small, thin piece is easier to chew and swallow.
- Keep it inside the treat budget. All treats together, seaweed included, should make up less than 10% of your cat's daily calories (WSAVA treat guide). Your vet team can tell you what that looks like for your cat.
- Make it occasional. A treat, not a daily topping.
If your cat throws it back up or gets an upset stomach, that's your answer: skip it next time.
Can Cats Eat Seaweed With Salt or Seasoning?
No. Most seaweed snacks are roasted with oil and salt, and many come flavored with soy sauce, garlic, onion, wasabi or chili. A cat is a lot smaller than the person the snack pack was made for.
Two things make seasoned seaweed risky:
- Salt. Too much salt can cause vomiting, diarrhea, depression, tremors, seizures and even death in pets (ASPCA).
- Garlic and onion. Cats are the most susceptible species to onion and garlic poisoning, and concentrated forms such as powders and dried flakes are the usual culprits (Merck Veterinary Manual). Those are exactly the forms used in seasoned snacks. They damage red blood cells, and the signs usually show up a few days later.
Our list of human foods cats can't eat covers the other usual kitchen suspects.
Can Cats Eat Roasted Seaweed?
Only if it's plain. "Roasted seaweed" on a snack packet almost always means seaweed roasted with oil and salt, sometimes sesame or other flavorings. Plain roasted nori with nothing added is fine as a tiny piece; the snack-aisle kind is not.
Can Cats Eat Dry Seaweed?
It depends on the seaweed. Dried plain nori is the same nori as above. Dried kelp, kombu, kelp flakes and kelp powder are a different story because of iodine. Dried kombu came in at around 2,350 µg of iodine per gram in the same analysis, and kelp flakes at over 8,000 (Zava and Zava). Keep those for the soup pot.
Seaweed, Iodine and Your Cat's Thyroid
Iodine is a mineral the thyroid gland uses to make its hormones. Seaweed can be very high in it, and the amount varies a lot from one species to the next.
You may read that seaweed "causes hyperthyroidism" in cats. That overstates it. Hyperthyroidism (an overactive thyroid) is common in middle-aged and older cats, and its most common cause is a benign, hormone-producing thyroid tumor (Merck Veterinary Manual). Researchers have studied whether iodine in cat food plays a part, but a direct link to too much or too little dietary iodine hasn't been shown (Edinboro and colleagues, Journal of Feline Medicine and Surgery). Still, the thyroid runs on iodine, so iodine-heavy extras like kelp are best left to your vet.
It matters most for cats already being treated. One treatment option is a prescription iodine-restricted diet, and it only works if the cat eats nothing else: other foods or treats make it ineffective (Merck). A seaweed snack is the opposite of what that cat needs. Other cats with hyperthyroidism are managed with medicine such as methimazole, surgery or radioiodine, chosen by your vet.
Signs of an overactive thyroid to mention at your next vet visit include weight loss despite a big appetite, restlessness, drinking and urinating more, vomiting, diarrhea and a fast heart rate (Merck). These come from the thyroid, not from a nibble of nori, but they are a good reason to get an older cat checked.
Kelp in Cat Food and Supplements
You may spot kelp or "dried kelp" on a cat food label. Pet food makers use it as one way to add iodine, alongside iodized salt or sea salt (Edinboro and colleagues). In a complete and balanced food, that kelp is part of a formula built as a whole, so there's nothing you need to add.
Kelp supplements and powders sold "for pets" are different. They stack extra iodine on top of a diet that already contains it, and the amount in seaweed varies a lot. Don't add one unless your vet recommends it for your cat. WSAVA counts supplements as treats, too.
Sushi, Beach Seaweed and Hijiki
- Sushi rolls. The nori is the least of it. Rolls come with rice, soy sauce, seasonings and often raw fish. Cornell does not recommend raw animal products for cats because they can carry infectious disease (Cornell). Keep the takeout box out of reach.
- Wild seaweed on the beach. You can't tell the species, it's coated in salt and sand, and you don't know where it has been. One UK vet has described treating a dog for a suspected gut blockage after it ate seaweed on a beach (Totnes Times vet column). Few cats get to the beach, but if yours does, don't let it graze the tide line.
- Hijiki. Food Standards Australia New Zealand names hijiki among the seaweed products that can contain high levels of inorganic arsenic (FSANZ). Not one to share.
Which Cats Should Skip Seaweed Altogether?
Ask your vet before offering any seaweed if your cat:
- eats a prescription or therapeutic diet, for example an iodine-restricted thyroid diet, a kidney diet or a heart diet. WSAVA notes that conventional treats may not suit cats on a vet-prescribed diet and advises asking your vet team (WSAVA);
- has hyperthyroidism or another thyroid problem;
- has any ongoing medical condition or food sensitivity.
For everyone else, plain nori is optional. Your cat won't miss it.
My Cat Ate Seasoned Seaweed: What Should I Do?
A stolen corner of plain nori usually needs nothing more than watching for an upset stomach. A salted or seasoned snack is different: even if your cat ate just one sheet, call your vet or a poison line for advice. They will judge whether the amount matters for your cat, so you don't have to.
- Take away what's left and keep the packet. The ingredient list is what your vet will ask about.
- Call your vet or a pet poison line now, before signs appear. ASPCA Animal Poison Control: (888) 426-4435. Pet Poison Helpline: (855) 764-7661, open 24/7. A fee may apply.
- Have the details ready: what it was, roughly how much, when, and your cat's weight.
- Don't try to make your cat vomit or give any home remedy unless the vet or poison line tells you to.
Watch for vomiting, diarrhea, stumbling, tremors, seizures or collapse after salt (Pet Poison Helpline), and for weakness, pale or yellow gums, dark urine or collapse in the days after garlic or onion (Merck). Any of those means your cat needs to be seen today.
Sources
- Cornell Feline Health Center: Feeding Your Cat
- WSAVA: Feeding Treats to Your Cat
- Merck Veterinary Manual: Disorders of the Thyroid Gland in Cats
- Merck Veterinary Manual: Garlic and Onion (Allium spp) Toxicosis in Animals
- ASPCA Animal Poison Control: People Foods to Avoid Feeding Your Pets
- Pet Poison Helpline: Salt
- Zava TT, Zava DT. Assessment of Japanese iodine intake based on seaweed consumption in Japan. Thyroid Research, 2011
- Edinboro CH, Scott-Moncrieff JC, Glickman LT. Feline hyperthyroidism: potential relationship with iodine supplement requirements of commercial cat foods. Journal of Feline Medicine and Surgery, 2010
- Food Standards Australia New Zealand: Arsenic
- Totnes Times: Kelp, the seagrass won't pass (vet column by Joe Shams)
This article is for general information and is not a substitute for advice from your veterinarian.
