Cats famously adore tuna, which is exactly why it needs limits. Canned tuna for people is just fish: it lacks nutrients a complete cat food provides, and diets built around fish for human consumption have caused real deficiency problems in cats — including vitamin E-related illness. Cornell's feline health experts note that canned fish products meant for people have been linked to neurological problems in some cats when fed as a staple. Tuna in cat food is a different matter, because it is supplemented to be balanced.
Mercury is the second reason for moderation. Tuna is a large predatory fish and accumulates more mercury than smaller fish; a cat eating tuna frequently, given its small body size, is far more exposed than a human eating the occasional sandwich. Veterinary guidance lands on a simple rule: keep human-grade tuna to a small spoonful as an occasional treat, choose water-packed with no seasoning or oil, and let a complete cat food do the everyday work. A last, practical note: some cats get so fixated on tuna that they start refusing their regular food — another good reason to keep it rare and special.
What to do
No emergency action is needed for a bite of plain tuna. If tuna has been a daily staple, transition your cat back to a complete cat food and mention it at your next vet visit — especially if you notice poor coat, lethargy, or appetite changes.
Sources
This page is informational and is not veterinary advice. If your pet has eaten something risky, contact your vet or an animal poison control line right away.