A small amount of tuna, occasionally, is fine for cats. It's not toxic, it's high in protein, and most cats go absolutely berserk for it. The problem isn't a single serving — it's making tuna a regular part of a cat's diet, which causes a few distinct and legitimate problems.
The mercury problem
Tuna — especially albacore — is one of the higher-mercury fish. Mercury accumulates in tissue over time, and cats are small animals. A human can clear mercury more efficiently than a cat can. Regular tuna consumption in cats has been linked to mercury toxicity, which can cause neurological symptoms: loss of coordination, balance issues, abnormal gait. This isn't theoretical — it happens, and it's called "tuna toxicosis" in veterinary literature.
A bite here and there? Not a concern. Tuna every day or as a primary food source? That's where it becomes a real issue.
The nutritional gap problem
Tuna is not a complete food for cats. It's high in protein but lacks several nutrients cats need — most notably vitamin E and certain fatty acids that cats can't produce themselves. A cat eating mostly tuna can develop a condition called steatitis (yellow fat disease), caused by vitamin E deficiency. It's painful and not something you want your cat to deal with.
Regular cat food — wet or dry — is formulated specifically to meet cats' nutritional needs. Tuna isn't.
The preference problem
This one is maybe the most practically annoying. Cats fed tuna regularly often develop what's sometimes called "tuna addiction" — they become so habituated to the intense flavor and smell that properly balanced cat food seems unacceptable by comparison. Then they'll refuse to eat their regular food, hold out for tuna, and you've accidentally created a situation where you're either giving in or watching your cat stage a hunger strike.
Vera, my cat, once held out for 18 hours because she'd gotten tuna two days in a row and decided that's what food was now. I learned from that.
If you do give tuna
Use tuna packed in water, not oil. A small amount — maybe a teaspoon — as an occasional treat. And occasional means maybe once a week at most, not every day. Plain cooked chicken is honestly a better treat option if you want to give your cat something high-protein and exciting — lower mercury, more nutritionally balanced, and it won't create the same fixation issues.