The short answer
Plain tuna packed in spring water, drained, is fine as a small treat. It isn't a complete diet: it lacks enough vitamin E and other nutrients cats need, and diets heavy in tuna have been linked to vitamin E deficiency (steatitis, or "yellow fat disease").
Which tuna
- Best: canned tuna in spring water, no salt added
- Avoid: tuna in brine (salty), in oil (extra fat), or with onion, garlic or flavourings
- Raw tuna: not recommended. Raw fish carries parasites and bacteria and contains thiaminase, which destroys vitamin B1.
How much and how often
A teaspoon or two, once or twice a week, is a sensible limit for an average cat. Tuna is calorie-dense and cats love it, so it's easy to overdo. Keep all treats under 10% of daily calories; the cat food calculator shows your cat's budget.
Mercury and picky eating
Tuna is higher in mercury than most fish, which is another reason to keep it occasional. Many cats, given tuna regularly, begin turning down their normal food, which is a hard habit to break. Cat foods with tuna as an ingredient are formulated to be complete and are a different matter.
Frequently asked questions
Can kittens eat tuna?
Tiny amounts are fine occasionally, but kittens need a complete kitten food to grow properly. Don’t let tuna replace meals.
Can cats drink the water from canned tuna?
Spring water from unsalted tuna is fine in small amounts. Skip brine, which is too salty.