When we do a Custom Meal diet consultation, we often find owners are feeding their cats fish as the primary protein in their diet.  The motivations behind this choice are well intentioned, and usually rooted in feeding whatever it takes to get a finicky cat to eat.

Cats evolved in deserts of Africa (why they have a low thirst drive, but that’s a topic for another day). They didn’t feed on fish. Your kitty’s natural prey are small land dwellers like mice, reptiles, insects, and birds.

There are some issues with feeding fish to cats (and dogs).  Tuna is typically the main fish being fed.  Tuna is a predator fish, top of the food chain, eating smaller fish.  This means they accumulate more mercury than plankton feeders lower on the food chain like anchovies, sardines, and Sockeye salmon.

Then you have the issue of fish used in pet (dog and cat) food.  Not only is it what’s discarded from human food, but it is heavily preserved during the manufacturing process using a chemical preservative called ethoxyquin, a known cancer-causing agent.  (You want to use human-grade fish if you’re going to feed it to your cat or dog.)

Since fish isn’t a natural food for cats many problems pop up when fed as the primary protein for long periods.  Fish is one of the most highly allergenic foods for felines. Allergies cause systemic inflammation leading to asthma.  Fish fed in high amounts can also lead to thiamine deficiency [reference], which can cause loss of appetite, seizures, and even death.

Long-term ingestion of fish in cat food can also deplete vitamin E resources. Vitamin E deficiency known as steatitis, can also be life-threatening.  Seafood is a very rich source of iodine, but cats aren’t designed to process a lot of iodine could be linked to hyperthyroidism.  

Fish isn’t all bad.  In small amounts it’s enticing and gives diet variety.  Using as a secondary protein to spice up some food or an occasional treat, is fine.  Plus fish is rich source of natural, healthy Omega-3s. Fish is an all-things-in-moderation food for dogs and cats.

But if you’re one of those people who open a can of tuna packed in oil, day after day for your cat, you could be creating some serious problems you and your cat are going to have to deal with in the future.